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 http://ad.usno.navy.mil/ucac/ .
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 skycov.ps).
- 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).
skycov.ps = 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] ------------------------------------------------
explanations: ------------- 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
examples: --------- Declination: 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