You are here: Home / USNO / Astrometry / Optical/IR Products / UCAC
The USNO websites,,,,,, and are undergoing modernization efforts. The expected completion of the work and the estimated return of service is the first half of 2022, subject to change due to potential impacts of COVID-19.

USNO CCD Astrograph Catalog (UCAC)

 Last update: 2013 Mar 11



UCAC is an astrometric, observational program, which started in February 1998 at CTIO. All sky observations were completed at the Naval Observatory's Flagstaff Station in May 2004, and data reduction was completed in July 2009. The UCAC3 catalog was released on August 10, 2009 at the IAU General Assembly in Rio de Janero, Brazil. Bug fixes, reduction improvements, use of NPM data for proper motions and inclusion of APASS 5-band photometry lead to the final UCAC4, released in August 2012.


UCAC4 News:

  • 2013 Mar 10, UCAC4 streak objects: Some objects in the UCAC4 catalog are already classified as "streak objects". In an effort to identify artifacts in the catalog this issue has been further investigated by others; see for example here.
  • 2013 Feb: Data for a small number of high proper motion stars have been corrected. See the following files:

    • readme_fixHPM = description of corrections
    • = program for modifying UCAC4 files
    • = a Python 3 version of the fix (works with Windows)
    • modifs.c = C-code for the update
    • modifs.exe = executable which should work on almost all Windows computers
    • fixhpm.ascii = text table with data and explanation of affected stars
  • 2012 Aug: A number of UCAC- and URAT-related talks were presented in the Commission 8 (Astrometry) science sessions at the August 2012 International Astronomical Union (IAU) General Assembly. These may be downloaded from the link here.  
  • 2012 Aug: UCAC4 was released in August 2012. It is available from CDS as Vizier catalog I/322; DVDs have been shipped to all customers on file.  To receive a free copy of the UCAC4 catalog on DVD please send a request to brenda.hicks[at] with "UCAC4" in subject and your full name and postal mailing address in the body of your e-mail.
  • 2011 Jul: By mid 2011 a first version of the UCAC4 was produced. It was sent to a number of testers worldwide. Feedback comments were received and by the end of 2012 an improved version of UCAC4 was produced which has further been investigated. Now we have an almost final version for about 113 million stars.

    UCAC4 does not use any Schmidt plate data. Proper motions of faint stars north of Dec = -20 deg are now based on NPM data. Besides 2MASS near-IR photometry (as in previous releases) UCAC4 now includes APASS 5-band photometry. The APASS data release #6 is expected to become available in May 2012 to complete the all-sky coverage. It has been decided to wait with the UCAC4 release to include APASS DR6 data. Thus the public release of UCAC4 is scheduled for June 2012.

    The UCAC4 data will be sent to CDS Strasbourg at that time and a double-sided DVD with the UCAC4 release files will be sent to addresses on our distribution list (UCAC2, UCAC3 customers). In order to assist users and 3rd party software vendors to interface with UCAC4, the following files are now available:

    • readme_u4v5 = text file with explanations
    • z001 = binary sample data of first zone
    • z001.asc = ASCII sample data (fixed format) of same data
    • u4dump.f = Fortran code to dump binay zone file(s) to ASCII
    • u4access.f = Fortran code for interactive data retrieval incl. index
    • u4sub.f = subroutine file needed by the above



  • UCAC3 was released 2009 August 10 at the IAU General Assembly in Rio. DVDs were shipped throughout the month of September to recipients on the distribution list (see the readme_u3 file.). The data are also available from the CDS.

    The observational program is complete. The last survey fields were taken successfully on 17/18 May 2004. A final all-sky plot is presented in the final monthly progress chart. 

    The Bright Star Supplement (BSS) to UCAC2 is now available. It contains exactly 430,000 stars from Hipparcos and Tycho-2 data to fill in the sky area missed by UCAC2 plus the all sky bright stars not in UCAC2. The files can be obtained from (new location since mid 2006) anonymous ftp, /users/nz/bss/ , please start with the "readme" file, which is also available here. A plot shows the sky distribution of all BSS stars.

    UCAC3 official star numbers: While the MPOS number (last column on each data record) mainly provides a means to identify known high proper motion stars, the primary star identification number should be of the form:


    The "3UC" is constant and indicates the UCAC3 catalog. The 3 digit "zzz" number is the zone the star is in, followed by a dash and a 6 digit number which is the record number of the star on that zone. Thus the official designation of the star 42 in zone 7 would be 3UC007-000042.


  • UCAC2: was released at the IAU GA in Sydney. Over 1200 copies have been distributed so far.

    The catalog introduction file (readme.txt) is available. It contains a detailed description of all files, formats and notes for the user.

    The paper describing the UCAC2 was published in AJ, 127, 3043-3059 (May 2004) and a pre-print is also available from (astro-ph) as entry 0403060.

    The UCAC2 is now available from (Vizier, Strasbourg) as entry I/289.

    A color plot shows the sky coverage of UCAC2.

    UCAC2 CD-ROM files access code in C/C++ is now available from (Project Pluto).

  • The history of other news notes has been relocated to a separate page.