Information on Service 'The fourth U.S. Naval Observatory CCD Astrograph Catalog (UCAC4)'

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UCAC4 is a compiled, all-sky star catalog covering mainly the 8 to 16 magnitude range in a single bandpass between V and R. Positional errors are about 15 to 20 mas for stars in the 10 to 14 mag range. Proper motions have been derived for most of the about 113 million stars utilizing about 140 other star catalogs with significant epoch difference to the UCAC CCD observations. These data are supplemented by 2MASS photometric data for about 110 million stars and 5-band (B,V,g,r,i) photometry from the APASS (AAVSO Photometric All-Sky Survey) for over 50 million stars. UCAC4 also contains error estimates and various flags. All bright stars not observed with the astrograph have been added to UCAC4 from a set of Hipparcos and Tycho-2 stars. Thus UCAC4 should be complete from the brightest stars to about R=16, with the source of data indicated in flags.

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This data release is available free of charge on a double sided DVD or through astronomical data centers.

UCAC4 is a compiled, all-sky star catalog covering mainly the 8 to 16 magnitude range in a single bandpass between V and R. Positional errors are about 15 to 20 mas for stars in the 10 to 14 mag range. Proper motions have been derived for most of the about 113 million stars utilizing about 140 other star catalogs with significant epoch difference to the UCAC CCD observations. These data are supplemented by 2MASS photometric data for about 110 million stars and 5-band (B,V,g,r,i) photometry from the APASS (AAVSO Photometric All-Sky Survey) for over 50 million stars. UCAC4 also contains error estimates and various flags. All bright stars not observed with the astrograph have been added to UCAC4 from a set of Hipparcos and Tycho-2 stars. Thus UCAC4 should be complete from the brightest stars to about R=16, with the source of data indicated in flags. UCAC4 also provides a link to the original Hipparcos star number with additional data such as parallax found on a separate data file included in this release.

The proper motions of bright stars are based on about 140 catalogs, including Hipparcos and Tycho, as well as all catalogs used for the Tycho-2 proper motion construction. Proper motions of faint stars are based on re-reductions of early epoch SPM data (-90 to about -20 deg Dec) and NPM (PMM scans of early epoch blue plates) for the remainder of the sky. These early epoch SPM data have also been combined with late epoch SPM data to arrive at proper motions partly independent from UCAC4 (Girard et al. 2011). The NPM data used in UCAC4 are not published. No Schmidt plate data are used in UCAC4.

The unpublished plate measure data obtained by StarScan from the AGK2, the Hamburg Zone Astrograph, the USNO Black Birch Astrograph, and the Lick Astrograph have contributed to considerable improvement in proper motions for stars mainly in the 10 to 14 mag range (down to the UCAC limit for Lick data); however, these data do not cover all sky.

Recources permitting, USNO plans to release the individual CCD observations (RA,Dec at epoch of each CCD observations) in the future. Please contact if you are interested in obtaining this set of about 50 GB data. We will likely request that interested users provide an external disk drive for the data release.

Differences between various versions of UCAC and how to get the data
initial results covering small sky area in the south (obsolete)
-90 to about +50 deg Dec, 48 mill.stars, restricted to good astrometry sources, systematic astrometric error corrections performed differently than for UCAC4 (mostly obsolete by now)
first all-sky catalog, new pixel reductions, more elaborate systematic error corrections, push for faint limit, not as clean as UCAC2 data, several bugs affecting ~ 1 % of stars (obsolete)
improved version of UCAC3 = final release

UCAC3 features a number of major differences with respect to UCAC2:

  • complete sky coverage
  • re-reduction of the pixel data with better modeling
  • double stars are resolved to the limit of the data
  • significantly improved photometry from CCD data
  • slightly deeper limiting magnitude with larger number of stars/area
  • reduced systematic errors of CCD observations
  • the addition of several new catalogs for improved proper motions
  • photometry in the B, R, and I bands from the SuperCosmos project
  • minor planet observations have been sorted out
  • identification of more high proper motion stars
  • match with 2MASS extended sources and LEDA galaxies

UCAC4 is largely based on UCAC3 (same pixel data reductions), however, has a number of critical improvements over UCAC3:

  • bug fixes (e.g. missing stars, multiple entries, mag.eq. corrections)
  • use NPM data to derive proper motions of faint stars north of -20 Dec
  • final tweak of systematic error corrections brings it closer to UCAC2
  • photometry in the B, V, g, r and i bands from APASS for 50 mill.stars
  • use APASS photometry to calibrate ("flatten") instrumental mags
  • removal of photometric bias as function of CCD x-coordinate
  • add brightest stars from FK6, Hipparcos and Tycho-2 catalogs
  • link to Hipparcos star numbers and inclusion of Hipparcos 2007 release data like parallaxes
  • cross reference to Tycho-2 star numbers

In August 2011 a pre-release UCAC4-beta catalog was constructed and distributed to several colleagues worldwide. The UCAC team appreciates the very helpful feedback provided by these reviewers. Unfortunately due to the deployment of the URAT (Zacharias & Gaume 2010) instrument, completion of the UCAC was delayed further. A journal paper (Zacharias et al. 2012) describing the UCAC4 release is in preparation. The UCAC4 release paper should be cited whenever UCAC data are utilized.

In July 2012 the UCAC4 data are sent to CDS and it is expected that the UCAC4 catalog will become on-line shortly. The UCAC4 double-sided DVD automatically will be sent to all addresses on our distribution list (people who received or requested UCAC2 or UCAC3) as fast as our resources allow. If you are not on that list or did not receive the DVD say by October 2012, please send a short e-mail request to, put "UCAC4" in the subject line and your complete postal mailing address in the body of the message (text format).

The UCAC4 comes free of charge. Technical questions may be addressed to (Norbert Zacharias) or (Charlie Finch). The latest update on astrometry related projects at USNO can be found at: .

Details about observations and reductions

Observations and Instrumentation

The UCAC is an observational program, using the U.S. Naval Observatory Twin Astrograph and a 4k by 4k CCD camera, covering just over one square degree per frame with a scale of 0.9"/pixel. The red-corrected, 20 cm aperture, 5-element lens of the astrograph provides a 9 degree diameter field of view (designed for photographic plates), thus only a fraction is utilized with our CCD camera, centered on the optical axis. The same lens in a new tube assembly is now used with a 476 million pixel LN2 cooled camera for the URAT program (Zacharias 2004, Zacharias & Gaume 2010).

The 4k CCD is a thick, Kodak device with 9 micrometer square pixels. The camera, made by Spectral Instruments, is Peltier cooled to -18 C. The raw data are severely affected by a low charge transfer efficiency (CTE) of our otherwise cosmetically excellent CCD chip. To mitigate this problem, a relatively warm operating temperature is used, causing a significant dark current.

Observations started in January 1998 at Cerro Tololo Interamerican Observatory (CTIO) in Chile, where the entire southern sky and about half of the northern sky were observed. In October 2001 the instrument was moved to the Naval Observatory Flagstaff Station (NOFS) in Arizona where it completed the northern sky in 2004.

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 both a long (100 to 150 sec) and a short (20 to 30 sec) exposure. Extensive quality control routines led to the rejection of over 15% of the frames taken. All raw (pixel) data were saved and archived.

Observations were made in a single bandpass (579-642 nm), thus the UCAC magnitudes are between Johnson V and R. No attempt has been made to obtain high quality, photometric data from the CCD observations. In fact, observations were made during nights with thin cirrus clouds.

