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\begin{document}
\title{AC 2000:  The Astrographic Catalogue on \newline
 The Hipparcos System}
\author{Sean E. Urban, Thomas E. Corbin and Gary L. Wycoff}
\maketitle

\begin{center}
\Large
\vspace*{1.0 in}
Catalog of Positions Derived from the Astrographic Catalogue
Measures.  Positions are on the Hipparcos System (J2000.0) at the
Epochs of Observations.

\vspace*{1.0 in}
United States Naval Observatory \\
Washington, D. C.
\end{center}
\normalsize
\newpage
\tableofcontents
\newpage
\listoftables
\vspace*{4.5in}
\footnotesize
\noindent
The CD-ROM cover is an illustration of the Henry brothers using the 
Paris Astrograph, originally appearing in E. Mouchez's ``La Photographie
Astronomique'' (Paris 1887).  The picture was reproduced from General History
of Astronomy, Volume 4 A (Gingerich 1984).
\normalsize
\newpage

\section{Introduction}
The AC~2000 is a catalog of 4,621,836 stars covering the entire sky.  The data
are from the images measured and published as part of the Astrographic
Catalogue (AC).
The positions are on the Hipparcos reference frame (ESA 1997), having originally
been
reduced plate-by-plate using the Astrographic Catalog Reference Stars
(ACRS;  Corbin \& Urban 1988, Corbin \& Urban 1990).
Each of the 22~zones of the AC has been reduced independently,
since telescopes, observing techniques and measurement methods varied.
This document describes the history behind the Astrographic Catalogue, the 
methodology employed in the plate reductions, the technique used to 
bring the data to the system defined by Hipparcos, the resulting catalog
and information about each of the participating observatories.

\section{The Carte du Ciel}

The Carte du Ciel was an international effort begun more than a century ago to
determine positions better than 0.5~arcsec for all stars brighter than 11th
magnitude using
photographic plates and, using another set of plates, to
publish charts representing the relative positions of all stars of 14th
magnitude and brighter.  The charts proved to be very expensive to photograph
and reproduce, so many institutions did not complete this part of the work.
However, the astrographic program
designed to measure all stars to 11th magnitude was completed.  Actually,
the original goal of 11th magnitude was generally surpassed.  In fact, some
observatories routinely measured stars as faint as 13th magnitude.
These plate measures, as well as the formulae used to transform them to 
equatorial
coordinates, have been published in what is known as the Astrographic Catalogue
(AC). 

In total, 20 observatories from around the world participated in 
exposing and measuring the plates.  Each was assigned a specific zone, between
two parallels of declination, to photograph.  In order to compensate for    
any plate defects, each area of the sky was to be photographed twice, using
a two-fold, corner-to-center overlap pattern.  This pattern was continued 
even at the zone boundaries; each observatory's plates would overlap with
those of the observatories responsible for the adjacent zones.
The participating observatories agreed to standardize the type of telescope,
so each plate photographed had a similar scale of approximately 60 arcsec/mm.
The measurable areas of the plates were 2 $\times$ 2 degrees, so the 
overlap pattern consisted of plates that were centered on every
degree band in declination, but offset in right ascension by two degrees.
The first plates in the even degree bands were centered at right ascension
0 hours 0 minutes; the first plates in the odd degree bands were centered
with right ascension several minutes higher (corresponding to approximately
one degree).

In addition to the overlap pattern and type of telescope to use, the 
observatories also agreed to expose a grid, 
called a {\em r\'{e}seau}, on each plate.  It was originally  used to monitor
emulsion shifts.
After the shifts were demonstrated to be quite small, the practice of exposing
a r\'{e}seau on each plate was continued because it aided in the measuring
of the star positions by letting the measurer refer each image position to
the grid lines.  Each r\'{e}seau unit was approximately 5 mm.
The r\'{e}seau orientation defined the plate's $x,y$ coordinate system.

All participating observatories (with the exception of Vatican) used one of
two measuring methods, {\em short-screw} or {\em eyepiece scale}.
In both methods, a set of spider wires was centered over an image, then
the distance traveled by
the slides carrying the spider wires was read.  With the short-screw
method, this distance was read off of the screws used to move the
slides.  With the eyepiece scale method, this distance was inferred by a
scale in the focal plane of the microscope.  In general, the eyepiece
scale method was faster, but less precise. 

Although telescope type, plate size, and use of a r\'{e}seau were standardized,
many other factors, such as reference catalog used,
reduction technique and printing formats were left up to the individual 
institutions. 

\section{Published Data}

Most observatories published their results in several volumes; each consisting
of measures from plates centered on the same degree of declination.
Generally, each line in the printed volumes consists of data from one star,
including its measured
$x,y$ value, a measure of brightness (magnitude or diameter), the plate
number and a running number on the plate.  
Other data concerning epoch of exposure, hour angle at mid-exposure,
air temperature, barometric pressure, r\'{e}seau used, observer,
measurer and measuring machine are usually provided in separate tables.
Additionally, provisional
plate constants used to transform the $x,y$ measures to standard coordinates
are supplied.

The published data have been transferred to machine-readable
form via double-keypunching (that is, typing each record twice to remove most 
keypunching mistakes) including errata found in the published volumes.
An additional literature search has been conducted on all
zones to increase the probability that all published errata not found with the
original volumes have been corrected.  Each page of each volume was searched 
for pertinent notes.  If important notes were found, they were keypunched and
the measures to which they refer were flagged.  A summary of the keypunching
information can be found in Table~\ref{tab:keypunch}.

\section{Preparing the Data for the Reduction Software}

It was necessary to prepare the data to put it in a standard format
required by the plate    
adjustment software.  The preparation process was broken down into three
discrete tasks: matching the images with reference stars; 
matching images with those of the same star on another plate; and
converting the data to a standard format.

\subsection{Matching images with the reference stars}

  The reference catalog used throughout the individual plate reductions of the 
Astrographic Catalogue was the Astrographic Catalog Reference Stars (ACRS;
Corbin \& Urban 1988, Corbin \& Urban 1990).
This was compiled with the distinct purpose of reducing the AC 
measures.  The ACRS is a dense set of reference stars, about eight per 
square degree, containing data from over 150 different catalogs specifically
included to strengthen the positions and proper motions at early epochs and
thus providing accurate positions for the AC plates.  The ACRS is on the 
system of FK5. A typical ACRS at the epoch of an AC plate will have a positional
error of about~200 to 250 mas per coordinate.

Equatorial coordinates for all AC images were computed from the rectangular
coordinates via the published plate constants.  The ACRS data were then
brought to the average epoch of the zone and transformed to the AC equinox,
B1900.0.  A positional match was then made between the AC and ACRS.  Checks were
made to ensure that {\em illegitimate} matches have not taken place.  An example
of an illegitimate match is an ACRS star that matches with two separate
AC images that are on the same plate.  Also at this stage, all plates were
checked to ensure that each contains an adequate number of
identified reference stars.  
Fewer than expected matched reference stars on a plate may indicate problems
with the published constants or plate centers.  These plates were investigated
and the problems were corrected.

\subsection{Matching images with those on other plates}

The same equatorial coordinates computed in the previous step
were used to identify images of the same
stars that lie on different plates.  Images within 2.0 arcsec were generally
identified as the same star at this stage.
Illegitimate matches were investigated and treated appropriately.
Each image was then assigned an {\em internal star number} that is unique
for each star in the zone, regardless of the number of plates on which it
appears.  The data were verified to ensure that all images marked with an ACRS
number have the same
internal star number, and visa versa.  No two images on one plate were allowed
to have the same internal star number.

\subsection{Converting data to a standard format}
The data were combined in one, standard format file.  This file contained
all pertinent information about each plate and image.  Plate information
such as plate centers, sidereal time of exposure, metrology data, measuring
machine and measurer, r\'{e}seau used, emulsion type, and epoch of exposure were
included, if they were published.
Data pertaining to each star, that is the $x,y$ values, image diameter
or magnitude,
internal star number, and ACRS identifier, were included.
A conversion from the published $x,y$ units to units of millimeters, along
with a translation of the coordinates so the origin is in the approximate
plate center, was performed when necessary.  The persons responsible for 
the zone preparations can be found in Table~\ref{tab:people}.

\section{Preliminary Reductions and Investigation of Plate Models}

The core of the plate reduction software was the same as that used in the
reductions of the Cape Photographic Catalogue 2 data
(Zacharias et al. 1992).  For all but the Potsdam, Perth and Sydney zones,
an eight-constant plate model, consisting of four
orthogonal terms ($a,b,c,$ and $d$), two non-orthogonal terms ($e$ and $f$) and
two tilt terms ($p$ and $q$) was initially used, as shown in
Eqs.~\ref{eq:xmodel1} and~\ref{eq:ymodel1} .
For the Potsdam, Perth and Sydney data, the same plate model was used except 
the corrections corresponding to the two tilt terms ($p$ and $q$) were
pre-applied to the data and were not solved for on individual plates.
\begin{equation}
\xi = ax + by + c + ex + fy + x^2p + xyq
\label{eq:xmodel1}
\end{equation}
\begin{equation}
\eta = ay - bx + d - ey + fx + xyp + y^2q
\label{eq:ymodel1}
\end{equation}
At this step, no corrections to the published
$x,y$ values were applied. Investigations of radial distortions,
tangential distortions,  magnitude equation, coma, periodic measuring errors
and r\'{e}seau dependent systematic errors were investigated 
following the procedures outlined previously (Urban \& Corbin 1996;
Urban et al. 1996).  Since an uncompensated systematic error may 
cause a star to be an outlier, only reference stars with residuals
over five times the standard deviation of unit weight of the solution
were removed while various plate models were investigated.

\subsection{Corrections to the $x,y$ values}

Results of investigations of systematic errors may lead to applying a
correction to the published star measures prior to final plate constant
determinations.
 
To investigate if radial distortion exists in the data, the reference star
radial residuals were plotted against distance from the plate center.
If a radial distortion was present, the data points were described by a 
non-zero function.  Additionally, the radial residuals were examined for 
magnitude, measurer and measuring machine dependence.
A similar investigation was conducted looking for
the existence of a tangential distortion, however none was found in any of 
the zones.

The presence of a magnitude equation, which is a systematic offset of
stars based solely on
their brightness, was investigated by plotting the reference stars' residuals
with respect to magnitude.  This was performed separately for both the $x$ and 
$y$ coordinates.  Also,
the existence of a magnitude equation that is dependent on the $x,y$
measures was also investigated.  Additionally, magnitude equation varying 
with measurer and plate epoch was considered.  Note that a magnitude
equation outside the range of the reference stars is extremely difficult
to find (Eichhorn 1974).  It is possible to compensate for a magnitude equation 
in the magnitude range of the reference stars, but it is unwise   
to extrapolate those corrections to field stars that may be two or three
magnitudes fainter.

