First U.S. Naval Observatory CCD Astrograph Catalog (UCAC1)   March 2000
1) Introduction
2) Summary of the project
3) Properties of the catalog and important notes for the user
4) Files contained on this CD-ROM
5) Data formats
6) Description of utility software
7) References
1) Introduction
This is the "readme" file of the UCAC1 distribution.
The UCAC1 is a high density, highly accurate, astrometric catalog of
over 27 million stars in the Southern Hemisphere in the magnitude 
range of about 8 to 16.  This is the first release of an ongoing project
to observe the entire sky.  For up-to-date information see our Web page
at .
The UCAC1 is a preliminary catalog.  By avoiding all "problem cases" it
is not complete.  Positions, proper motions and errors are provided,
together with approximate magnitudes.  The positional precision is 
about 20 mas for 9 to 14 mag and 70 mas at 16th mag.  The errors of the
proper motions are very heterogenous, ranging from 1 to 35 mas/yr.
The project and this catalog are described in detail in a paper prepared  
for The Astronomical Journal (AJ), Zacharias et al.(2000).  A version of 
that and other related papers will be placed on our Web page.
The data are freely available; however, any publication utilizing these 
data should make proper reference to USNO and the paper mentioned above.
Included with the CD-ROM is also some utility software.  This is not
required to use the data; however, this software serves as an example
for a fast access to the data.  No guarantee or support is provided.
The main data are compressed (gzipp'ed and tar'ed), thus software to
uncompress the data is required by the user.  The CD should work on
a variety of platforms and operating systems, including Linux and Unix,
Mac and MS Windows.
2) Summary of the project
a) Observations
The UCAC is a new observational program, using the U.S. Naval Observatory
Twin Astrograph and a 4k by 4k CCD camera, covering just over 1 square
degree per frame with a scale of 0.9"/pixel.  The CCD is a thick, Kodak
device with 9 micrometer pixel size.  The camera, made by Spectral 
Instruments, is peltier cooled to about -18 C.  Observations started in 
January 1998 at Cerro Tololo Interamerican Observatory (CTIO) and are 
continuing.  This first catalog contains data observed through 07 November
1999 at which time about 80% of the Southern Hemisphere was completed.
Full sky coverage is expected by early 2003 after a re-location of the
instrument to the Northern Hemisphere planned for early 2001.
A 2-fold, center-in-corner overlap pattern was adopted on a 0.5 degree
grid, starting at the South Celestial Pole.  Each field was observed 
with a long (100 to 150 sec) and a short (20 to 30 sec) exposure.
An extensive quality control rejected over 10% of the frames taken.  
All raw (pixel) data are saved.
Observations are made in a single bandpass (579-642 nm) and UCAC 
magnitudes are between Johnson V and R.  No attempt has been made to 
provide good photometric data.  In fact, observing progresses even during
nights with cirrus clouds.  The red-corrected, 20 cm aperture, 5-element
lens of the astrograph provides a 9 degree field of view (designed for 
photographic plates), thus only a fraction is utilized with our CCD
camera, centered on the optical axis.
The telescope is being actively guided, using the other, visually-
corrected lens of the twin-astrograph with an ST-4 autoguider.  
Operation is automated using a PC and a single board computer. 
An HP-Unix workstation is used for on-line reductions.
b) Reductions
A combination of Hipparcos, Tycho-1 and ACT data were used as the 
reference star catalog for the CCD frame reductions.  For each individual
frame about 10 to 40 reference stars were used.  For various reasons no 
successful solution for 245 CCD frames (out of 79,931) was obtained and 
thus these frames were excluded from this preliminary catalog.  Individual
problem stars, particularly multiple stars, are excluded as well.
Saturated images of stars were not used, thus no bright stars are
included.  Depending on atmospheric conditions, the cut-off at the
bright end is around magnitude 8.  Weighted mean positions were produced
from the individual positions.  Only stars with 2 or more images are
included in this catalog.  Stars with a formal standard error exceeding
200 mas in either coordinate were excluded.