The telescope was actively guided with an ST-4 autoguider mounted behind the second, visually-corrected lens of the twin-astrograph. Operation was automated using a PC and a single board computer. An HP-Unix workstation was used for on-line reductions to obtain quality control statistics in near real-time.

Reductions of UCAC Observations

The UCAC4 positions of the CCD observations are based on the Tycho-2 reference stars (Hoeg et al. 2000), similar to the UCAC2 catalog (Zacharias et al. 2004). However, the 2MASS was utilized to probe for systematic errors in the CCD data, particularly magnitude equations and coma-like errors caused by the poor CTE of the detector (Finch, Zacharias & Wycoff 2010). Image centering is based on a modified Lorentz profile model which matches the observed PSF better than a Gaussian function (Zacharias 2010).

For UCAC4 the pixel reduction results of UCAC3 was adopted with the only change being the identification and elimination of close multiple images on the same CCD frame. Pixel processing included double star fits and real aperture photometry. "Flip" observations (telescope on West and East side of pier for the same calibration fields) were utilized to fix the overall magnitude equations.

Saturated images of stars were propagated through the pipeline into the output catalog. Thus similar to UCAC3, UCAC4 contains many more bright stars than UCAC2. However positional results are not as reliable as for unsaturated images and the user is urged to pay attention to flags and the meaning of various data columns.

To improve this situation, a new feature was introduced with UCAC4: For stars brighter than UCAC model or aperture magnitude 8.5 and if no "good" image from UCAC CCD is available or the object is flagged as blended image or the position difference to Tycho-2 is larger than 50 mas in either coordinate, the FK6/Hipparcos/Tycho-2 astrometric data are used instead of the UCAC observational data. Furthermore, UCAC4 was supplemented by FK6/Hipparcos/Tycho-2 data for all bright stars not observed with the UCAC astrograph. The source of data is indicated by a flag in UCAC4. Nevertheless the purpose of those supplement stars is to make the user aware of these bright stars. No claim is made to have the best astrometric and photometric data available for those stars, or even have the identification right in all cases in UCAC4. Users of bright stars ought to look at a variety of catalogs to compare all information available.

As before, some positions (can happen to stars of all magnitudes) are based on a center-of-mass centroiding when the least-squares fit did not succeed. Those observations are identified by number of used images (nu) equal to 0. The positions of those stars should be used with caution.

Data used to derive Proper Motions

A master list of exactly 181,895,143 mean positions from CCD data was matched against the various other catalogs. This number is larger than before due to a lower threshold used to accept individual images from CCD pixel data reductions.

Proper motions of bright stars (R ~8 to ~12.5) were derived using a combination of ground-based photographic and transit circle catalogs, and included satellite observations from the Hipparcos and Tycho-2 catalogs. In addition, the U.S. Naval Observatory measured about 5000 astrograph plates on the StarScan machine to derive about 9 million positions for stars mainly in the 10 to 14 magnitude range, including the complete set of AGK2 (Bonn and Hamburg zones, +90 to -2.5 deg Dec), as well as about 30 % of the sky covered by the USNO Black Birch (south) and Hamburg Zone astrograph (north) programs.

For the faint stars (~12.5 to ~16.5), data from the first epoch plates of the Yale Southern Proper Motions (SPM, van Altena et al. 1999) are utilized. These plates were measured on the Precision Measuring Machine (PMM) at USNO Flagstaff Station by D. Monet. A complete re-reduction of the data was performed in a joint USNO - Yale University effort utilizing the StarScan pipeline for the pixel reductions and the Yale software to obtain RA,Dec coordinates (Girard et al. 2011). The SPM data used for UCAC4 are based on the early epoch (about 1970) blue and visual plates covering the about -90 to -20 deg declination sky area. For the rest of the sky an unpublished star catalog (Girard, private comm.) based on NPM 1st epoch (about 1950) blue plates was used. These plates were also measured on the PMM and processed with the StarScan and Yale Univ. pipelines. Only for the SPM data original pixel data of plate scans were available. The NPM data shows larger systematic errors than the SPM data. However, the epoch difference to UCAC is larger for NPM than for SPM resulting in about an equal amount of estimated, remaining systematic errors of a few mas/yr for UCAC4 proper motions based on those data.

When deriving UCAC4 proper motions from all individual epoch positions, estimates of systematic errors for each catalog entered the weights. The number of stars in UCAC4 which are also in major other catalogs used for the proper motions are as follows:

120487 Hipparcos (including bright supplement stars)
2506683 Tycho-2 (including bright supplement stars)
4373790 AC2000
279570 AGK2 Bonn
982815 AGK2 Hamb
4682287 Zone Astrograph
3492601 Black Birch Astrograpph
1104138 Lick astrograph selected fields
68887550 NPM Lck1
57355612 SPM YSJ1
Computation of Proper Motions

The computation of proper motions is performed similarly to the procedure used for the UCAC2 and Tycho-2 catalogs. All input catalogs were reduced to the ICRF utilizing Hipparcos data or some denser, interim catalog that follows the system of Hipparcos. Standard errors for each position are estimated. These error estimates and RMS added, estimated systematic errors are used as weights to compute a mean position and proper motion by a weighted, least-squares adjustment procedure. Estimates of errors for UCAC4 positions and proper motions are provided.

Note, while calculating proper motions, no attempt was made to correct data for parallaxes. This will lead to slightly inferior results for few stars with high parallax if it involves observations from largely different parallax factors.

Errors in proper motions of the bright stars (to R ~12) run from about 1 to 3 mas/yr benefited by the large epoch spans involved. For the fainter stars using SPM and NPM data, typical errors are 2 to 6 mas/yr.

Contrary to UCAC2, and similar to UCAC3, not all stars in UCAC4 have proper motions. Stars observed by the astrograph made it into the UCAC4 release catalog if any one of the following applies:

  • at least 2 matching positions from different CCD frames were obtained,
  • star was matched with a 2MASS entry,
  • proper motions could be obtained in combination with any other early epoch catalog.

The "plots" folder of the UCAC4 distribution DVD contains all-sky plots with color coded proper motions for the RA*cosDec and Dec component, respectively. Mean proper motions for stars in 0.5 by 0.5 deg boxes are shown after cutting the low/high 15% of proper motions in each box.

For more details please see also the next section and the upcoming UCAC4 journal paper, as well as the published UCAC2 and UCAC3 journal papers (also found on the UCAC4 distribution DVD).

High proper motion (HPM) stars

Note 1: this includes also stars with not so large a proper motion if they are present in the below mentioned external surveys and catalogs aiming at discovering high proper motion stars.

Note 2: automatic processing of high proper motion stars is prone to mismatches with early epoch data and blended images cause issues. Likely there will be some false data in UCAC4 for hight proper motion astrometry. In particular, we do not claim completeness of UCAC4 data with respect to high proper motion stars. Most stars with a proper motion of about 200 mas/yr or smaller should be included in UCAC4, as well as a hand-picked sample of top high proper motion stars. However, coverage of the range in between could be significantly incomplete.