The presence of coma, a change in scale based on magnitude, is uncovered
in two ways.  First, investigating the $x,y$-dependent magnitude equation will
reveal coma if it exists within the range of the reference stars. 
Second, using fainter stars, a plot of the difference between the mean
position (computed from overlapping plates) and an individual image position
vs. plate coordinates of the individual position will, using data from hundreds
of stars and different magnitude ranges, reveal the presence of coma.  If 
coma was found, it was examined for variations with respect to plate epoch.

Errors caused by the measuring mechanism were investigated 
by analyzing the residuals of reference stars as a function of
location on the $x$ and $y$ measuring apparatus (usually a screw or an eyepiece
scale).  The presence of an error with a period 
corresponding to the length of the measuring screw or eyepiece scale is quite
likely caused by the machining of that part, but may also be a function of the
measurer.  In either event, these errors can be corrected.

Additionally, the presence of a remaining field distortion pattern
can be examined by using plots similar to those produced while investigating
coma.  These patterns were investigated for dependence on magnitude, r\'{e}seau,
measuring machine and measurer. 

Using the information from these investigations, a plate model was     
developed for each of the AC zones.
Equations~(\ref{eq:xprime})
and~(\ref{eq:yprime}) show the corrected $x,y$ values used in the final plate
adjustments.
\begin{equation}
x^\prime = x+\rm RD + \rm TD + \rm ME_{ \mit x} + \rm MC_{ \mit x}
      + \rm S_{ \mit x} \mit mx + \rm MA_{ \mit x} + \rm FDP_{ \mit x}
\label{eq:xprime} 
\end{equation}
\begin{equation}
y^\prime = y+\rm RD + \rm TD + \rm ME_{ \mit y} + \rm MC_{ \mit y}
      + \rm S_{ \mit y} \mit my + \rm MA_{ \mit y} + \rm FDP_{ \mit y}
\label{eq:yprime}
\end{equation}
In Eqs.~(\ref{eq:xprime}) and~(\ref{eq:yprime}),
RD is the correction applied to compensate for radial
distortion; TD is the correction to compensate for tangential distortion;
ME and MC are corrections to compensate for magnitude equation and 
$x,y$-dependent magnitude equation; S is the correction for coma; MA corrects for
measuring apparatus errors, and FDP compensates for any remaining field distortion
pattern.  The variable $m$ is the computed magnitude.  By substituting 
$x^\prime, y^\prime$ for $x,y$ in Eqs.~\ref{eq:xmodel1} and~\ref{eq:ymodel1},
the 8 constants for each plate were computed.
The significant systematic errors found and corrected in each zone are listed in 
Table~\ref{tab:const}.

\section{Investigation of Discordant Data}
Once a suitable plate model was determined, the computed positions were
used to investigate problems such as mismatched images, blended images
of multiple stars and typographical errors.

\subsection{Incorrectly matched images from overlapping plates}

Images may be incorrectly matched in a variety of ways.  Images are originally
matched in the way described in the subsection ``Matching images with those on other
plates.''  Positions computed from the provisional plate constants can 
change significantly due to the new reductions. So, images earlier believed to
be from two stars because their positions were dissimilar may now
be recognized as coming only from one if their new positions are closer.
To investigate this possibility, images closer together than a certain
distance were investigated.  The distance chosen depended on the
range in plate epochs (since proper motion will then be a factor) as well as
accuracy of the $x,y$ measures.
Images that fall outside the expected precision may be a result of 
a double star, poor measurements of the same
star or good measurements of a star that has moved due to proper motion.
Deciding which was the case was often difficult because no 
comparable catalog with the epoch of the AC plates exists with which
to check the stars in question.  The Preliminary Version of the Third 
Catalogue of Nearby Stars (Gliese \& Jahreiss 1991) aided in
detecting high proper motion stars, but this catalogue, as its name states,
contains only nearby stars.  The  Luyten's Two-Tenths Catalogue and its
supplement (Luyten 1979, 1980; Luyten \& Hughes 1980) has also been used in
aiding in the identification of high proper motion stars.
The Washington Double Star
catalog (WDS; Worley \& Douglass 1996), the internationally recognized source
of visual double star data, was also used but is limited because of its
incompleteness over the AC magnitude range and its occasional use of AC data.
However, certain criteria were followed in deciding if two images were from
the same or different stars.  These criteria were zone   
specific, but for most zones images were identified as coming from the same 
star if an image was within 3.5~arcsec of a mean position and there was no
indication of duplicity.
Images from different stars, but incorrectly identified as coming from 
the same star, will result in a high standard deviation in right ascension
or declination, $\sigma_{\alpha}$ or $\sigma_{\delta}$.  Stars with the largest
$\sigma_{\alpha}$ and $\sigma_{\delta}$ were investigated for this possibility.

\subsection{Duplicate entries}
Virtually every zone has mistakenly printed some of its measures more than
once.  These were easily found since the data in question was either exactly
the same as another record (in cases of true duplication) or the resulting
star positions of two records were ridiculously close for the telescope scale
and typical seeing.  In general, images closer than 1.0~arcsec that appear 
on the same plate were suspected of being duplicate entries.  In cases of this
type, generally only one of the measures was kept.

\subsection{Blended images}

Images of double stars that are blended on one plate but discrete on another
require special treatment.  Two possible problems exist under these 
circumstances.  First, if the blend is identified as one of the separate
images, then the computed separation of the double star will be smaller than
it is in actuality (since the blend will fall near the photocenter).
Second, if the blend is not identified with either
discrete image, then three sets of coordinates will be computed where only
two stars exist.

Under the first scenario (a blend is matched with a discrete image),
the data will consist
of a multiple star system with at least one star having a large 
standard deviation of position, $\sigma_{\alpha}$ or $\sigma_{\delta}$.
To investigate this possibility, the area around each star was searched for
the presence of another star. If a nearby star was
found, then the data were examined for a blend if $\sigma_{\alpha}$ or
$\sigma_{\delta}$ of either star exceeded a certain amount (usually 
1.0~or 0.9~arcsec).  Additionally, any two 
stars closer than about 3.0 arcsec, or any triple or quadruple systems,
were examined.

Under the second scenario, (a blend is not identified with either of the 
discrete images), then a star system
will appear to have one more member than it really has.  To minimize this
occurrence, an area with radius of about 10~arcsec
around each star was searched for the presence of two other stars.
Additionally, an area with radius of about 15.0~arcsec was searched for three 
other stars.  If multiples were found, they were examined to ensure that
images were identified correctly.  In the situation described above, blends
are discarded.

\subsection{Large proper motion candidates}

Due to the span of plate epochs, images from large proper
motion stars may be displaced from one another by several arcsec and hence
not be matched as coming from the same star.  To minimize this occurrence,
a search in the Luyten's Two-Tenths Catalogue and its supplement
was made to identify high proper 
motion stars.  An additional search for large proper stars in the Third
Catalogue of Nearby Stars was also made.  Stars in 
the final catalog having large standard deviations of position are known 
high proper motion stars.

\subsection{Typographical errors}
A bright star may only appear to have one image if one of its records
contains a typographical error.  To find and correct this, all bright,
single image
stars were investigated. (The exact magnitude limit depends on the zone. 
Typically all stars down to 11.5 are investigated.)

The process involved generating positions for these images if one
altered one of the digits of the printed $x$ or $y$ value, then performing 
a search around these pseudo-positions.  If another image was
found at one of the locations, a typographical error may be present.  The
Twin Astrograph Catalog (TAC; Zacharias et al. 1996),
was used to determine which of the two, if either, should be changed
for all zones north of, and including, Tacubaya.  For the more southerly zones
which are not covered by TAC, the  Tycho Input Catalogue (Halbwachs et al. 1994)
has been used.
If a corresponding star was not found in the TAC or TIC, then a search in the 
the Digitized Sky Survey using the Skyview interface was made to determine 
which, if either, contains a typographical error. 

To minimize the possibility of a printing error in the magnitude data, 
a standard deviation of the magnitude, $\sigma_{ \rm mag}$, was generated
from the computed magnitudes for every star appearing on more than one plate.
The stars with the 
largest $\sigma_{ \rm mag}$ were investigated.  Additionally, range checks were
made on the original data as well as the final computed magnitudes to ensure
that all values were reasonable.

\subsection{Investigation of other potential problems}

To investigate the possibility of a few plates being adjusted incorrectly,   
the positions of the 500 stars with the largest $\sigma_\alpha$ and
$\sigma_\delta$ were plotted for each zone.
Additionally, the percentage of stars on each plate whose $\sigma_\alpha$
and $\sigma_\delta$ are in the highest 1\% of that zone were computed.  Plates
that have more of these than expected were investigated.

Positions of all stars were plotted to ensure that the data contain no missing
areas, such as a block of data having not been typed or accidentally 
discarded.  Positions of stars having only one image, and positions of stars
having multiple images, were plotted separately to investigate the 
possibility of missing plates.

To avoid the presence of non-stellar objects in the final catalogue, a search
in the New General Catalogue of Nebulae
and Clusters of Stars (NGC), the Index Catalogue (IC), and the Second Index
Catalogue (Sinnot 1988) was made.  The Digitized Sky Survey, using the 
Skyview interface, was used to graphically show potential NGC and IC objects
present in the data.  Those records found to be non-stellar NGC or IC objects
have been discarded.

\section{Final Plate Model and Weights}

It was necessary to ensure that the plate model was still valid because it was
originally developed on the data that included erroneous identifications
and typographical errors.  Once any needed revisions are made, plate weights
can be assigned.  For most of the AC zones, a plate's weight is
the inverse of the variance of the positions of the stars which it contains,
after removing the stars with the highest 1\% $\sigma_{\alpha}$ and
$\sigma_{\delta}$.  The removal of these stars prior to computing the plate
weights reduced the possibility that a few, high proper motion stars  
adversely affected the weighting of an entire plate.

No grating was used in the AC program, so bright stars are over-exposed on the
plates and should not be used in the final adjustment nor be included in the 
final catalog.  All stars with mean computed magnitudes brighter than
4.0 have been removed prior to final plate adjustments.  Reference stars
with a residual larger than 3.0 times the standard deviation of unit weight
of the plate solution were removed from determining the plate parameters
and the plate adjustment is performed again.  These stars are included in the
final catalog, but are treated as field stars.  

\section{Linking of Individual Zones}

 The Astrographic Catalogue was observed in discrete zones in the sky, and
the reductions, by necessity, were made on individual zones.  However it 
is desirable to link these together to make one, cohesive catalog.
To do this, stars in common to adjacent zones were identified.  Blended
images, high proper motion stars, and typographical errors were investigated
as described above.  Each star was assigned a new internal star number.
All images were combined to yield weighted mean right ascension and 
declinations in the FK5 system at the weighted mean epoch of observation for
the equinox J2000.0.