Proper motions of bright stars (8 to 12.5) were derived using a 
combination of ground-based astrometric catalogs, Hipparcos, and 
Tycho-2 positions.  Those proper motions are very accurate with internal
errors of about 2 mas/yr mainly due to the large epoch differences
involved.  For the fainter stars the USNO A2.0 (Monet, 1998) was used
as first epoch.  Typical errors for these stars are estimated to be
10 to 35 mas/yr, primarily depending on the epoch of the photographic 
plates used in the USNO A2.0 catalogue.  Unfortunately the epoch 
difference between the A2.0 in the South and the UCAC1 data is very small
(sometimes less than 10 years) causing large errors in the proper motions.
3) Properties of the Catalog and important notes for the user
- Sky coverage: from South Celestial Pole to between -21 and -5.5 degree
 declination.  The northern limit is a function of right ascension 
 (see the plot
- Completeness:  the UCAC1 is NOT complete, neither in the above 
    mentioned sky coverage, nor for individual stars:
  -- A few areas (mainly in extended, dark clouds) have long OR short
       OR both exposures missing (see reduction section above).
       Depending on how many other, overlapping frames are available, 
       this could mean:
       a) faint stars (>14mag) are missing, or,
       b) bright stars (<10) are missing, or,
       c) all stars are missing
      Only a small fraction of the sky (in the area covered here) will
      be affected, maybe about 100 isolated spots of 0.5degree width.
  -- Multiple stars within about 3 arcsec are excluded in every case.
  -- All "problem stars", e.g. with large internal errors in the
    image combine step are excluded.
  -- In case no unique match for the proper motion determination could
       be made, the star was removed from the current catalog, resulting
       in a loss of about 13% of the stars.
  -- If a star's proper motion was found to be larger than 200 mas/yr 
       and it is based on only 2 catalogs (i.e. UCAC1 and A2.0), the star
       was excluded from the present catalog.  A large excess of these
       "high proper motion" stars (330,000) were found, most of which 
       are likely to be artifacts of the currently adopted matching routine.
- Galaxies: the UCAC1 contains some galaxies, particularly at the 
    faint end.  No flag indicating a galaxy or star is provided with this
    preliminary catalog; however, extended objects are very unlikely to be
    listed in the UCAC1 due to the detection and reduction procedure adopted
    for UCAC1.
- Magnitude range: about 8th to 16th magnitude in a 579-642 nm 
    bandpass which is between visual (V) and red (R).  The limiting 
    magnitude typically varies by +- 0.3 mag, can be more in some 
    areas.  The bright cut-off varies even more. 
- Magnitudes are only approximate with an estimated error of about
    0.3 mag for the majority of stars and much worse in some cases.
    The relative precision over <0.5 degree and medium magnitude range
    can be much better, maybe 0.1 mag.  Note: some data were obtained
    in non-photometric conditions.  Magnitudes are only provided for
    identification purposes.
- Positions are on the Hipparcos system, i.e. the International
    Celestial Reference System (ICRS), which is consistent with "J2000".
- Positions are given at the mean epoch of observation, which ranges
    from Feb.1998 to Nov.1999. 
- For the majority of stars, the proper motions are very preliminary,
    intended to update the star positions to a recent epoch.  In some
    cases it might be advisable not to use the proper motions at all.
- For the Tycho-2 (and other AC) stars in UCAC1 the provided proper 
    motions are very precise (1 to 3 mas/yr) and accurate (systematic
    errors typically 0.5 mas/yr or less).
- Identification: explicit star numbers are not provided.  It is
    recommended to use the running line number over all data (index 
    files, see below) and use an 8-digit after the string "1UCAC" 
    (compliant with IAU recommendations).  Example: "1UCAC00000001" 
    would be the first star in the first zone, while "1UCAC27425433" 
    is the last star in our current catalog.  Also, the position of a 
    star can be used as identifier in any case.
- The data are arranged in declination zones of 0.5 degree width
    starting at the South Pole.  There is a separate file for each zone
    in the directory "u1" after you uncompress the "u1.tar.gz" (see below).
    The individual files are sorted by right ascension, one line for
    each star.