Stars with high proper motions were handled specifically. First a catalog of 1.8 million stars was constructed from published proper motions. In the North the LSPM-North Catalog (Lepine & Shara 2005) of 61977 new and previously known high proper motion stars having proper motions greater than 0.15"/yr was used. In the South many smaller surveys along with the Revised NLTT Catalog (Salim & Gould 2003) were used, which produced 17730 unique high proper motion stars greater than 0.15"/yr. In both the North and South a supplement list of proper motion stars greater than ~0.15"/yr from the Tycho-2 and Hipparcos catalogs were used to fill in any gaps. In chronological order, the smaller southern surveys used include:

  1. 7 papers covering various portions of the southern sky by Wroblewski and collaborators (Wroblewski & Torres 1989, 1991, 1994, 1996, 1997; Wroblewski & Costa 1999, 2001),
  2. UK Schmidt Telescope survey plates of 40 survey fields by Scholz and collaborators (Scholz et al. 2000),
  3. The Calan-ESO survey (Ruiz et al. 2001)
  4. SuperCOSMOS-RECONS proper-motion survey of the entire southern sky (Henry et al. 2004; Subasavage et al. 2005a, 2005b; Finch et al. 2007; Boyd et al. 2011),
  5. the Southern Infrared Proper-Motion Survey (SIPS; Deacon et al. 2005),
  6. Lepine's SUPERBLINK survey of a portion of the southern sky (Lepine 2008) and
  7. UCAC3 proper motion survey (Finch et al. 2010, 2012).

Then we identified these stars in our CCD observations in a 2-step approach. For each individual exposure we establish a list of HPM stars which could be present in that field. HPM star positions were calculated for the epoch of that exposure and then matched with the individual RA,Dec observations of that exposure to identify and flag HPM stars on each exposure (object type = 3).

Contrary to UCAC3, this time a UCAC4 based solution for mean position and proper motion was attempted for all stars including the HPM stars. The results were analyzed as follows:

The position and proper motion solution obtained by the above procedure was substituted by zero proper motion and the mean CCD data position at mean observational epoch for the following cases:

  • PM solution failed or shows large errors (>= 500 mas, >= 50 mas/yr)
  • derived PM is larger than 500 mas/yr in either component
  • derived central epoch is earlier than 1947
  • RA or Dec position difference to CCD data mean position is >= 3 arcsec

These stars are thus added to the group of "no proper motion" stars, i.e. those which did not match up with other catalogs to even begin the proper motion calculation. All stars where then checked against the external set of HPM stars. The PM from the external catalog was used for stars with no UCAC4 PM solution and for those where the difference in PM for either component exceeded 40 mas/yr. Thus we trust the external catalog data more than the UCAC4 derived proper motions in those cases.

Comparions of UCAC4 based astrometric solutions of stars with about 100 mas/yr or more with external data (2MASS, thanks Rae Stiening) indicated unrealistic proper motions for many such stars in UCAC4 data. Plots of distribution of number of stars as function of UCAC4 proper motion bins showed an overdensity around 100 and 200 mas/yr which could be traced back to blended images or wrong idendifications of stars with NPM and SPM early epoch data in crowded fields. NPM and SPM plates were exposed twice (long and short) with offset between exposures and using an objective grating, enhancing the chances of confusion in crowded fields. In order to mitigate this problem in UCAC4 new object flags were introduced as follows. All stars with object type = 0,1,2 (i.e. those with UCAC4 based solution for proper motions, excluding above described special high proper motion stars handling, Hipparcos and Tycho data, and supplemented stars) and a proper motion larger than 80 mas/yr in either coordinate were picked from the UCAC4. This is a total of about 2.8 million stars. These were matched with the PPMXL (Roeser et al 2010) using a match radius of 4 arcsec with both catalog positions at epoch 2000. The UCAC4 stars not matched with PPMXL in this way were assigned the new object type = 8 (2029306 stars). For the matched stars the proper motion difference vector lenght was calcualted and a threshold set to flag discrepant proper motions with object type = 9. The threshold was set as the lower of 80 mas/yr and (3 times the combined formal errors + 5 mas/yr systematic error floor). This way 689470 stars in UCAC4 obtained the ojt = 9 flag, while 140887 stars were found to have consistent proper motions from this set of stars and no change to UCAC4 data flags were made.

Obviously stars flagged with ojt = 8 or 9 should be handled with caution. Many of the ojt = 8 objects can be bogus stars and many of the ojt = 9 stars are likely affected by blended images. Unless external data are consulted to verify UCAC4 results these stars should not be used.

Photometric data

For each detected image 2 instrumental magnitudes were derived. First, the model magnitude is based on the flux volume of the best-fit image profile model which is also used for the center position result. Second, an aperture photometry magnitude is calculated.

In addition to the systematic error corrections applied for UCAC3 (Finch et al. 2010), for UCAC4 a bias correction was applied as a function of the pixel x-coordinate of the image. This bias is caused by the poor charge-transfer efficiency (CTE) of the CCD used for UCAC observations, resulting in image elongation as a function of x. Different bias models were derived for the model and aperture photometry.

The APASS 5-band photometry of over 9 million stars (DR2) was utilized to "flatten" the UCAC instrumental magnitudes. The raw instrumental magnitudes are not linear with offsets up to about 0.5 and 0.3 mag at the bright and faint end, respectively. From color-color plots a linear model was adopted to predict UCAC bandpass magnitudes from APASS r and V. The predicted minus observed (model and aperture mags handled separately) UCAC magnitudes were then plotted as fucntion of magnitude and inter- polation polynomials derived. These corrections were then applied to all UCAC instrumental magnitudes.

All corrected, instrumental magnitudes were then transformed into the system of Tycho-2 by a simple zero-point correction (Finch et al. 2010), the same way as with UCAC3 data. Photometric results were averaged for stars with multiple CCD observations to arrive at the UCAC4 catalog entries. Nights with poor photometric quality were flagged and not used in the average, unless no other data are available.

Note, that "poor photometric" quality for UCAC4 means "really bad" nights, with significant transparency variations due to clouds. The goal was to obtain approximate magnitudes, maybe reliable on the 10% level with differential magnitudes good to about 5%. UCAC is not a photometric catalog.

The UCAC4 data are supplemented by 2MASS near-IR magnitudes and APASS 5-band optical photometry up to its data release DR6 (June 2012).

Properties of the catalog and important notes for the user

Sky coverage

UCAC4 is an all-sky catalog with at least about 40 stars per square deg anywhere on the sky. The average density of this catalog is over 2000 stars per square deg. See a color-coded all-sky plot called showing the logarithm of the number of stars per box of 0.5 by 0.5 deg on the sky. Blue is low density, red high with green and yellow in between.


UCAC4 like UCAC3 is more complete than UCAC2, including previously omitted "problem" stars and double stars, many of which could be new discoveries. A paper is in preparation about a sample of new double stars found in UCAC4 and the rate of confirmation by speckle observations. For preliminary results see (Hartkopf et al. 2010). The separation limit for double stars in UCAC4 varies as function of brightness of the components and brightness difference. However, some doubles with separations of under 2 arcsec in UCAC3 have been confirmed as real. Those cases are rare and objects within 2 arcsec of each other have generally been merged to at least a blended image in UCAC4. Some 12,000 sources with a separation of less than 2 arcsec remain in UCAC4, of which about half entered through the Hipparcos/Tycho-2 supplement data, which includes double star annex catalogs. Stars fainter than R = 10 with separations larger than 2 arcsec are likely real double stars in UCAC4.

UCAC4 also contains observations of some bright stars, as they happened to make it through the pipeline. However, poor data have been substituted by FK6, Hipparcos and Tycho-2 data (in that order of available data). For reduction details on this see section 2b above. Thus UCAC4 should be complete from the brightest naked eye stars to about 16th mag.