\section{Conversion of the AC Magnitudes}
The Astrographic Catalogue contains non-uniform magnitude measures, in part
because of different techniques used by participating observatories.  Many of 
the published magnitudes are unreliable, especially for the faintest and 
brightest stars.  Thus, it is desirable to transform the magnitudes to 
some kind of well-known (or often used) system.  The plates used
were most sensitive in the blue spectral region, so a logical choice
of systems was the Tycho B, since the Tycho catalog (ESA 1997) contains about
one million stars covering much of the AC magnitude range.

Each zone was treated independently.  Only stars identified in Tycho as being
single and with negligible variability were used for the calibration.
Differences between AC and
Tycho B magnitudes as a function of AC magnitude were computed.  Polynomial
expressions describing the results were computed via least-squares fitting.
Some extrapolation of these polynomials was required because, in general,
the Tycho catalog is not as faint as the AC.  (Usually an extrapolation
of less than 0.5 magnitudes was used.  Beyond that the corrections were held 
constant).  Corrections based on 
these functions were applied to each observation within the zone.  Thus the
magnitudes contained in AC~2000 should, in a broad sense, be close to that
of Tycho~B.

\section{Conversion to the Hipparcos System}

The conversion of the FK5 based Astrographic Catalogue to the system of
Hipparcos was necessary because Hipparcos is now recognized as the standard
reference frame in the optical wavelength, following the recommendation of the
IAU Working Group on Reference Frames.  The conversion
was made in three steps.  
\begin{enumerate}
\item{Stars in common between the AC and 
Hipparcos were identified.  Systematic differences between the two 
catalogs were found and applied by the following method.
For each AC position, all AC observations of Hipparcos stars within a 2~degree
radius were identified.  A weighted mean residual was computed using
the differences between the observations and the Hipparcos catalog positions
at the epochs of the AC observations.  The weighting function used was
parabolic;
those closest to the central AC star were given the most weight while 
those near 2~degrees away were given the least.  At this stage, no residuals
exceeding 1500~mas were used.  The weighted mean residual
was then applied to the central AC observation.}

\item{After step~1, it is possible that a magnitude equation remains in the 
AC positions due to the presence of one in the ACRS or a remaining one
in the original AC measures.  Thus, each of the 22~zones was analyzed
and corrected independently.  For each zone, mean residuals (AC-Hipparcos) as 
a function of
magnitude were computed and described by a least-squares fit to a polynomial
function.  Since individual observations of any star may contain different
measured AC magnitudes, each observation was corrected based
on the value of the polynomial function.}
\item{A repetition of step~1 was necessary for convergence.  During this step,
no residuals exceeding 750~mas were used.}
\end{enumerate}

\section{Cross-Referencing Information}

In order to aid users and to facilitate future work on the 
Astrographic Catalogue, most stars from the ACRS, Hipparcos, and Tycho 
Catalogs have been identified.  This cross-reference is not intended to be
100\% complete, however the vast majority of stars from these catalogs
are identified.   

Identification of the ACRS stars was a natural result of the original plate
reductions.  Even ACRS stars not used in the plate reductions
due to large residuals are identified.

Identification of the Hipparcos stars was necessary for the conversion to the
Hipparcos system.  To make the identification, only stars in the Hipparcos
catalog 
identified as being single or whose solutions for the components are given
were matched (No stars with '*' or '+' in the AstroRef field were used; only
multiples with 'C' in MultFlag were used).   Using these, the Hipparcos
positions were moved to the epoch of the AC stars.  A search area with a 
radius of six arcsec around the AC star was used.
If there was a one-to-one match, that is 
one AC star and one Hipparcos star in the search area, then the Hipparcos
number is added to the AC record.
If there was not a one-to-one match, for example one Hipparcos star and two AC 
stars,  then a ratio of the distances between the Hipparcos and the
closest AC star and the Hipparcos and the farther AC star was
computed.  If the value of the ratio is 0.2 or less and the magnitude difference
between the close AC and Hipparcos star was less than 1.5 magnitudes, then 
the  Hipparcos number was added to the closer AC record.  Otherwise, the
matching was considered ambiguous and no cross identification was made.

To match the Tycho catalog, no Tycho
stars where duplicity is indicated nor extremely low astrometric
quality entries were matched (No stars with 'D','R' or 'S' in the MultFlag field
or stars with '9' in the Q field were used).  Using a similar procedure as 
the Hipparcos matching, the Tycho stars were brought to the the AC star's epoch
by the application of the Tycho proper motions.  An area with a radius of 15
arcsec around each AC star was searched.  If more than one Tycho star was found
in the search area, a ratio test (using .2 as the cutoff) of the differences
was utilized.
For the Tycho stars not matched at this point,  another search was made but
without application of any proper motions.   A search area with a radius
of 20 arcsec was used.  Again, a ratio test of .2 was applied for all 
non-singular matches.
For the Tycho stars still not matched, a search in the New Luyten's Two Tenths
catalog was made.  The Luyten's proper motion was then applied to the Tycho 
stars to bring them to the epoch of the AC positions.  A search area with a
radius of 15~arcsec around each AC star was used.  Once again, if more than one
star fell within the search area, a ratio test was used to determine which
observations were really the same star. 

\newpage
\section{Description of AC~2000}

\subsubsection{Right ascension}

The mean right ascension for each star as computed from its weighted images,
in units of hours, minutes and seconds of time, referred to the Hipparcos
system (J2000.0) at the weighted mean epoch of observation.

\subsubsection{Declination}

The mean declination for each star as computed from its weighted images,
in units of degrees, minutes and seconds of arc, referred to the Hipparcos
system (J2000.0) at the weighted mean epoch of observation.

\subsubsection{Magnitude}
Each magnitude listed here is the average of all the images for that star.
They should roughly correspond to the Tycho B system.
See section ``Conversion of the AC Magnitudes'' for details.

\subsubsection{Epoch}
The mean epoch for each star as computed by its weighted images, in years.

\subsubsection{Number of images used}
The number of individual images used to compute position, magnitude, epoch and
standard deviation of position.

\subsubsection{Standard deviation of weighted mean}
The standard deviations of weighted means, $\sigma_{\overline{\alpha}} $ and 
$\sigma_{\overline{\delta}} $ are computed for every star with more than one 
image.  The formula used is:
\begin{equation}
\sigma^2_{\overline{x}} = \frac{\overline{x^2} - \overline{x}^2}
{N^\prime - 1}
\end{equation}
where
\begin{equation}
x = \alpha_i, \delta_i
\end{equation}
and 
\begin{equation}
N^\prime \equiv N \left[ \frac{\langle w \rangle^2}{\langle w^2 \rangle} \right] 
\end{equation}
where $w$ is the weight of an individual observation.  A description of the 
derivation of this formula can be found elsewhere (Germain 1997).

\subsubsection{AC 2000 number}
This number is used in the reduction process to
identify all images of the same star that may appear on different
plates.  This is generated at the U.S. Naval Observatory and
added on to the original $x,y$ data.  When the $x,y$ data are
released, these numbers can be used to link the data to the final catalog.

\subsubsection{ACRS number}
If a star has been identified as being in the Astrographic Catalog Reference
Stars (Corbin et al. 1991), then the ACRS identifier is given in this field.
If the star has not been identified as such, this field is blank.

\subsubsection{Hipparcos number}
If a star has been identified as being in the Hipparcos Catalogue
then the Hipparcos number is provided.
See section ``Cross-Referencing Information'' for details.  

\subsubsection{Tycho number}
If a star has been identified as being in the Tycho Catalogue,
then the Tycho identifier is provided.
See section ``Cross-Referencing Information'' for details.  

\subsubsection{Verification Flag}
A '1' in this field indicates the star has a single image and is not
found in Hipparcos, Tycho, ACRS or the Hubble Guide Star Catalogue 1.2.
These ``stars'' may not exist but instead may be the result of typographical
errors, plate defects or other such blunders.

\newpage

\section{Participating Observatories}

Work from the observatories that participated in the
photographing and measuring of the Astrographic Catalogue data are summarized
below.  Some important characteristics of each zone can be found in
Table~\ref{tab:obschar1} and in Appendix~A.  The telescope characteristics
agreed upon was a normal astrograph with an aperture of roughly 33~cm and
a focal length of 3.43~m.  This created a scale close to
60~arcsec/mm on the plates.  All observatories, with the exception of 
Nizamiah, used this type of instrument.

\subsection{The Royal Observatory at Greenwich}

The Royal Observatory at Greenwich was an original participating institution
in the Astrographic Catalogue project, sending representatives to the
International Congress on Astronomical Photography held in Paris in 1887.  This
meeting outlined the plans for the Carte du Ciel project.  Funding was provided
shortly following this meeting.  A telescope built by Sir Howard Grubb
following the design agreed upon in Paris was delivered in May 1890.        
Greenwich centered its plates between +65 and +90 degrees, with five plates 
taken on the pole in different orientations. 
In total, 1153~plates were exposed and measured in this zone.  Three exposures
were made on each plate lasting six minutes, three minutes and 20~seconds.
The telescope was moved 20~arcsec between the exposures, so the six and
three minute exposures are offset from each other in declination, the six 
minute and 20~second exposure are offset in right ascension.
The plate epochs span from 1892 to 1905.

After initially measuring some plates using micrometer screws, Professor H.H.
Turner realized that a different measuring technique was required if the job
were to be completed in a timely fashion.  At his request, an eyepiece scale
type
measuring machine was built and greatly reduced the time to measure a plate.
Many other observatories adopted this measuring procedure.
Systematic measuring of the plates
began in October~1894.  A duplex micrometer, which is a measuring machine capable 
of measuring two plate simultaneously, was put into use in February~1895. 
This aided the identification of images of the same star but on different
plates since it was possible to arrange the plates in the machine so that 
the same field of sky was coincident under the measuring apparatus. (Remember
that each plate overlapped surrounding plates so that each area of sky appears
on at least two plates.)  All plates were measured in two orientations, with
the plates being rotated 180~degrees between measurements.  In all degree bands
except the +65, +66, and +67~degree bands, the same measurer was used for 
both orientations.  Both the six minute and three minute exposures were 
measured for all stars that appeared on the 20~second exposure.
Details can be found in the introductions to the published volumes
(Christy \& Dyson 1904-1932).

\subsection{The Vatican Observatory}

The Vatican Observatory, located in Vatican City, was founded
in 1888 while the Carte du Ciel program was in its infancy.
Vatican staff members realized that
participation in this program would immediately give their
young observatory international recognition.
Pope Leo XIII commissioned Father Francesco Denza and Father Giuseppe Lais
to attend the Astrographic Congress and enroll the Vatican as one of the 
participating institutions.