4) Files contained on this CD-ROM
This CD-ROM contains the following files:
 readme.txt = This file (with CR/LF end of line convention).  = PostScript file of a plot showing the sky coverage of
                the UCAC1 data.  Areas not covered by dots are not 
                included in this release.  However, areas covered with
                dots need not necessarily have stars in UCAC1 (see
                remarks about completeness above).
 u1.tar.gz  = Main data in compressed format.  You need software to
                uncompress this file.  For MS Windows you may use the
                "WinZip" utility.  On a Unix (Linux) machine issue the
                following command, after you copied the file to disk:
                tar -xvzf u1.tar.gz
                The uncompressed data are about 1.8 GB, consisting of
                the directory "u1". This contains the 169 individual
                zone files "z001" to "z169" which are in plain ASCII,
                in a format compatible with your machine.
 unix       = Directory with utility software (see below), and index
                files for zones and boxes (see below).  All files in this
                directory have 1 byte as end of line marks, which is 
                compatible with Unix computers.
 win        = This directory contains the same files as "unix", however
                using 2 bytes (CR and LF) as end of line markers, which
                is compatible with MS Window ASCII files.
Files within directories "win" and "unix":
 zone.txt   = Table of contents of main data giving a list of zones.
                It is recommended to look into this file next.
                Each line of the table gives statistics for one
                declination zone.
 index.txt  = ASCII table of full index, organized in declination
                zones and sections of 0.1 hour width along right ascension.
 cat2da.f   = Fortran program to convert the "u1" zones into a single,
                unformatted, direct access files of 600 MB
 catread.f  = Fortran program to access the file created by cat2da.f
The utility software is not required to make use of the UCAC1 data.
The software serves as an example of efficient use of the UCAC1 data
with fast, random access to arbitrary areas in the sky.
5) Data Formats
z001 to z169 -->  main data, all integer, example:
        RA       SPD  mag  sx  sy no   ep    pmx    pmy spx spy nc
   1236845   1232368 1591  27  90  2 1566   -117    -30 118 128  2
   4085292    292994 1382  17  23  8 1537    -39    -15 117 118  2
   5017477   1421948 1577   3  39  2 1570   -629    174 117 119  2
   7335326   1552433 1579  43  26  4 1530    477    270 119 118  2
  All columns are separated by at least 1 blank, with no blank entry in
  any column, thus format free input (in Fortran  READ* ) is possible.
  col format item    unit      range of values   
   1-10  I10   RA    mas       [  1154 1295999821] 
  11-20  I10   SPD   mas       [ 57965  304195851] 
  21-25  I5    mag   1/100     [   570       1715] 
  26-29  I4    sx    mas       [     1        200] 
  30-33  I4    sy    mas       [     1        200]
  34-36  I3    no              [     2         37] 
  37-41  I5    ep   1/1000 yr  [  1121       2852] 
  42-48  I7    pmx 1/10 mas/yr [-18759      65382] 
  49-55  I7    pmy 1/10 mas/yr [-57756       9936] 
  56-59  I4    spx 1/10 mas/yr [     6        347] 
  60-63  I4    spy 1/10 mas/yr [     5        310] 
  64-66  I3    nc              [     2         26] 
  RA  = right ascension, in the system of ICRS (J2000), at the epoch of 
        observation, given in column "ep"
  SPD = south pole distance = declination + 90 degree, same system and
        epoch as RA (see also example below)
  mag = approximate magnitude in UCAC bandpass (579-642 nm), which is 
        between V and R, non-photometric, for identification only
  sx, sy = internal standard error of RA*cosDC and DC (SPD) respectively
  no  = number of observations (images per star) from the astrograph data
        used for the mean position given here
  ep  = mean epoch of astrograph observation minus 1997,
          note: this is not the mean UCAC1 epoch based on all catalogs
  pmx, pmy = proper motion in RA*cosDC and declination, respectively
  spx, spy = standard error of pmx, pmy, respectively; the larger value of
             either the model prediction or the error from the scatter 
            (applicable for 3 and more catalogs) is given here
  nc  = number of catalog positions used in the proper motion determination
    DC = SPD - 324000000 (in mas) 
    SPD= 1232368 mas --> DC = -322767632 mas = -322767.632 arcsec = 
    -89.6576756 degree = -89 39'27.632"
  Epoch:  1566  means 1.566 years after 1997.0, thus 1998.566
zone.lst  --> explanations are given on the file itself
index.asc --> 1 line for each box (0.5 deg, 0.1 hour), example:
       zn RAi stars      i0 
     z001   1     3        0
     z001   2     2        3
     z001   3     4        5
     z001   4     2        9
  col format item    range             explanation
   1                             letter "z" to match zone file names
   2- 4  I3   zn     [1   169]   zone number (see file zone.lst)
   5- 8  I4   RAi    [1   240]   RA index (0.1 hour wide boxes)
   9-14  I6   stars  [0 10208]   number of stars in that box
  15-23  I9   i0  [0 27758257]   ID number of star before first in that box
6) Description of utility software
cat2da.f : - read  u1/z001  to u1/z169  ASCII files (1.8 GB)
           - output unformatted, direct access file (0.6 GB)
           - compile and run interactively, 
               you need to provide path and file names 
           - output some statistics and  index.asc, index.unf  files
           - Note: the std.errors in proper motions will be rounded 
               to the next even number in 1/10 mas/yr units
catread.f : - program to access unformatted, direct access file,
                which is created by the program  cat2da.