The following table gives some general statistics about UCAC4 stars:

113780093 total number of stars in UCAC4 (incl. supplement stars)
109921682 with 2MASS identification
106689821 with proper motions
81897551 with 2 epoch PM
27245403 with 3 or more epoch PM
80806744 with 2 or more images from "good fit" CCD observations
48323349 matched with UCAC2
54690 matched with LEDA galaxies
76020 matched with 2MASS extended source catalog
8925 supplemented stars (no CCD obs.)
121350 UCAC4 entries with a matched Hipparcos star ID
104681 UCAC4 entries with CCD obs. substituted by FK6/Hip/Tycho-2

If the computed position error of a star exceeds 500 mas in either coordinate it was set to 500 mas but the star was kept, if at least 2 observations from different CCD observations were matched or the star is either in the 2MASS, SPM or NPM data files. Similarly, the error in proper motion was truncated to 50 mas/yr but respective stars kept in UCAC4 by the same criteria as for large positional errors. Obviously all large error objects need to be handled with caution, and some of these are simply nonexistent.

This approach, different to UCAC2, was taken in order to drive up the completeness of UCAC3 and UCAC4 at the expense of slightly greater contamination. For UCAC4 also a lower threshold than for UCAC3 was adopted in image size for failed image profile fit objects. In particular, the larger limits in positional errors deemed acceptable for UCAC3 and UCAC4 accommodate the fainter limiting magnitude, which was possible mainly due to superior handling of dark subtraction in the pixel data. This allowed many low signal-to-noise, real stars to enter the catalog, although with expected large random errors, as compared to UCAC2.

Reference frame

The astrometry provided in UCAC4 is on the Hipparcos system, i.e. the International Celestial Reference System (ICRS), as represented by the Tycho-2 catalog. Contrary to earlier UCAC releases, the UCAC4 processing included a down-weighting of Tycho-2 stars with V-magnitude fainter than 11.5 by a factor of 1.5 w.r.t. their nominal weight. Any possible deviations of the Tycho system with respect to Hipparcos or the extragalactic radio reference frame are still under investigation.

Positions in UCAC4 are given at the standard epoch of Julian date 2000.0, thus the UCAC4 is a compiled catalog. In order to be able to calculate positional errors at any epoch, the central epoch, i.e. the weighted mean epoch of the data (UCAC + early epoch other catalogs) is given. At the central epoch (which varies from star to star and is also different for RA and Dec) the positional error has its smallest value; the one given in the catalog for "sigma position". In most cases this central epoch will be close to the UCAC observational epoch due to the relatively large weight given to the UCAC observations. However, a fair number of stars have a vastly different mean epoch, ranging back to about 1947. Proper motion solutions with central epoch earlier than that were defined as invalid and substituted by other catalog results if available, or only the observed CCD position is reported with no proper motion.

The proper motions are given at the central epoch. Positional errors of stars increase according to the errors in the proper motions when going forward or backward in time from the central epoch.


UCAC4 observational data (CCD exposures) covers the magnitude range of about R = 8 to 16.3 in a 579-642 nm bandpass. However, data from FK6, Hipparcos, and Tycho-2 were used to supplement UCAC4 to create a star catalog complete from brightest stars to about 16th magnitude. The UCAC bandpass is between visual (V) and red (R). The limiting magnitude can vary by about +-0.3 mag from field to field.

UCAC4 gives center fit-model magnitudes as well as aperture photometry derived from the same pixel data reductions already performed for UCAC3. Systematic errors in these magnitudes are believed to be below 0.1 mag, which is a significant improvement over the UCAC2 release. Non-linearity of the instrumental magnitudes were calibrated out using APASS photometry for UCAC4. Tycho-2 stars (excluding the faint end) were used to determine the zero-point of the corrected, instrumental magnitudes on a frame by frame basis. However, UCAC observations often were performed in non- photometric sky conditions. Mean magnitudes were derived from CCD frames with indications of "acceptable photometric quality", which is about 50% of the observations.

Additional photometry

The UCAC4 observational data are supplemented with 5-band photometry (B,V, g,r,i) from the APASS project (Henden, private comm.) as well as with IR photometry (J,H,K_s) from the Two Micron All Sky Survey, 2MASS (Skrutskie et al. 2006). In addition, magnitudes errors and some flags are provided. For more details see and .

Non-stellar data

The UCAC4 contains some galaxies, particularly at the faint end. No flag indicating a galaxy or star has been derived from the CCD pixel data; however, a cross reference to the LEDA galaxies (Paturel et al. 2005) and the 2MASS extended source catalog (Skrutskie et al. 2006) has been made and corresponding information is put into the UCAC4 release. The procedure is the same as for UCAC3.

Observations of asteroids have been identified and taken out of the main UCAC4 catalog. Individual epoch observations of about 1000 asteroids at multiple epochs have been prepared for publication, but are not part of the UCAC4 release. The observing schedule explicitly avoided all minor planets brighter than about R = 12, except for special observing campaigns to derive masses of asteroids.

Star identification numbers (name)

Official UCAC4 star ID numbers should be utilized for identification purposes and for communication with the UCAC team: the UCAC4 star number is of the following format:


where zzz is the 3 digit zone number (form 001 to 900) and nnnnnn the the 6-digit running record number along the zone file. The cross- identification to UCAC2 stars follows the same pattern with zone and record number of the UCAC2 release given in the UCAC4 data.

Note, zones numbers in UCAC4 run from 1 to 900 (0.2 deg wide), while zone numbers in UCAC2 run from 1 to 360 (0.5 deg wide), in both cases beginning at the South Celestial Pole (i.e. in order of declination).


In a project such as this that has spanned over more than a decade and two continents, many people have been involved. Here we list people and their main contributions to the project.

Norbert Zacharias
Principle Investigator, observer, responsible for UCAC position reductions (from pixels to positions on the sky), observing schedule, and external position comparisons, re-processing of pixel data and development of reduction pipeline, Star-Scan plate measure reductions.
Ted Rafferty
former project lead, instrumentation specialist, observer, main author of the astrograph history document.
Charlie Finch
re-reduction of CCD x,y to RA,Dec, including systematic error control, high proper motion stars, match to 2MASS and photometric calibration.
Marion Zacharias
quality control, observer, early position reductions, extragalactic link program.
Terry Girard
SPM data reductions, from x,y plate data to RA,Dec, incl. systematic error handling (magnitude equations) and all related reduction pipeline development.
Bill van Altena
for many years of collaboration in astrometry.
Burton Jones, Bob Hanson, and Arnold Klemola
NPM data.
Arne Henden
for sharing unpublished APASS data and photometric consulting.
Dave Monet
measurements of SPM and NPM plates on PPM at NOFS.
Nigel Hambly
for collaboration on high proper motion stars and big contributions to UCAC3 proper motions.
Gary Wycoff
proper motions data preparation, star identifications (up to UCAC3).
Sean Urban
proper motions of UCAC2 release, preparation of other position catalogs including systematic error removal and compiling a merged catalog of Hipparcos and Tycho data.
Valeri Makarov
reduction of AGK2 and other StarScan plate measures from global x,y to individual RA,Dec positions.
Jennifer Bartlet
assisting in taking out original PMM x,y measures mapping parameters to arrive at raw x,y data of NPM plates
Brian Mason, Bill Hartkopf
new double star speckle observations with USNO 26in and data reductions.
Bill Hartkopf and Marion Zacharias
preparing the history section of the UCAC4 DVD release.
David Hall
verification of tape archive, early data handling.
Marvin Germain
telescope control system software development, software-to-hardware interface including electronics.
Ellis Holdenried
telescope control system software maintenance and extension of user interface for telescope operation.
Lars Winter
basis for astrometric CCD reduction software.
Greg Hennessy
computer administration support (USNO-DC)
Danilo Castillo
observer, Cerro Tololo, Chile
Mauricio Martinez
observer, Cerro Tololo, Chile
Sergio Pizarro
observer, Cerro Tololo, Chile
Oscar Saa
telescope manager, Cerro Tololo, Chile
Trudy Tilleman
observer, U.S. Naval Observatory Flagstaff Station, assist with data reductions.
Stephanie Potter
observer, U.S. Naval Observatory Flagstaff Station.
Dominic Marcello
observer, U.S. Naval Observatory Flagstaff Station.
Gary Wieder
telescope upgrades, relocation, maintenance, and troubleshooting.
John Pohlman
telescope upgrades, maintenance, and troubleshooting.
John Bowles
telescope upgrades and maintenance (USNO-DC).
Dave Smith
telescope upgrades and maintenance (USNO-DC).
Tie Siemers
telescope upgrades and maintenance (USNO-DC).
Mike Divittorio
telescope maintenance and troubleshooting (NOFS).
Steve Sell
telescope maintenance and troubleshooting (NOFS).
Albert Rhodes
telescope maintenance and troubleshooting (NOFS).
Blaize Canzian
network and computer system support (NOFS).
Sumit Dutta
summer student working on separating out minor planet observations and streak artifacts.
Aleida Young
summer student working on identification of high proper motion stars.
Danley Hsu
summer student working on UCAC3 release, and providing the UCAC3 access code, assist with checking of UCAC4.