After being accepted as a participant, the Vatican commissioned
the Henry brothers of France to build the telescope and P. Gautier to build
a machine to measure the stars on the plates.
Father Denza describes the finished telescope:
``The instrument consists of two parallel telescopes:  The photographic
telescope
with an aperture of 33 cm and a focal length of 3.43 m; and the finding 
telescope or collimator with 20 cm aperture and a focal length of 3.6 m.  Both
are housed in a metal tube with a rectangular cross section of 37 by 68 cm.
Both objectives are fixed on the same block of bronze at one end of the 
tube and at the other end is the photographic plate holder and the eyepiece 
of the collimator.  A thin metallic diaphragm separates the two telescopes.
...The photographic objective is a doublet of flint and crown and it is
both achromatic and aplanic for the most intense chemical rays of the 
spectrum.''  (Denza 1891).

The telescope was installed in the Leonine Tower in 1891.  This tower,
located on the highest point of Vatican Hill,
was originally constructed in 840 AD under Pope Leo IV as a 
defense against the Saracen invasions.  It is about 20 meters above 
ground (about 100 meters above sea level) with walls about 4.5 meters in
thickness.  

The Vatican was assigned the strip of sky between +55 and +64 degrees on 
which to center the plates.  In order to achieve the two-fold sky coverage
1040 plates would be needed.  (Actually, 1046
plates were exposed and measured.)  The job of photographing and developing
the plates was 
carried out primarily by one person, Father Lais, who worked on this for
over 25 years until the time of his death in 1921.  Lais's aid, Carlo
Diadori, completed the photographing in 1922.                   
For the first several years
that Father Lais was working at the telescope, no plates were being measured.

Father Johann Hagen,
appointed director of the Observatory
in 1906, was committed to seeing the project completed.  He soon
realized that the machine built by Gautier to measure the
stars was too slow.  After investigating different measuring 
techniques employed by other institutions, he decided on the use of an eyepiece
grid.  In the method used by Vatican, a 10 $\times$ 10 mm area of a plate,
corresponding to 2 $\times$ 2 r\'{e}seau intervals, was magnified in a 
microscope along with a grid which is segmented into 0.05 mm steps.
The edge of the grid was aligned with the edge of the
2 $\times$ 2 r\'{e}seau interval
area.  The location of a star within the magnified area was read from its 
apparent position on the grid.  This method was indeed 
efficient, allowing the Vatican to be one of the first observatories to complete
its assigned zone despite utilizing minimal manpower.
However, the accuracy was not as high as can be obtained with the
measuring techniques used at other participating institutions.
Vatican was the only observatory in the Astrographic
Catalogue program to use the eyepiece grid technique.
Also during his trips to other observatories,
Father Hagen saw extensive use
of women in measuring the plates, freeing the full-time, male
astronomers from this time-consuming, repetitive task.
He brought in three nuns from the Instituto di
Maria Bambina to measure the plates.  These nuns worked from 1910 to 1921 and
measured the vast majority of the Vatican data.  The Oxford University 
Observatory agreed to compute the plate constants used to convert the
rectangular measures to equatorial coordinates.

For additional information concerning the history of the Vatican Observatory
and its personnel, see the book \underline {In the 
Service of Nine Popes} (Maffeo 1991).  Additional details regarding the 
Vatican's participation in the Astrographic Catalogue project 
can be found in the introductions to the data (Vatican 1914-1928), written
in Italian.

\subsection{The Catania Observatory}

The Catania University Observatory, located in Catania, Sicily, centered
its plates between +47 and +54 degrees.
In total, 1010 plates were exposed and measured in this zone.
The epochs span from 1894 to 1932, but over 95\% were exposed prior to 1906.
All the plates were measured using the short-screw method.  Additional 
details can be found in the introductions to the published data
(Catania 1907-1963).

\subsection{The Helsingfors Observatory}

The Helsingfors Observatory, located in Helsinki, Finland, centered its plates
between +40 and +46 degrees.
In total, 1008 plates were exposed and measured in this zone.
The epochs span from 1892 to 1909, but over 94\% were exposed prior to 1897.
All the plates were measured using the short-screw method.
One screw was used; the plates
were rotated 90 degrees to measure both $x$ and $y$ coordinates.  The plates
measured early in the work had images from both the longest and middle exposures
measured in one orientation.  Later (after 1896), only the images from the 
longest exposure were measured, but the plates were rotated 180 degrees between
measurement so these images were measured twice (this also helped remove any
bias a measurer may have).  
Various aspects of the work are detailed in the introduction to Volume~1
(Helsingfors 1903-1937).

\subsection{The Potsdam Observatory}

In 1887, at the meeting of the International Congress which established the
Astrographic Catalogue, the zone with plate centers from +39 to +32 degrees was
assigned to the Potsdam Observatory.  Potsdam
began photographing the plates by~1893 and 1226~plates were
exposed by the end of 1900.  Each plate had two exposures of 5~minutes duration.
Unfortunately, the measurements of the plates lagged
behind the exposures. By the start of World War I only 406 plates were
measured.  Following the war, Potsdam announced it could no longer continue
with the project, and a re-photographing of its zone was made by Oxford,
Uccle, and Hyderabad.  The remaining plates were never measured.
An allied bomb during World War II destroyed virtually the entire set of 
plates (Dick 1988, 1990).

In total, 406 plates were measured and published as the Potsdam zone.
These plate are scattered throughout the zone, so many are not overlapped
by others.  All were measured using one of two short-screw 
type measuring machines.  The plates were measured in one direction only;
the plates were not rotated.
Various aspects of the work are detailed in the introductions
of the printed volumes (Potsdam 1889-1915).

\subsection{The Nizamiah Observatory, Hyderabad}

In 1887, at the meeting of the International Congress which established the
Astrographic Catalogue, the
zone from -17 to -23 degrees was assigned to the Observatory of Santiago.
By 1900, the work was still not progressing, so a proposal to establish an
observatory in Montevideo was made.  This, too, did not progress so 
Santiago asked to re-undertake the project.  At the same time, the 
Nizamiah Observatory, located in Hyderabad, India, offered to work on this 
zone as well.  So, in 1909 there were two observatories offering to work on
the -17 to -23 zone.  A resolution passed by the Congress in 1909 assigned the
-17 to -20 zone to Hyderabad.  After completing the photographing and 
measuring of these four bands in 1920, the International Astronomical Union
recommended that Hyderabad continue photographing down to -23~degrees
declination.
This work was completed in 1928.  This zone between -17 and -23 degrees is
known as the Hyderabad South zone.  Hyderabad also observed a section of 
the sky in the Northern hemisphere that Potsdam was originally assigned.  This
zone, between +36 and +39 degrees, is known as the Hyderabad North zone.

In total, 1260 plates were exposed and measured in the Hyderabad South zone.
The epochs span from 1914 to 1928, with only a handful taken after 1923.
For the Hyderabad North zone, 592 plates were exposed and measured
The epochs span from 1928 to 1937, with just a very few taken after 1934.
The telescope used was not one of the Henrys' design, as all the other AC
participation observatories used.  Instead of a 13~inch, the Hyderabad 
instrument, built by Cooke and Sons of York, had an aperture of only 8~inches.
Its objective was described as a
``patent photo-visual lens''.  The smaller aperture meant longer exposures
were required to achieve the desired magnitude limit set for the AC.  
The telescope's focal length was 133~inches.
All the plates were measured using one of four eyepiece scale 
type measuring machines, all built by Cooke and Sons.
Various aspects of the work are detailed in the introductions
of the printed volumes (Edinburgh 1918-1930, London 1934-1946).

\subsection{The Uccle Observatory}

The Royal Observatory of Belgium, located in Uccle, was assigned the zone
with plate centers running from +34 to +35 degrees.  Although Uccle
was not an original participating observatory in the AC project, it became
one because the Potsdam Observatory, originally assigned to cover 
this area, was unable to fulfill its commitment.  The      
telescope used was of similar design as the Henry brothers, but built by
Gautier.  In total, 320 plates 
were exposed between 1939 and 1950.  The epochs of the plates are spread fairly
uniformly, except for a lack of plates exposed between mid-1943 and mid-1945.
The measurements took place at the Paris Observatory with the use of three
short-screw measuring machines.
The r\'{e}seau used was one from the Toulouse Observatory.
An introduction can be found in Volume~1 of the printed catalog
(Paris 1960,1962).

\subsection{The Oxford Observatory}

The University Observatory at Oxford was originally assigned the zone +25 to
+31 degrees on which to center the plates.  This is known as the Oxford~I
zone.  An additional
zone with plates centered on +32 and +33 degrees declination was photographed
at Oxford after Potsdam announced they would not be able to complete their
assigned area (+32 to +39).  This two degree band is referred to as the
Oxford~II zone.

The telescope used was the same
design as the Henry brothers' instrument located in Paris.  The Oxford 
lens was made by Sir Howard Grubb and attached to an existing Grubb $12
\frac{1}{4}$ inch, which was utilized as a guiding instrument.  All plates
of the Oxford~1 zone
were taken between mid-1892 and 1910, with over 80\% exposed by the 
end of 1903.  These plates were measured mostly by boys from the New College
Choir School, and Mr. T. J. Moore, a gardener who was interested in astronomy.
The measuring apparatus was designed with an eyepiece scale, similar
to that employed
by the Greenwich Observatory in their AC work, with the exception that only one
plate was measured at a time whereas Greenwich measured two.

Mr. F.A. Bellamy of University Observatory at Oxford supervised much of the
work in photographing the 320 plates required to complete the Oxford~2
zone.  He employed the same techniques used in the Oxford~1 zone
and used the same Grubb refractor operated 30 years earlier.  
Mr. Bellamy would have photographed the entire zone himself, however he died 
with 32 fields left unobserved.  These 32 fields were exposed at the 
Royal Observatory at Greenwich by Mr. H.G.S. Barrett with the telescope used
for the Greenwich Zone (+65 to +90).  These last 32 plates were the only
plates in the Astrographic Catalogue that did not have a
r\'{e}seau exposed on them, however they were measured with one clamped on
the glass.
All plates of the Oxford~2 zone were exposed between 1930 
and 1936. (Actually there are two plates that appear to have typographical
errors in their epochs.  One is date in 1918; the other in 1930, but 8 months
prior to any other plate.)   Mr. Barrett also supervised the 
measurement and reduction of these plates at Oxford, using the
eyepiece scale technique.
However World War~II intervened preventing
the determination of the plate constants for 92 of the fields.  After the
war, the constants for these remaining plates were determined under Dr. H.
Kox at the Hamburg Observatory at Bergedorf.
A detailed introduction covering the participation of the
University Observatory at Oxford in the Astrographic Catalogue project
can be found in Volume~1 of the Oxford~I zone catalog (Turner 1906-1911),
as well as in the book \underline {The Great Star Map} also by Turner.  An 
introduction to the 
Oxford~II zones can be found in Volume~1 of the Oxford~II zone catalog
(Paris 1953-1954).