            - interactive, option to specify a box in RA,DC or range
                of star numbers
            - includes subroutine, which retrieves the data
            - random access on the sky, using  index.unf  file
                based on 0.5 degree by 0.1 hour large boxes,
                with 2975 stars/record (64KB)
No guarantee can be made for this software, neither can any kind of
support be provided.  Users are encouraged to take segments of these
examples and built their own software.
7) References
ESA 1997, "The Hipparcos and Tycho Catalogues", European Space Agency,
     publication SP-1200
Gauss,F.S. Zacharias,N. Rafferty,T.J. Germain,M.E. Holdenried,E.R.
Pohlman,J.W. and Zacharias,M.I., 1996,
     "A new astrometric survey of the Southern Hemisphere",
      Bull.AAS 28, No.4, p.1282
Hoeg,E., Fabricius,C., Makarov,V.V., Urban,S., Corbin,T., Wycoff,G.,
Bastian,U., 2000, "The Tycho-2 Catalogue of the 2.5 million brightest
      stars", in press Astron.& Astroph. (short paper)
Hoeg,E., Fabricius,C., Makarov,V.V., Bastian,U., Schwekendieck,P.,
Wicenec,A., Urban,S., Corbin,T., Wycoff,G., 2000, 
     "Construction and verification of the Tycho-2 Catalogue", 
     submitted to Astron.& Astroph. (long paper)
Monet,D. et al., 1998, USNO-A2.0 catalog, CD-ROM (USNO, Washington DC)
Urban,S.E., Wycoff,G.L., Makarov,V.V., 2000, "Comparison of Tycho-2 
      with ACT and Hipparcos", in preparation for AJ
Zacharias,N. 1997, "Astrometric Quality of the USNO CCD Astrograph (UCA)",
      AJ 113, 1925
Zacharias,N., Germain,M.E., Rafferty,T.J., 1997,
      "UCAC-S: a New High Precision, High Density Astrometric
       Catalog in the Southern Hemisphere",
   in: Proceedings "Hipparcos Venice 97", ESA publication SP-402, p.177
Zacharias,N. Zacharias,M.I., 1999,
      "Data Structure and Software of the UCAC-S Project",
   in: Proceedings of ADASS VIII, Eds. D.M.Mehringer,  R.L.Plante and
       D.A.Roberts,  ASP Conf.series 172, p.345, San Fransico 
Zacharias,N., Urban,S.E., Zacharias,M.I., Hall,D.M., Wycoff,G.L.,
Rafferty,T.J., Germain,M.E., Holdenried,E.R., Pohlman,J.,
Gauss,F.S., Monet,D., Winter,L., 2000,
      "The first US Naval Observatory CCD Astrograph Catalog",
      in preparation for AJ
Zacharias,N., Zacharias,M.I., Urban,S.E. and Hoeg,E., 2000,
      "Comparing Tycho-2 astrometry with UCAC1", submitted to AJ