This list includes items cited in the above text and is supplemented by listing all publications related to UCAC production and presentations. Many of the papers listed here can be found in the "papers" subdirectory on the distribution DVD with the aim to document the history of the project.

Boyd,M.R., Winters,J.G., Henry,T.J., et al. 2011,
"The solar neighborhood XXV. Discovery of new proper motion stars with 0.40 "/yr > mu > 0.18 "/yr between declinations -47 and 00", AJ 142, 10
Deacon,N. R., Hambly,N. C., Cooke,J. A. 2005
"Southern infrared proper motion survey. I. Discovery of new high proper motion stars from first full hemisphere scan" A&A, 435, 363
ESA 1997,
"The Hipparcos and Tycho Catalogues", European Space Agency, publication SP-1200
Finch,Charlie T., Henry,Todd J., Subasavage,John P., Jao,Wei-Chun,
Hambly,Nigel C., 2007, "The Solar Neighborhood. XVIII. Discovery of New Proper-Motion Stars with 0.40" yr-1 > µ >= 0.18" yr-1 between Declinations -90° and -47°", AJ, 133, 2898
Finch,C., Zacharias,N., Girard,T., Wycoff,G.L., Zacharias,M.I. 2009,
"UCAC3 is coming!" AAS
Finch,C., Zacharias,N., Wycoff,G., 2010,
"UCAC3 atrometric reductions", AJ 139, 2200
Finch,C., Zacharias,N., Henry,T., 2010
"UCAC3 Proper Motion Survey. I. Discovery of New Proper Motion Stars in UCAC3 with 0."40 yr-1 > µ = 0.0018 yr^-1 between Declinations -90 degrees and -47 degrees", AJ 140, 844
Finch, C.T., Zacharias, N., Boyd, M.R., Henry, T.J., Hambly, N.C., 2012
"UCAC3 Proper Motion Survey. II. Discovery Of New Proper Motion Stars In UCAC3 With 0.40" yr^-1 > mu >= 0.18" yr^-1 Between Declinations -47 deg and 00 deg", in press ApJ, 745, 118
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
Girard,T. M., Platais,I., Kozhurina-Platais,V., van Altena, W. F.,
Lopez, C. E. 1998, "The Southern Proper Motion Program. I. Magnitude-Equation Correction", AJ 115, 855.
Girard,T.M., van Altena,W.F., Zacharias,N., Viera,K., Casetti-Dinescu,D.I.,
Castillo,D., Herrera,D., Lee,Y.S., Beers,T.C., Monet,D.G., Lopez,C.E. 2011, "The Southern Proper Motion Program IV. The SPM4 Catalog", AJ 142, 15
Hartkopf,W.I., Mason,B.D., Wycoff,G.L., Finch,C.T., Zacharias,N., 2010,
"Double Stars in the UCAC3 Catalog", BAAS 41,280, AAS Meeting #215,419.24
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", A&A 355L, 27 (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", A&A 357, 367 (long paper)
Hambly,N. C., MacGillivray,H. T., Read,M. A., Tritton,S. B.,
Thomson,E. B., Kelly,B. D., Morgan,D. H., Smith,R. E., Driver,S. P., Williamson,J., Parker,Q. A., Hawkins,M. R. S., Williams,P. M., Lawrence,A. 2001, "The SuperCOSMOS Sky Survey - I. Introduction and description", MNRAS, 326, 1279
Hambly,N. C., Irwin,M. J., MacGillivray,H. T. 2001,
"The SuperCOSMOS Sky Survey - II. Image detection, parametrization, classification and photometry", MNRAS, 326, 1295
Hambly,N. C., Davenhall,A. C., Irwin,M. J., MacGillivray, H. T. 2001,
"The SuperCOSMOS Sky Survey - III. Astrometry", MNRAS. 326, 1315
Henry,Todd J., Subasavage,John P., Brown,Misty A., Beaulieu,Thomas D.,
Jao,Wei-Chun, Hambly,Nigel C. 2004, "The Solar Neighborhood. X. New Nearby Stars in the Southern Sky and Accurate Photometric Distance Estimates for Red Dwarfs", AJ. 128, 2460
Jones, B. F., Hanson, R. B., and Klemola, A. R., 2000,
"Lick Northern Proper Motion Program: NPM2", AAS meeting 196 abstract 53.02
Kovalevsky,J., Lindegren,L., Perryman,M.A.C. et al. 1997,
"The Hipparcos Catalogue as a realisation of the extragalactic
reference system", A&A 323, 620
Lepine,S., Shara,M.M. 2005,
"A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)", AJ. 129, 1483
Lepine,S., 2008
"New High Proper Motion Stars from the Digitized Sky Survey. Iv. Completion of the Southern Survey and 170 Additional Stars with mu > 0.45'' yr", AJ, 135, 2177
Paturel,G., Vauglin,I., Petit,C., Borsenberger,J., Epchtein,N.,
Fouque,P., Mamon,G. 2005, "A catalog of LEDA galaxies with DENIS measurements" A&A, 430, 751
Platais,I., Girard,T. M., Kozhurina-Platais,V., van Altena,W. F.,
Lopez,C. E., Mendez,R. A., Wen-Zhang Ma, Ting-Gao Yang, MacGillivray,H. T., Yentis,D. J. 1998, "The Southern Proper Motion Program. II. A Catalog at the South Galactic Pole", AJ, 116, 2556
Rafferty,T., Zacharias,N. 1999,
"USNO CCD Astrograph Catalog - South", Acta Historica Astronomiae, 6, 175-178
Roeser,S., Demleitner,M., Schilbach,E. 2010,
"The PPMXL catalog of positions and proper motions on the ICRS. Combining USNO-B1.0 and the two micron all sky survey(2MASS)", AJ 139, 2440
Ruiz,Maria Teresa, Wischnjewsky,Marina, Rojo,Patricio M.,
Gonzalez,Luis E. 2001, "Calan-ESO Proper-Motion Catalog", ApJ 133, 199
Salim S., Gould A. 2003
"Improved astrometry and photometry for the Luyten catalog. II. Faint stars and the revised catalog.", AJ, 582, 1011
Scholz,R.-D., Irwin,M., Ibata,R., Jahreiß,H., Malkov,O. Yu. 2000,
"New high-proper motion survey in the Southern sky", A&A 353, 958
Skrutskie,M. F., Cutri,R. M., Stiening,R., Weinberg,M. D., Schneider,S.,
Carpenter,J. M., Beichman,C., Capps,R., Chester,T., Elias,J., Huchra,J., Liebert,J., Lonsdale,C., Monet,D. G., Price,S., Seitzer,P., Jarrett,T., J.D. Kirkpatrick, J. Gizis, E. Howard, T. Evans, J. Fowler, L. Fullmer, Hurt,R., Light,R., Kopan,E. L., Marsh,K. A., McCallon,H. L., Tam,R., Van Dyk,S., Wheelock,S. 2006, "The Two Micron All Sky Survey (2MASS)", AJ, 131, 1163
Subasavage,John P., Henry,Todd J., Hambly,Nigel C., Brown,Misty A.,
Jao,Wei-Chun 2005, "The Solar Neighborhood. XII. Discovery of New High Proper Motion Stars with µ >= 0.4" yr-1 between Declinations -90° and -47°", AJ, 129, 413
Subasavage,John P., Henry,Todd J., Hambly,Nigel C., Brown,Misty, A.,
Jao,Wei-Chun, Finch,Charlie T. 2005 "The Solar Neighborhood. XV. Discovery of New High Proper Motion Stars with µ >= 0.4" yr-1 between Declinations -47° and 00°", AJ, 130, 1658
van Leeuwen, F. 2007,
"Hipparcos, the new reduction of the raw data",
Springer Science Library, Vol. 350