\subsection{The Paris Observatory}

The Paris Observatory agreed to photograph the zone with plate centers 
between declinations
+18 and +24 degrees.  The telescope used was the original Henry brothers'
instrument, after which all other AC telescopes were supposed to be patterned.
In total, 1261 plates were photographed and measured.  The measuring technique
employed at Paris was the short-screw method, which was the
most accurate utilized with the AC plates.
The epoch range of the plates are
from October 1891 through November 1927, however only 7 plates were exposed
after 1907 and all but 100 were exposed prior to 1900.
An introduction to the Paris Observatory's participation in the Astrographic
Catalogue can be found in the introductions to the individual volumes
(Paris 1902-1932).

\subsection{The Bordeaux Observatory}

The Bordeaux University Observatory, located in Floirac, France,
was assigned the zone between +11 and +17 degrees
declination on which to center its plates.  The telescope used was a similar
design to the Paris instrument and was built by the Henry brothers.
In total, 1260 plates were exposed between 1893 and 1925, all but five being
taken before 1913.  The plates were measured at Bordeaux using the 
short-screw method.
An introduction to the Bordeaux Observatory's participation in the
Astrographic Catalogue project can be found in Volume~1 of the published data
(Paris~1905-1934).

\subsection{The Toulouse Observatory}

The Toulouse University Observatory agreed to participate in the Astrographic
Catalogue project by taking plates centered between +5 and +11 degrees 
declination.  In total, 1260 were exposed and measured.  The epochs vary
from 1893 to 1935, and were taken in three fairly distinct groupings.  Most
were exposed before the end of 1910.  Another set is taken between 1918 and
1922.  The last few plates are scattered between 1930 and 1935.  All the 
plates were measured with using a short-screw type measuring machine,
however not all plates were measured at Toulouse.  Ninety plates were measured
at the Bordeaux University Observatory and 36 were measured at the Paris
Observatory.
Various aspects of the work are detailed in the introductions
of the printed volumes (Paris 1903-1948).

\subsection{The Algiers Observatory}

The Algiers Observatory was assigned the zone between -2 and +4 degrees on
which to center its plates.  All 1260 plates were exposed 
between 1891 and 1911.  The plates were measured using the 
short-screw method.  
Details about the Algiers Observatory's participation in the
Astrographic Catalogue project can be found in the introduction to the
catalogs (Tr\'{e}pied 1903,Paris 1903-1924).

\subsection{The San Fernando Observatory}

The Naval Observatory of San Fernando (Spain) was assigned the area between
-3 and -9 degrees declination on which to center its plates.  The telescope
used was built by Gautier, with the objective made by the Henry brothers.
All of the 1260 plates were exposed between 1891 and 1917.  Over 1000 were 
taken before 1899, then only a few per year until 1917.  The plates were
measured using a short screw micrometer.  
An introduction to San Fernando's participation in the Astrographic Catalogue
project can be found in Volume~1 of the published data (San Fernando 1921-1929).

\subsection{The Tacubaya Observatory}

At the meeting of the International Congress which established the
Astrographic Catalogue, the
zone from -10 to -16 degrees was assigned to the National Astronomical 
Observatory of Tacubaya, located near Mexico City, Mexico.   
In total, 1260 plates were exposed and measured in the Tacubaya zone.
(Actually, one of the plates had an incorrect and unknown plate center and
has been discarded from the reductions, leaving a total of 1259 plates).
All but five plates were exposed between 1900 and 1912.  The five later 
plates were exposed between 1926 and 1938.
All the plates were measured using the eyepiece scale method.
An introduction of the history
of the work can be found in Volume~1 part~1 of the -15 degree zone
(Tacubaya, 1913-1962).

\subsection{The Cordoba Observatory}

At the 1887 Paris meeting, the
zone from -24 to -31 degrees was assigned to the La Plata Astronomical   
Observatory.  In~1900, the zone was re-assigned to Cordoba.  The telescope
was installed at the end of~1901.  In~1908, Dr. Thome, the Director of Cordoba,
died and Dr. Perrine was named new director.  After some investigations,
Perrine decided  to re-observe all areas.  He discovered that the telescope
was out of focus and many of the plates were impaired, and that the plates
were centered on apparent place coordinates, not at those defined at
equinox 1900.  Plates of this series were exposed starting in~1909 and 
finished by the end of~1913.  These plates were measured between~1909 and
1920.  In total, 1360~plates were exposed as part of the Astrographic 
Catalog.

The telescope used was one of the Henrys' design and build, following the 
standard with a 33~cm
aperture and 3.47~m focal length.  In the introduction written by 
Perrine, he states that the guiding is difficult because the guide scope has
only a 19~cm aperture, as opposed to the more conventional 25~cm in use
by most of the participating observatories.  The mount was built by Gautier.
Some plates showed a ``triangular distortion'' that was traced to a warp in the
ring which held the lens in place.  This ring was replaced in~1911.  The 
lens, from August~9,~1910 until the end of the program, was stopped down to 
11~ inches.  This, according to Perrine, greatly improved the image quality.

Virtually all plates were taken and developed by R. Winter or F.P. \linebreak Symonds.
Four exposures on each plate were made; two long exposures of the same
duration (both of 5~or 6~minutes) one medium exposure (of~60 to 90~seconds)
and one short exposure (of~5 to 8~seconds).  The telescope was moved in 
declination between exposures.  In order to expedite the work, two measuring
machines of the short-screw type were retro-fitted with eyepiece 
scales.  As a result, 140~plates were measured using the short-screw
method, the remaining 1220~plates were measured using the eyepiece
scale method.  In total, five different 
measuring machines were used, allowing each measurer to have his or her 
own machine.  
The r\'{e}seaux used were supplied by Gautier and Prin; four were used
throughout the work.
Investigations of two of the r\'{e}seaux were made in 
Paris and the deviations were found to be negligible; no corrections
for the r\'{e}seaux were applied to the measures.
Only stars within one degree in right ascension and declination of the plate
center were measured.  All stars having three images were measured
unless images ran together, which was the case of 
the brightest stars.  Four measures were made on each star; a measure was made
on both of the long exposures in both orientations of the plate.
(Following an initial measurement of all stars, the plate was reversed 180
degrees and all stars re-measured.)
In general, measures in the direct and reverse
orientations of the plate were made on the same day by the same measurer.
In total, 37~man-years went into measuring the plates.
Various aspects of the work are detailed in the introduction of 
Volume~26 of the Observatory Results (Cordoba 1925-1934).

\subsection{The Perth Observatory}

At the International Congress which established
the Astrographic Catalogue, the Observatory
of Rio de Janeiro was assigned to photograph the area between -32 and -40
degrees declination.  In 1900, the work at Rio had not progressed and so the
Perth Observatory undertook the task.  The telescope used was from Sir 
Howard Grubb, and was of similar design to other telescopes used for the
AC work.  
Although observing was progressing, no resources were available to measure the
plates.  At this time, the Perth Observatory was primarily a meteorological
station, and the meteorological work took precedence.  This changed 
in 1908 when the Australian Federal Government established the 
Australian Weather Bureau.  About this time, four women were hired as plate  
measurers.  Professor Dyson of the Edinburgh Observatory
offered assistance in the measuring.  Perth accepted this offer and started
sending those plates of the -40~degree zone.
By 1915, the Edinburgh 
Observatory completed its commitment by measuring all of the plates centered
on the -40, -39 and -38 degree
band.  This area is known as the Perth-Edinburgh AC zone.
Observing continued at Perth until 1919, at which time all areas had been
photographed.  Personnel at Perth measured all plates between -37 and -32
degrees; this is known as the Perth zone.
In total, 432~and 944~plates make up the Perth-Edinburgh and Perth zones,
respectively.

All plates had three exposures taken, one of 4~minutes, 2~minutes and
13~seconds, or that of 6~minutes, 3~minutes or 20~seconds.  The change in
exposure times took place following a 1909 meeting of key personnel from
different observatories participating in the AC.  At that meeting, many
people expressed concern about the uniformity of limiting magnitudes on
different plates.  Many of the Perth plates were re-examined and found
to be unsatisfactory.  The areas affected were re-observed and a method 
of ensuring more uniformity was developed.

In general, no guiding of the instrument other than the sidereal drive
was made, as it was found unnecessary.  All plates were the brand
Ilford ``special rapid''.  All were measured using an eyepiece scale machine,
similar to the Oxford Observatory's.  For all but 5~plates, the measurements
done at Perth were made in two orientations of the plate by the same
person; the plate being rotated 180 degrees between measurements.  In total,
10~measurers were used at Perth.
For the plates measured at Edinburgh, images were measured in two orientations
of the plate, with the plate
being rotated 180 degrees between measurements.  For the direct orientation,
the second exposure (either 2~or 3~minutes) was measured; the longest 
exposure was measured with the plate in reverse orientation.  Quite often
more than one measurer was used on each plate. 
Various aspects of the work can be found in the introduction to Volume~23
of the Perth-Edinburgh data (Perth 1922, Paris 1949-1952) and in Volumes
1~and~17 of the Perth data (Perth 1911-1921).

\subsection{The Royal Observatory at the Cape of Good Hope}

The Royal Observatory at the Cape of Good Hope, South Africa, took the zone
between -41 and -51 on which to center its plates.  In all, 1512 plates 
were exposed and measured.  The epochs range from 1897 to 1912, with 97\%
of them being exposed prior to 1906.  The telescope used was built by 
Sir Howard Grubb.  Two machines were
used to measure the plates, both were designed by David Gill and built by
Repsold of Hamburg (Gill 1898).
Both machines employed the short-screw measuring method and
were of similar design.
An introduction to the participation of the Royal Observatory at the 
Cape of Good Hope in the Astrographic Catalogue
project can be found in Volume~1 of the published data (London 1913-1926) .

\subsection{The Sydney Observatory}

H.C. Russell, the Government
Astronomer at Sydney, was in attendance at the 1887
meeting of the International Congress.  He committed the Sydney observatory
to photograph the sky between declinations -52 and -64 degrees.
The lens used for the project was built by Howard
Grubb of Dublin, and it followed the general design established by the 
International Congress.  The lens was
delivered in December of 1890.  Most of the telescope was built in Sydney. 
The observing program began in earnest 
in 1892.  The telescope was moved twice in the course of the AC work; first 
from inside Sydney to Redhill (located about 12 miles from Sydney) in 1899,
and then back to its original location in 1931.  The observing was left  
unchanged until 1912, when W.E. Cooke took over the project.  He was unsatisfied
with the quality of many plates and eventually rejected and rephotographed many
of the areas.  Until this time, plates were being sent to Melbourne for 
measurement, but under Cooke the Sydney observatory began to measure their own
photographs.  Prior to Cooke's arrival, plates were positioned so the center
of the plate was in the sharpest focus (The astrograph used in photographing
the plates did not have a flat field of focus, so some areas of the plate
are in focus while others are not.  This is true for all the telescopes used
in the Astrographic Catalogue work).  Cooke altered this and made the ring
about 50~arcsec from the center the place with the sharpest focus.  In 1926,
Cooke retired and James Nagle took over.  He altered the place of best focus
in a ring about 40~arcsec from plate center.  Nagle died in 1941, and 
H.W. Woods took over.  Some plates were found unsatisfactory or missing.  The
remaining areas were photographed between 1944 and 1948.
In total, 1400~plates were taken as part of the Astrographic Catalogue.
All plates exposed between 1890 and 1930 were taken by James Short.

Plate measuring did not start for about six years after the first plates 
were taken.  Evidently there was a suggestion about having all plates from 
all participating observatories sent to Paris for 
measurement.  This plan was not ever put in place, but the Australians liked 
this idea so they decided that Melbourne would be used to measure both the
Sydney and Melbourne plates.  As mentioned above, this changed with Cooke's
arrival and Sydney started measuring their own plates.  There were four
short-screw measuring machines used in Melbourne, and two eyepiece
scale machines used at Sydney.
From this point on, all stars were measured twice with the plates being
rotated 180 degrees between measurements.
Various aspects of the work are detailed in the introduction of 
Volume~53 of the data (Sydney 1925-1971).

\subsection{The Melbourne Observatory}

In 1887, following the meeting which established
the Astrographic Catalogue, the Victorian government agreed to have the 
Melbourne Observatory participate in the project.  Melbourne was assigned
the zone from -65 to -90 degrees.  The telescope used was built by Howard
Grubb of Dublin, and it followed the general design established by the 
International Congress.  The telescope was
delivered in December of 1890.  The observing program started about one year
later in January~1892 and continued until 1927 (Actually, one plate was 
exposed in 1940).  Over 80\% of the plates were exposed prior to 1898.
In total, 1149~plates were taken as part of the Astrographic Catalogue.
All plates had three exposures on them of 5 minutes,
2.5~minutes and 20~seconds duration, with the exception of the plates taken
prior to February~26,~1892, whose exposures were slightly longer.  The
r\'{e}seau was exposed on the plates shortly after the plates were removed
from the telescope.
In total, six different r\'{e}seaux were used during the
program; three were supplied by Gautier and three were made at Melbourne.

Plate measuring did not commence in earnest until November 1898, when six
women were hired at Melbourne.  Two measurers were used for each plate, one
taking the northern half and one the southern.  The plates were rotated 
180 degrees and the stars were remeasured by the same person.
All reference stars on each plate were measured
by both measurers.
Four measuring machines were used throughout most of the work; all used
short-screws for the star measurements.
The Melbourne staff tried using
an eyepiece scale measuring machine for the plates but found the measuring
error too high.

Periodic and progressive screw errors were investigated.  None were applied,
as they were found to be negligible.  (Further reductions performed at the
U.S. Naval Observatory as part of the new reductions show this not to be
true.)  Investigations into errors of the r\'{e}seaux were made and these
corrections were applied to the data prior to publishing.
Various aspects of the work are detailed in the introduction of 
Volume~1 of the data (Melbourne 1926-1929; Paris 1955-1958; Sydney 1963).

\newpage
\addcontentsline{toc}{section}{Bibliography}
\begin{thebibliography}{}
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Vol 1 through 8
\bibitem{Chr1}
Christy, W.H.M., and Dyson, F.W., 1904-1932, Astrographic Catalogue 1900.0,
Greenwich Section Vol 1 through 6
\bibitem{Corbin1}
Corbin, T.E. and Urban, S.E., 1988, IAU Symposium 133 Mapping the Sky,
ed. S. Debarbat, J.A. Eddy, H. K. Eichhorn and A. R. Upgren,
Kluwer, Dortrecht, p. 287 
\bibitem{Corbin2}
Corbin, T.E., and Urban, S.E., 1990, IAU Symposium 141 Inertial Coordinate
 System on the Sky, ed. J. Lieske and V.K. Abalakin
 Kluwer, Dortrecht p. 433.
\bibitem{Corbin3}
Corbin,T.E., Urban,S.E.,Warren,W., 1991, Astrographic Catalog Reference
Stars, NASA, NSSDC 91-10
\bibitem{Cord1}
Cordoba, 1925-1934, Resultados del Observatorio Nacional Argentino, Vol 26
through 34
\bibitem{Denza}
Denza, F., 1891, Cenni Strorici sulla Specola Vaticana, Pubblicazioni della
Specola Vaticana (1891) Fascicolo II, p. 88
\bibitem{Dick1}
Dick, R.W., 1988, Info Bull of CDS, {\bf 34} 155
\bibitem{Dick2}
Dick, R.W., 1990, Info Bull of CDS, {\bf 38} 19
\bibitem{Edin1}
Edinburgh, 1918-1930, Astrographic Catalogue 1900.0 Hyderabad Section,
Vol 1 through 7 
\bibitem{Eichhorn}
Eichhorn, H., 1974, Astronomy of Star Positions, Fredrick Ungar, p. 286
\bibitem{ESA1}
ESA, 1997, The Hipparcos and Tycho Catalogues, SP~1200
\bibitem{Germain}
Germain, M.E., 1997, Variance of a Weighted Mean with Astrometric Applications,
in preparation
\bibitem{Gill}
Gill, D., 1898, MNRAS {\bf 59} 61
\bibitem{Gliese}
Gliese, W., and Jahreiss, H., 1991, Preliminary Version of the Third Catalog
of Nearby Stars
\bibitem{Halbwachs}
Halbwachs J.L., Baessgen G., Bastian U., Egret D., Hoeg E.,
van Leeuwen F., Petersen C., Schwekendiek P., Wicenec A., 1994, A\&A {\bf 281}
25
\bibitem{Hels1}
Helsingfors, 1903-1937, Catalogue Photographique du Ciel zone de Helsingfors
Vol 1 through 8
\bibitem{London2}
London, 1913-1926, Cape Astrographic Zones, Vol 1 through 11
\bibitem{London1}
London, 1934-1946, Astrographic Catalogue 1900.0 Hyderabad Section,
Vol 9 through 12
\bibitem{Luy1}
Luyten, W.J., 1979, 1980, New Luyten Catalogue of Stars with Proper Motions
Larger than Two Tenths of an Arcsecond (Minneapolis:  University of Minnesota) 
\bibitem{Luy2}
Luyten,  W.J. and Hughes,  H.S., 1980,  Proper Motion Survey with the
Forty-Eight Inch Schmidt Telescope.  LV. First Supplement to the NLTT Catalogue
(Minneapolis: University of Minnesota)
\bibitem{Maffeo}
Maffeo, S. 1991, In the Service of Nine Popes:  100 years of the Vatican 
Observatory, Vatican Observatory Foundation
\bibitem{Mel1}
Melbourne, 1926-1929, Melbourne Astrographic Catalogue 1900.0, Vol 1 through 3
\bibitem{Paris3}
Paris, 1902-1932, Catalogue Photographic du Ciel, Vol 1 through 7
\bibitem{Paris6}
Paris, 1903-1924, Observatoire D'Alger Catalogue Photographique du Ciel, Vol~1
through~7
\bibitem{Paris5}
Paris, 1903-1948, Observatoire du Toulouse, Catalogue Photographique du Ciel,
Vol 1 through 7
\bibitem{Paris4}
Paris, 1905-1934, Observatoire de Bordeaux, Catalogue Photographique du Ciel,
Vol 1-7
\bibitem{Paris7}
Paris, 1949-1952, Astrographic Catalogue 1900.0, Perth Section, Vol 1 
through 3
\bibitem{Paris2}
Paris, 1953-1954, Astrographic Catalogue 1900.0, Potsdam-Oxford Section, Vol 1-2
\bibitem{Par8}
Paris, 1955-1958, Melbourne Astrographic Catalogue 1900.0, Vol 4 through 7
\bibitem{Paris1}
Paris, 1960 and 1962, Catalogue Photographique du Ciel, Zone Uccle-Paris,
Vol 1 and 2
\bibitem{Per1}
Perth, 1911-1921, Astrographic Catalogue 1900.0, Perth Section, Vol 1
through~24
\bibitem{Per2}
Perth, 1922 , Astrographic Catalogue 1900.0, Perth Section, Vol 33
\bibitem{Pot1}
Potsdam, 1889-1915, Publicationen des Astrophysikalishen Observatoriums zu
Potsdam, Photographische Himmelskarte Catalog, Vol 1 through 7
\bibitem{SF1}
San Fernando, 1921-1929, Cat\'{a}logo Astorgraphico para 1900.0,
secci\'{o}n del Observatorio de Marina de San Fernando, Vol 1 through 8
\bibitem{Sin1}
Sinnot, R.W., 1988,  NGC 2000.0, The Complete New General Catalogue and Index
Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer, ed. R. W. Sinnott,
Sky Publishing Corporation and Cambridge University Press 
\bibitem{Syd1}
Sydney, 1925-1971, Astrographic Catalogue 1900.0, Sydney Section, Vol~1
through~53
\bibitem{Syd2}
Sydney, 1963, Melbourne Astrographic Catalogue 1900.0, Vol 8
\bibitem{Tacu1}
Tacubaya, 1913-1962, Catalogo Astrofotografico 1900, Vol. 1 through~7
\bibitem{Trepied}
Tr\'{e}pied, C. 1903, Observatoire D'Alger Catalogue Photographique du Ciel,
Introduction
\bibitem{Turner1}
Turner, H.H., 1906-1911, Astrographic Catalogue 1900.0, Oxford Section, vol 1
through 7
\bibitem{Turner2}
Turner, H.H., 1912, The Great Star Map, John Murray
\bibitem{Urban1}
Urban, S.E. and Corbin T.E., 1996, A\&A {\bf 305} 989
\bibitem{Urban2}
Urban, S.E., Martin, J.C., Jackson E.S. and Corbin, T.E., 1996,
A\&AS {\bf 118} 163
\bibitem{Vatican}
Vatican, 1914-1928, Catalogo Astrografico 1900.0, Sezione Vaticana, Vol 1-10
\bibitem{Worley}
Worley, C.E. and Douglass, G.G., 1996, The Washington Double Star Catalog
\bibitem{Zach1}
Zacharias, N., de Vegt, C. Nicholson, W. and Penston, M.J., 1992,
\newline A\&A {\bf 254} 397
\bibitem{Zach2}
Zacharias, N., Zacharias, M.I., Douglass G.G., and Wycoff, G.L., 1996,
AJ {\bf 112} (5), 2336
\end{thebibliography}

\newpage
\appendix
\section{Notes on Participating Observatories Data Characteristics}
\begin{itemize}
\item{Greenwich notes:}
\begin{itemize}
\item{Five plates are centered on the pole.}
\item{A duplex micrometer was used; two plates were measured simultaneously.
  Formats of the  published volumes reflect this.}
\end{itemize}