Veron-Cetty & Veron 2006, A&A 455, 773

Wroblewski, H. and Torres, C. 1989
"New proper-motion stars south of declination -40 deg and right ascension between 00 H and 04 H 30 M", A&A 78, 231
Wroblewski, H. and Torres, C. 1991
"New proper-motion stars south of declination -40 deg and right ascension between 04h 30m and 16h 00m", A&A 91, 129
Wroblewski, H. and Torres, C. 1994
"New proper-motion stars south of declination -40deg and right ascension between 16h and 24h", A&A 105, 179
Wroblewski, H. and Torres, C. 1996
"New proper-motion stars with declination between -5deg and -30deg and right ascension between 0h and 9h.", A&A 115, 481
Wroblewski, H. and Torres, C. 1997
"New proper-motion stars with declination between -5d and -30d and right ascension between 9h and 13h 30m`", A&A 122, 447
Wroblewski, H. and Costa, E. 1999
"New high proper motion stars with declinations between -5(deg) and -30(deg) , and right ascensions between 13h 30m and 24h", A&A 139, 25
Wroblewski, H. and Costa, E. 2001
"High proper motion stars with declinations between -30o and -40o, and right ascensions between 00 h and 10 h 40 m", A&A 367, 725
Urban,S. E., Corbin,T. E., Wycoff,G. L., Martin,J. C., Jackson,E. S.,
Zacharias,M. I., Hall,D. M. 1998, "The AC 2000: The Astrographic Catalogue on the System Defined by the HIPPARCOS Catalogue" AJ, 115, 1212
Urban, S.E., Wycoff, G.L., Makarov, V.V., 2000,
"Comparisons of Tycho-2 Catalogue Proper Motions with Hipparcos and ACT", AJ 120, 501
Urban,S.E., Corbin, T.E., Wycoff, G.L., Makarov, V.V., Hoeg, E.,
Fabricius, C., 2000, "The AC 2000.2 Catalogue", AAS meeting 199, abstract #129.04
Urban,S.E., Zacharias,N., Wycoff,G.L, 2004,
"The UCAC2 Bright Star Supplement", Vizier on-line data catalog I/294A, 2004yCat.1294....0U
van Altena, W. F., Girard, T. M., Platais, I., Kozhurina-Platais, V.,
Ostheimer, J., Lopez, C. E., and Mendez, R. A., 1999, "The Yale/San Juan Southern Proper Motion Program", AAS DDA meeting #31, #10.04
Wielen,R., Schwan,H., Dettbarn,C., Lenhardt,H. Jahreiss,H., Jahrling,R.,
1999, "Sixth Catalogue of Fundamental stars (FK6)", Veroeff. Astron. Rechen-Inst. Heidelberg 35, 1
Winter, L. 1999
Ph.D thesis, University of Hamburg (in German), CCD image reductions
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
"Extension of the optical reference frame: ground based",
IAU GA 1997, JD7, invited review talk in Highlights of Astronomy, Vol.11A, p.300-303, ed. J.Andersen, Kluver Acad.Publ. 1998
Zacharias,N. Corbin,T.E. Zacharias,M.I Rafferty,T.J.
Seidelmann,P.K. Gauss,F.S., 1998 (BAAS 30 No.4, p.1368) "High Precision Astrometry for the Hubble Deep Field - South", poster preseted at the AAS meeting #193, Austin, TX, Jan. 1999
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 Francisco
Zacharias,N. Urban,S.E. 1999,
"The first year of the UCAC-S project", AAS, DDA meeting #31, paper #10.05
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", AJ 120, 2131
Zacharias, N., Zacharias, M.I., Urban, S.E. and Hoeg, E., 2000,
"Comparing Tycho-2 astrometry with UCAC1", AJ 120, 1148
Zacharias,N., Rafferty,T.J., Urban,S.E., Zacharias,M.I., Wycoff,G.L. 2000,
"The UCAC as input catalog for FAME", proceed. IAU Coll. 180, p.80, Washington, DC, Eds. K.J.Johnston, D.D.McCarthy, B.J.Luzum, G.H.Kaplan
Zacharias,N., Zacharias,M.I., Rafferty,T.J. 2001,
"Status of the UCAC project", abstract, DDA meeting #32, Houston, TX
Zacharias, N., 2002,
"Astrometric surveys in support of large telescopes", Proceed. SPIE 4836, 279, Eds. T.A. Tyson & S. Wolff
Zacharias,N., Zacharias,M.I., Urban,S.E., Rafferty,T.J. 2002,
"UCAC2: a new high precision catalog of positions and proper motions", abstract, AAS meeting 199, Washingtion DC, Jan. 2002
Zacharias,N., Zacharias,M.I. 2002,
"Systematic error corrections for UCAC2 positions", presentation at the DDA meeting, Mt.Hood
Zacharias, N. 2003,
"The USNO CCD Astrograph Catalog (UCAC) Project and Beyond", in "Small Telescopes in the New Millennium II. The Telescopes We Use.", ed. T Oswald, Kluwer Acad. Publ
Zacharias, N., Urban, S., Rafferty, T., Holdenried, E., and Winter, L.,
2003, "First Results from AGK2 Plate Remeasurements", BAAS 35 #4, p.1036, abstract #6.01, 34th DDA meeting Ithaca, NY
Zacharias,N. 2004,
"Astrometric reference stars: from UCAC to URAT", proceed. 3rd Potsdam Thinkshop on robotic telescopes, AN 325, 631
Zacharias,N., Urban,S.E., Zacharias,M.I., Wycoff,G.L., Hall,D.M.,
Monet,D.G., Rafferty,T.J. 2004, "The Second US Naval Observatory CCD Astrograph Catalog (UCAC2)" AJ 127, 3043 (May)
Zacharias,N., Zacharias,M.I. 2007,
"The USNO CCD Astrograph Catalog (UCAC)", poster for USNO display (unpublished)
Zacharias,N., Finch,C., Wycoff,G., Hartkopf,W.
"Improving Hipparcos Proper Motions with UCAC", DDA meeting 2009, Virginia Beach, 2009, BAAS 41 No.2, p. 910, abstract #16.03
Zacharias, M.I., Zacharias, N. 2009,
"Significant radio-optical reference frame offsets from CTIO data", poster paper at the IAU XXVII Gen.Ass., Rio de Janeiro, Brazil
Zacharias,N., Finch,C., Girard,T., Hambly,N., Wycoff,G.
Zacharias,M.I., Castillo,D., Corbin,T. et al. 2009yCat.1315...0Z, data release of UCAC3, CDS catalog I/315, Aug 2009
Zacharias,N. 2010,
"UCAC3 pixel processing", AJ 139, 2208-2217
Zacharias,N., Finch,C., Girard,T., Hambly,N., Wycoff,G., Zacharias,M.I.,
Castillo,D., Corbin,T., DiVittorio,M., Dutta,S., Gaume,R., Gauss,S., Germain,M., Hall,D., Hartkopf,W., Hsu,D., Holdenried,E., Makarov,V., Martines,M., Mason,B., Monet,D., Rafferty,T., Rhodes,A., Siemers,T., Smith,D., Tilleman,T., Urban,S., Wieder,G., Winter,L, Young,A., 2010 "The Third US Naval Observatory CCD Astrograph Catalog (UCAC3)", AJ 139, 2184-2199
Zacharias,N. Gaume,R. 2010,
"UCAC and URAT: optical astrometric catalog observing programs", proceed. Journees meeting Paris 2010 (in press) Ed. N.Capitaine,
Zacharias,N., Finch,C., Zacharias,M.I., Girard,T. 2011,
"UCAC4 status", poster paper presented at the AAS 218, Boston,MA


You can access this service using:

This service is published as follows:

local means it is listed on our front page, ivo_managed means it has a record in the VO registry.

Spatial Coverage

Input Fields

The following fields are available to provide input to the service (with some renderers, some of these fields may be unavailable):

NameTable Head DescriptionUnitUCD
DEC Delta (ICRS) Declination (ICRS decimal) deg pos.eq.dec
hscs_pos Position/Name Coordinates (as h m s, d m s or decimal degrees), or SIMBAD-resolvable object N/A N/A
hscs_sr Search radius Search radius in arcminutes N/A N/A
maga m UCAC aperture magnitude mag phot.mag;em.opt.V
maxrec Match limit Maximum number of records returned. Pass 0 to retrieve service parameters. N/A N/A
objt Obj. Object type N/A src.class
RA Alpha (ICRS) Right Ascension (ICRS decimal) deg pos.eq.ra
responseformat Output Format File format requested for output. N/A meta.code.mime
SR Search Radius Search radius deg N/A
ucacid ID UCAC 2 identification number (UCAC4-zzz-nnnnnn) N/A;meta.main
verb Verbosity Exhaustiveness of column selection. VERB=1 only returns the most important columns, VERB=2 selects the columns deemed useful to the average user, VERB=3 returns a table with all available columns. N/A N/A

Default Output Fields

The following fields are contained in the output by default. More fields may be available for selection; these would be given below in the VOTable output fields.

NameTable Head DescriptionUnitUCD
_r Dist. Distance to cone center deg pos.distance
cdf Double? Combined double star flag Note 5 N/A meta.code.multip
cu1 #Cats Number of catalogs (epochs) used for proper motions N/A meta.number
dej2000 RA Declination at epoch J2000.0 (ICRS) Note 1 deg pos.eq.dec;meta.main
maga m UCAC aperture magnitude Note 2 mag phot.mag;em.opt.V
magB Mag. B B magnitude from APASS Note 13 mag phot.mag;em.opt.B
magg Mag. g g magnitude from APASS Note 13 mag phot.mag;em.opt.V
magH Mag. H 2MASS H magnitude mag phot.mag;em.IR.H
magi Mag. i i magnitude from APASS Note 13 mag phot.mag;em.opt.I
magJ Mag. J 2MASS J magnitude mag phot.mag;em.IR.J
magK_s Mag. K_s 2MASS K_s magnitude mag phot.mag;em.IR.K
magr Mag. r r magnitude from APASS Note 13 mag phot.mag;em.opt.R
magV Mag. V V magnitude from APASS Note 13 mag phot.mag;em.opt.V
objt Obj. Object type Note 4 N/A src.class
pmde PM(Dec) Proper motion in Dec deg/yr;pos.eq.dec
pmra PM(RA) Proper motion in RA*cos(Dec) deg/yr;pos.eq.ra
raj2000 RA Right Ascension at epoch J2000.0 (ICRS) Note 1 deg pos.eq.ra;meta.main
sigdec Err. RA Standard error at central epoch in Dec Note 6 deg stat.error;pos.eq.dec
sigmag Err. m error of UCAC magnitude Note 3 mag stat.error;phot.mag;em.opt.V
sigpmd Err. PM(Dec) Standard error of proper motion in Dec deg/yr stat.error;;pos.eq.dec
sigpmr Err. PM(RA) Standard error of proper motion in RA*cos(Dec) deg/yr stat.error;;pos.eq.ra
sigra Err. RA Standard error at central epoch in RA (*cos Dec) Note 6 deg stat.error;pos.eq.ra
ucacid ID UCAC 2 identification number (UCAC4-zzz-nnnnnn) N/A;meta.main

VOTable Output Fields

The following fields are available in VOTable output. The verbosity level is a number intended to represent the relative importance of the field on a scale of 1 to 30. The services take a VERB argument. A field is included in the output if their verbosity level is less or equal VERB*10.