\item{Vatican notes:}
\begin{itemize}
\item{One plate has a published epoch of 1891.}
\item{A measuring grid was used instead of eyepiece scale or screw.}
\item{Most plates were measured by one of three women.  Measurer ``E'', who measured
   about 50\% of the plates, was significantly worse than the other two.}
\item{Plate weights, computed at USNO, are based on measurer.}
\end{itemize}

\item{Catania notes:}
\begin{itemize}
\item{Over 95\% of the plates were exposed prior to 1906.}
\item{Errata are found throughout volumes, mostly at the end of each plate's
$x,y$ data.}
\item{Measures of large images (bright stars) are often listed many times
   (with slightly different values).}
\item{Multiple stars are often measured both as center of light AND individual
   images.}
\item{One plate has been taken with the same plate center as another, but only
   faint stars measured on the second.  Hence only reference stars near the
   edges were available on that plate.}
\item{Many typographical errors in the printed volumes were found by USNO.  These
   have been corrected.}
\end{itemize}

\item{Helsinki notes:}
\begin{itemize}
\item{Over 95\% of the plates were exposed before 1897.}
\item{Helsinki was under Russian rule during the time of exposures and
measurements of the plates.}
\end{itemize}

\item{Potsdam notes:}
\begin{itemize}
\item{The number of plates exposed was 1226, of which only 406~ were
measured and published.  No funds after World War~I were available to continue
the work.}  
\item{All but 34~plates were destroyed during World War~II.}
\item{In cases where stars were measured and published twice, USNO used the 
mean of the measures in the reductions.}
\item{Personal equation tables (describing observer dependent magnitude equation)
are published.  USNO developed its own corrections by observer that supersedes
the Potsdam tables.}
\item{Generally, only stars brighter than 11.0 were measured}
\item{Plate weights, computed at USNO, are based on measurer.}
\end{itemize}

\item{Hyderabad North notes:}
\begin{itemize}
\item{Nizamiah Observatory had the only instrument not of standard design.
  Objective diameter was 20~cm (not 33~cm) so longer exposures were required
  to achieve the desired limiting magnitude.  The plate scale approximately
  61~arcsec/mm.}
\item{This zone was photographed in response to the incompletion of the Potsdam
 region.}
\item{India was under British rule during the time of exposures and measurement
of the plates.}
\end{itemize}

\item{Uccle notes:}
\begin{itemize}
\item{This zone was photographed in response to the incompletion of the Potsdam
 region.}
\item{One plate was broken prior to diameter measurements.}
\item{Measurements were made at Paris.}
\item{No hour angle of exposure is given.}
\end{itemize}

\item{Oxford~II notes:}
\begin{itemize}
\item{This zone was photographed in response to the incompletion of the Potsdam
 region.}
\item{Thirty two plates in this zone were exposed and measured at Greenwich.  These
   are the only plates of the AC without a reseau printed on them.}
\item{One plate has a published epoch of 1918.}
\item{Kox (Hamburg Observatory) determined plate constants for 92 of the plates.
   All others were computed at Oxford.}
\end{itemize}

\item{Oxford~I notes:}
\begin{itemize}
\item{Some reseaux had rulings of 5.04 mm (5.00~mm was standard). 
   Introductions indicate which ones, but the introduction appears to be wrong
   for 3~plates.}
\end{itemize}

\item{Paris notes:}
\begin{itemize}
\item{Only 7~plates were exposed after 1907.}
\item{Corrections indicated in the ARI version (1991) of the keypunched data
 have been applied.}
\item{Additional typographic errors found in Bull. of CDS 12 (p32-40) by Dunham
   and Herget were applied, where needed.}
\item{One plate was printed with the $x,y$ measures of two plates (see end of
 Volume~2).}
\item{Nine plates were measured using {\em Baillaud method}, not utilizing the
   reseau.  These plates did not have field distortion pattern corrections
   applied during the reductions at USNO.}
\item{Notes indicate that several stars were  not measured to their full
 precision (0.1 microns).  Images were not kept if the $x,y$ values were not
 given to 1.0 micron or better.}
\end{itemize}   

\item{Bordeaux notes:}
\begin{itemize}
\item{Both Toulouse and Bordeaux have plates centered on +11 degrees.}
\item{Only five plates were taken after 1913.}
\item{Corrections indicated in the ARI version (1991) of the keypunched data
  have been applied.}
\item{Many typographical errors in the printed volumes were found by USNO.
  These have been corrected.}
\end{itemize}

\item{Toulouse notes:}
\begin{itemize}
\item{Both Toulouse and Bordeaux have plates centered on +11 degrees.}
\item{Ninety plates were measured at Bordeaux and 36~plates were measured at
  Paris.}
\item{Corrections indicated in the ARI version (1991) of the keypunched data
  have been applied.}
\item{Notes indicate that several stars were not measured to their full
   precision (0.1 microns).  Images were not kept if the $x,y$ values were not
   given to 1.0 micron or better.}
\item{The odd zones(+5,7,9 and 11) are split.  The first part (usually 0 to
   about 6h)
   do not have reseaux corrections applied, but do give information on which
   measuring machine (1 or 2) and which reseaux was used.  After the split,
   there is no information regarding micrometer or reseaux. 
   For the even zones, there is information regarding where the plates were
   measured and often what type of plate emulsions were used.  However, there
   is no data regarding which reseau was used.}
\end{itemize}

\item{Algiers notes:}
\begin{itemize}
\item{Algeria was under French rule during the AC data acquisition.}
\end{itemize}

\item{San Fernando notes:}
\begin{itemize}
\item{Y-coordinate increases to South}
\item{80\% of the plates were taken before mid-1898.}
\item{One plate was exposed with Saturn and 3 of its satellites.}
\item{Four records have footnotes saying ``body moved during exposure of the
   plate''.}
\end{itemize}

\item{Tacubaya notes:}
\begin{itemize}
\item{All but five plates were taken before 1913.}
\item{One of the plates was not used by USNO because it has an unknown plate
   center.}
\item{Many typographical errors in the printed volumes were found by USNO.
  These have been corrected.}
\item{The type is very poor in some of the volumes.}
\end{itemize}

\item{Hyderabad South notes:}
\begin{itemize}
\item{The Nizamiah Observatory had the only instrument not of standard design.
  The objective's diameter was 20~cm (not 33~cm) so longer exposures were
  required to achieve the desired limiting magnitude.  The plate scale
  is approximately 61~arcsec/mm.} 
\item{India was under British rule during this .}
\item{Five reseaux were used, some with non-standard rulings.}
   Rulings of~5.00, 5.04~and 4.985~mm used.
\item{Y increases to South.}
\end{itemize}

\item{Cordoba notes:}
\begin{itemize}
\item{1220 plates were measured using an eyepiece scale, 140~were measured with
  a short-screw.}
\item{Errata were found in Information Bulletin of CDS 22 79-86 (1982).}
\item{Lens was stopped down to 28~cm in August 1910 to give better images.}
\item{Reseaux with rulings of 5.00~mm were used for all plates except those
   numbered 2001~through~2290. These have 5.05~mm between rulings.  Note that
   this is slightly different than what is given on page~xviii of Volume~26.}
\item{Many typographical errors in the printed volumes were found at USNO.
   These have been corrected.}
\end{itemize}

\item{Perth notes:}
\begin{itemize}
\item{This zone covers only those plates photographed and measured at Perth.}
\item{A change of scale for plate~2063 through 2209 is present.  This was
  the result of someone altering the telescope focus.}
\item{A Scale value, usually given as a letter, is used as an estimate of
 magnitude for all but the brightest stars.}
\item{Very few typographical errors were found at USNO.}
\end{itemize}

\item{Perth-Edinburgh notes:}
\begin{itemize}
\item{This zone covers plates taken at Perth, but measured at Edinburgh,
 Scotland.}
\item{Very few typographical errors were found at USNO.}
\item{Y increases to South.}
\end{itemize}

\item{Cape notes:}
\begin{itemize}
\item{Images were measured with micrometer one have significantly lower mean
   errors than those measured with micrometer two.}
\item{Actual diameters were measured for many stars, but the faintest stars
   were given a ``density'' measurement, dependent on how dark each image
   appears. }
\item{South Africa was under British control during the AC work.}
\end{itemize}

\item{Sydney notes:}
\begin{itemize}
\item{The telescope was relocated twice during the AC work.}
\item{The plate location of sharpest focus altered during the work, depending
   on observatory director.}
\item{Early plates were sent to Melbourne for measurement, using screw method.
   Later, the plates were measured at Sydney with an eyepiece scale.}
\item{The plates were exposed in three discrete epoch spans, 1892 to 1901,
   1918 to 1929, and 1944 to 1948.}
\item{Many typographical errors in the printed volumes were found at USNO.
   These have been corrected.}
\item{Authors suspect a magnitude equation by measurer, but no identification
   of the measurers of individual plates are given.}
\item{Scale value as estimate of magnitude is given for the majority of images.}
\item{X increases to the West}
\end{itemize}

\item{Melbourne notes:}
\begin{itemize}
\item{Over 80\% of the plates were taken before 1898.}
\item{One plate has an epoch of 1940.}
\item{Additional Melbourne errata were found in Sydney Volume 53, pg 64.}
\end{itemize}
\end{itemize}


\newpage
\begin{table}    
\caption{Byte-by-byte description of AC~2000 using format requested by the
CDS.}
\begin{flushleft}
\begin{tabular}{lllll}
\hline\noalign{\smallskip}
   Bytes & Format & Units &    Label &  Explanations        \\
\noalign{\smallskip}
\hline\noalign{\smallskip}
   1- 2& I2   &  h     & RAh           &   Right Ascension (hours)   \\
   4- 5& I2   &  min   & RAm           &   Right Ascension (minutes) \\
   7-12& F6.3 &  s     & RAs           &   Right Ascension (seconds) \\
     14& A1   &  ---   & DE-           &   Declination (sign)  \\
  15-16& I2   &  deg   & DEd           &   Declination (degrees)\\
  18-19& I2   &  arcmin& DEm           &   Declination (minutes) \\
  21-25& F5.2 &  arcsec& DEs           &   Declination (seconds) \\
  27-31& F5.2 &  mag   & B(mag)        &   Magnitude from image diameters \\
  33-40& F8.3 &  yr    & Ep            &   Mean epoch of position \\
  42-43& I2   &  ---   & Num           &   Number of images used \\
  44-49& F6.3 &  arcsec& e\_RAs        &   ?Standard deviation of mean, RA \\
  50-55& F6.3 &  arcsec& e\_DEs        &   ?Standard deviation of mean, Dec \\
  57-64& I8   &  ---   & AC2000        &   AC 2000 Number \\
  66-71& I6   &  ---   & HIPP          &   ?Hipparcos Number\\
  73-84& A13  &  ---   & TYCHO         &   ?Tycho ID\\
  86-92& I7   &  ---   & ACRS          &   ?ACRS number \\
  93-93& I1   &  ---   & VER           &   ?Verification Flag \\
\noalign{\smallskip}
\hline
\label{tab:format}
\end{tabular}
\end{flushleft}
\end{table}

\begin{table}[h!]
\caption{Characteristics of each observatory's plates}
\begin{flushleft}
\begin{tabular}{llclrc}
\hline\noalign{\smallskip}
Zone        & Country   & declination& techn  & num of  &    epoch \\
            & or region & of centers &        & plates  &          \\
\noalign{\smallskip}
\hline\noalign{\smallskip}
Greenwich   & England   &  +90  +65 & scale &  1153 & 1892  1905 \\
Vatican     & Italy     &  +64  +55 & grid  &  1046 & 1895  1922 \\
Catania     & Sicily    &  +54  +47 & screw &  1010 & 1894  1932 \\
Helsinki    & Finland   &  +46  +40 & screw &  1008 & 1892  1910 \\
Potsdam     & Germany   &  +39  +32 & screw &  406  & 1893  1900 \\
Hyderabad N & India     &  +39  +36 & scale &  592  & 1928  1938 \\
Uccle       & Belgium   &  +35  +34 & screw &  320  & 1939  1950 \\
Oxford II   & England   &  +33  +32 & scale &  320  & 1930  1936 \\
Oxford~I    & England   &  +31  +25 & scale & 1188  & 1892  1910 \\
Paris       & France    &  +24  +18 & screw & 1261  & 1891  1927 \\
Bordeaux    & France    &  +17  +11 & screw & 1260  & 1893  1925 \\
Toulouse    & France    &  +11  +5  & screw & 1260  & 1893  1936 \\
Algiers     & Algeria   &  +4   -2  & screw & 1260  & 1891  1912 \\
San Fernando& Spain     &  -3   -9  & screw & 1260  & 1891  1918 \\
Tacubaya    & Mexico    & -10   -16 & scale & 1259  & 1900  1938 \\
Hyderabad S & India     & -17   -23 & scale & 1260  & 1914  1929 \\
Cordoba     & Argentina & -24   -31 & both  & 1360  & 1909  1913 \\
Perth       & Australia & -32   -37 & scale &  944  & 1902  1919 \\
Perth-Edinb.& Australia & -38   -40 & scale &  432  & 1903  1914 \\
Cape        & S. Africa & -41   -51 & screw & 1512  & 1897  1912 \\
Sydney      & Australia & -52   -64 & both  & 1400  & 1892  1948 \\
Melbourne   & Australia & -65   -90 & screw & 1149  & 1892  1928 \\
\noalign{\smallskip}
\hline
\label{tab:obschar1}
\end{tabular}
\end{flushleft}
\end{table}

\begin{table}[h!]
\caption{Numbers of images in each zone and institutions aiding in keypunching
the data, by zone}
\begin{flushleft}
\begin{tabular}{lccl}
\hline\noalign{\smallskip}
Zone        &  stars  &  images & keyed and verified \\
            &(thousand)&(thousand)&                  \\
\noalign{\smallskip}
\hline\noalign{\smallskip}
Greenwich   & 179 & 322 & USNO         \\
Vatican     & 256 & 480 & CIDA, Venezuela (verified USNO)  \\
Catania     & 163 & 320 & USNO          \\
Helsinki    & 159 & 284 & USNO          \\
Potsdam     & 108 & 143 & CIDA, Venezuela (verified USNO)   \\
Hyderabad N & 149 & 242 & USNO          \\
Uccle       & 117 & 159 & USNO          \\
Oxford II   & 118 & 161 & USNO          \\
Oxford~I    & 277 & 471 & Strasbourg    \\
Paris       & 254 & 436 & Strasbourg    \\
Bordeaux    & 224 & 355 & Strasbourg    \\
Toulouse    & 270 & 433 & Strasbourg    \\
Algiers     & 200 & 330 & Strasbourg    \\
San Fernando& 226 & 346 & USNO          \\
Tacubaya    & 312 & 518 & USNO          \\
Hyderabad S & 293 & 521 & USNO          \\
Cordoba     & 309 & 467 & Sternberg, Russia \\
Perth       & 228 & 402 & USNO          \\
Perth-Edinb.& 139 & 202 & USNO          \\
Cape        & 545 & 901 & USNO          \\
Sydney      & 431 & 743 & USNO          \\
Melbourne   & 218 & 392 & Univ of Fla., USNO  \\
\noalign{\smallskip}
\hline
\label{tab:keypunch}
\end{tabular}
\end{flushleft}
\end{table}

\begin{table}[h!]
\caption{Personnel involved in plate reductions}
\begin{flushleft}
\begin{tabular}{lll}
\hline\noalign{\smallskip}
Zone        &    prepared&  reduced \\
\noalign{\smallskip}
\hline\noalign{\smallskip}
Greenwich    &  Gary Wycoff, John Martin               & Sean Urban \\
Vatican     &  John Martin                            & Sean Urban  \\
Catania     &  John Martin, Gary Wycoff               & Sean Urban  \\
Helsinki    &  Harry Crull, Edward Jackson, David Hall & Sean Urban  \\
Potsdam     &  Marion Zacharias                       & Marion Zacharias  \\
Hyderabad N &  Edward Jackson                         & Sean Urban  \\
Uccle       &  Edward Jackson                         & Sean Urban  \\
Oxford II   &  John Martin                            & Sean Urban  \\
Oxford~I    &  John Martin                            & Sean Urban  \\
Paris       &  Edward Jackson                         & Sean Urban  \\
Bordeaux    &  Edward Jackson                         & Sean Urban  \\
Toulouse    &  David Hall                             & Sean Urban  \\
Algiers     &  Edward Jackson                         & Sean Urban  \\
San Fernando&  Gary Wycoff                            & Sean Urban  \\
Tacubaya    &  John Martin, Gary Wycoff               & Sean Urban  \\
Hyderabad S &  Edward Jackson, Sean Urban             & Sean Urban \\
Cordoba     &  Gary Wycoff                            & Sean Urban  \\
Perth       &  David Hall, Gary Wycoff, John Martin   & Marion Zacharias  \\
Perth-Edinb.&  David Hall, Gary Wycoff                & Sean Urban  \\
Cape        &  Sean Urban                             & Sean Urban  \\
Sydney      &  David Hall, Gary Wycoff                & Sean Urban  \\
Melbourne   &  Gary Wycoff                            & Sean Urban  \\
\noalign{\smallskip}
\hline
\label{tab:people}
\end{tabular}
\end{flushleft}
\end{table}

\begin{table}
\caption[Corrections applied to x,y data prior to final least squares
adjustment.  Also includes single image precisions by zone.]{Corrections applied
to x,y data prior to final least squares adjustment.  Also includes single
image precisions by zone.

\vspace*{.10in}
In the table, a ``Y'' indicates a correction was applied,
an ``n'' indicates it was not.
Values in parentheses indicates an additional dependence on those
characteristics.

\vspace*{.10in}
mg=magnitude, mr=measurer, mp=microscope, rs=reseau, ep=epoch, \\
x=x-coordinate, and y=y-coordinate.

\vspace*{.10in}
Note that Ox~II/Grn ``zone'' refers
to the 32~plates observed at Greenwich for the Oxford~II zone.}
\begin{flushleft}
\begin{tabular}{lllllllll}
\hline\noalign{\smallskip}
 Zone       &con& radial dist& Mag Eq.      & Coma  & Screw & FDP      &$\overline{\sigma}$ ra  &  $\overline{\sigma}$ dec \\
\noalign{\smallskip}
\hline\noalign{\smallskip}
Greenwich   & 8 &  Y(mg)     & Y            & Y     & n     & Y        & .29    &   .30 \\
Vatican     & 8 &  Y(mg)     & n            & n     & Y(mr) & Y(mp)    & .41    &   .43 \\
Catania     & 8 &  Y(mg)     & Y(x,y)       & n     & n     & Y(mg)    & .33    &   .31 \\
Helsinki    & 8 &  Y(mg)     & Y(x,y)       & Y     & Y(mr) & Y        & .24    &   .22 \\
Potsdam     & 6 &  n         & Y(mr)        & Y     & n     & Y        & .26    &   .25 \\
Hyder. N    & 8 &  Y         & Y(x,y)       & Y     & n     & Y(mg)    & .32    &   .29 \\
Uccle       & 8 &  Y(mg,x,y) & Y            & Y     & Y     & Y(mg)    & .32    &   .37 \\
Oxford~II   & 8 &  n         & Y(x,y)       & n     & n     & Y        & .32    &   .31 \\
Ox~II/Grn   & 8 &  Y         & n            & n     & n     & n        &        &       \\
Oxford~I    & 8 &  n         & Y(x)         & n     & n     & Y        & .32    &   .31 \\
Paris       & 8 &  Y(mg)     & Y            & n     & Y(mp) & Y(rs)    & .22    &   .21 \\
Bordeaux    & 8 &  n         & Y(ep)        & n     & Y     & Y(rs)    & .22    &   .21 \\
Toulouse    & 8 &  Y(mg,mp)  & Y(x,y)       & n     & Y(mp) & Y(rs)    & .30    &   .28 \\
Algiers     & 8 &  Y(mg)     & Y            & n     & n     & n        & .19    &   .19 \\
San Fer.    & 8 &  Y(mg)     & Y            & Y     & n     & Y(mg)    & .33    &   .34 \\
Tacubaya    & 8 &  n         & Y            & n     & n     & Y(mg)    & .27    &   .26 \\
Hyder. S    & 8 &  Y(mg)     & Y(x,y)       & Y     & n     & Y(mg,rs) & .33    &   .34 \\
Cordoba     & 8 &  Y(mg)     & Y(x,y)       & n     & Y(mp) & Y(mg)    & .35    &   .30 \\
Perth       & 6 &  Y         & Y            & Y     & n     & Y        & .33    &   .30 \\
Per-Edin.   & 8 &  Y(mg)     & Y(x,y)       & n     & Y     & Y(mg)    & .31    &   .30 \\
Cape        & 8 &  n         & Y(x,y)       & Y(ep) & Y(mp) & Y(mg,rs) & .31    &   .29 \\
Sydney      & 6 &  Y(mg)     & Y(mp,rs,x,y) & n     & Y(mp) & Y(mg,rs) & .48    &   .43 \\
Melbourne   & 8 &  Y         & Y            & n     & Y(mp) & Y(mg)    & .37    &   .35 \\
\noalign{\smallskip}
\hline
\label{tab:const}
\end{tabular}
\end{flushleft}
\end{table}
\end{document}