NameTable Head DescriptionUnitUCD Verb. Level
ucacid ID UCAC 2 identification number (UCAC4-zzz-nnnnnn) N/A;meta.main 1
raj2000 RA Right Ascension at epoch J2000.0 (ICRS) Note 1 deg pos.eq.ra;meta.main 1
dej2000 RA Declination at epoch J2000.0 (ICRS) Note 1 deg pos.eq.dec;meta.main 1
maga m UCAC aperture magnitude Note 2 mag phot.mag;em.opt.V 5
pmra PM(RA) Proper motion in RA*cos(Dec) deg/yr;pos.eq.ra 5
pmde PM(Dec) Proper motion in Dec deg/yr;pos.eq.dec 5
_r Dist. Distance to cone center deg pos.distance 10
sigmag Err. m error of UCAC magnitude Note 3 mag stat.error;phot.mag;em.opt.V 15
objt Obj. Object type Note 4 N/A src.class 15
cdf Double? Combined double star flag Note 5 N/A meta.code.multip 15
sigra Err. RA Standard error at central epoch in RA (*cos Dec) Note 6 deg stat.error;pos.eq.ra 15
sigdec Err. RA Standard error at central epoch in Dec Note 6 deg stat.error;pos.eq.dec 15
cu1 #Cats Number of catalogs (epochs) used for proper motions N/A meta.number 15
sigpmr Err. PM(RA) Standard error of proper motion in RA*cos(Dec) deg/yr stat.error;;pos.eq.ra 15
sigpmd Err. PM(Dec) Standard error of proper motion in Dec deg/yr stat.error;;pos.eq.dec 15
magJ Mag. J 2MASS J magnitude mag phot.mag;em.IR.J 15
magH Mag. H 2MASS H magnitude mag phot.mag;em.IR.H 15
magK_s Mag. K_s 2MASS K_s magnitude mag phot.mag;em.IR.K 15
magB Mag. B B magnitude from APASS Note 13 mag phot.mag;em.opt.B 15
magV Mag. V V magnitude from APASS Note 13 mag phot.mag;em.opt.V 15
magg Mag. g g magnitude from APASS Note 13 mag phot.mag;em.opt.V 15
magr Mag. r r magnitude from APASS Note 13 mag phot.mag;em.opt.R 15
magi Mag. i i magnitude from APASS Note 13 mag phot.mag;em.opt.I 15
magm m(M) UCAC fit model magnitude Note 2 mag phot.mag;em.opt.V 21
na1 #Img tot. Total number of CCD images of this star Note 7 N/A meta.number 25
nu1 #Img used Number of CCD images used for this star Note 7 N/A meta.number 25
cepra Ep(RA) Central epoch for mean RA yr time.epoch 25
cepdec Ep(Dec) Central epoch for mean Dec yr time.epoch 25
pts_key 2MASS 2MASS unique star identifier N/A 25
sigmagJ Err. Mag. J Error in 2MASS J magnitude Note 12 mag stat.error;phot.mag;em.IR.J 25
icqflgJ Flag J Quality flag for 2MASS J magnitude Note 11 mag meta.code;phot.mag;em.IR.J 25
sigmagH Err. Mag. H Error in 2MASS H magnitude Note 12 mag stat.error;phot.mag;em.IR.H 25
icqflgH Flag H Quality flag for 2MASS H magnitude Note 11 mag meta.code;phot.mag;em.IR.H 25
sigmagK_s Err. Mag. K_s Error in 2MASS K_s magnitude Note 12 mag stat.error;phot.mag;em.IR.K 25
icqflgK_s Flag K_s Quality flag for 2MASS K_s magnitude Note 11 mag meta.code;phot.mag;em.IR.K 25
sigmagB Err. Mag. B Error in B magnitude from APASS Note 14 mag stat.error;phot.mag;em.opt.B 25
sigmagV Err. Mag. V Error in V magnitude from APASS Note 14 mag stat.error;phot.mag;em.opt.V 25
sigmagg Err. Mag. g Error in g magnitude from APASS Note 14 mag stat.error;phot.mag;em.opt.V 25
sigmagr Err. Mag. r Error in r magnitude from APASS Note 14 mag stat.error;phot.mag;em.opt.R 25
sigmagi Err. Mag. i Error in i magnitude from APASS Note 14 mag stat.error;phot.mag;em.opt.I 25
spmflags SPM flags Yale SPM g and c flags Note 15 N/A meta.code;src 25
srcflags Src flags Source flags (see note) Note 16 N/A meta.code 25
diam2mass 2MASS diam Object size from 2MASS Note 19 deg phys.angSize;src 25
rnm ID Unique star identification number Note 20 N/A 25
zn2 UCAC2 Zone Zone number of UCAC2 Note 21 N/A 25
rn2 UCAC2 Number Running record number along UCAC2 zone Note 21 N/A 25

The UCAC4 release paper (in preparation) should be cited whenever UCAC data are utilized.

VOResource XML (that's something exclusively for VO nerds)

Note 1

Positions are on the International Celestial Reference System (ICRS) as represented by the Hipparcos / Tycho-2 catalogs. The epoch for the positions of all stars is J2000.0; the weighted mean catalog position was updated using the provided proper motions. The observational UCAC position is but one of several going into these values and is not given in the UCAC4; thus the original UCAC observation cannot be recovered from these data.

Note 11

For each 2MASS bandpass a combined flag was created (cc_flg*10 + ph_qual) consisting of the contamination flag (0 to 5) and the photometric quality flag (0 to 8).

0 cc_flg 2MASS 0, no artifacts or contamination
1 cc_flg 2MASS p, source may be contaminated by a latent image
2 cc_flg 2MASS c, photometric confusion
3 cc_flg 2MASS d, diffraction spike confusion
4 cc_flg 2MASS s, electronic stripe
5 cc_flg 2MASS b, bandmerge confusion
0 no ph_qual flag
1 ph_qual 2MASS X, no valid brightness estimate
2 ph_qual 2MASS U, upper limit on magnitude
3 ph_qual 2MASS F, no reliable estimate of the photometric error
4 ph_qual 2MASS E, goodness-of-fit quality of profile-fit poor
5 ph_qual 2MASS A, valid measurement, [jhk]snr>10 AND [jhk]cmsig<0.10857
6 ph_qual 2MASS B, valid measurement, [jhk]snr> 7 AND [jhk]cmsig<0.15510
7 ph_qual 2MASS C, valid measurement, [jhk]snr> 5 AND [jhk]cmsig<0.21714
8 ph_qual 2MASS D, valid measurement, no [jhk]snr OR [jhk]cmsig req.

For example icqflg = 05 is decoded to be cc_flg=0, and ph_qual=5, meaning no artifacts or contamination from cc_flg and 2MASS qual flag = "A" .

Note 12

The photometric errors from 2MASS were rounded by 1 digit here. These data were taken from the j_msigcom, h_msigcom, and k_msigcom columns of the 2MASS point source catalog.

Note 13

These data are from the AAVSO Photometric all-sky survey (APASS) DR6 plus single observation stars kindly provided by A.Henden. For bright stars the magB and magV columns contain the Hipparcos/Tycho Bt and Vt mags respectively, whenever there is no APASS B or V available and valid Bt or Vt mags were found. For the bright supplement stars the same was done. All thses cases are identified by the respective errors being NULL. For over 10,000 stars no Vt mag was available and the V mag from Tycho was used instead.

The following number of stars have entries in the APASS photometry:

51862350 some valid data in either of the columns
51070044 B mag
51861015 V mag
45799843 g mag
45615993 r mag
41491953 i mag
Note 14

The original data release distinguishes between error estimates from the APASS data release (for objects with at least two observations) and formal, S/N estimated errors. This distinction is not carried over here.

Note 15

The g-flag from the Yale San Juan first epoch Southern Proper Motion data (YSJ1, SPM) has the following meaning:

0 no info
1 matched with 2MASS extended source list
2 LEDA galaxy
3 known QSO

The c-flag from the Yale San Juan first epoch Southern Proper Motion data (YSJ1, SPM) indicates which input catalog has been used to identify stars for pipeline processing:

1 Hipparcos
2 Tycho2
4 2MASS psc
5 2MASS xsc (extended sources, largely (but not all!) galaxies)
6 LEDA (confirmed galaxies, Paturel et al. 2005)
7 QSO (Veron-Cetty & Veron 2006)
Note 16

Catalog match flags, compiled into a single number, formed as:

icf = hiptyc*10^8
  + AC2000*10^7
  + AGK2 Bonn*10^6
  + AGK2 Hamburg*10^5
  + Zone Astrog.*10^4
  + Black Birch*10^3
  + Lick Astrog.*10^2
  + NPM Lick*10^1
  + SPM YSJ1

The meaning of hiptyc is:

0 not a Hip. or Tycho star
1 Hipparcos 1997 version main catalog (not in UCAC4 data files)
2 Hipparcos double star annex
3 Tycho-2
4 Tycho annex 1
5 Tycho annex 2
6 FK6 position and proper motion (instead of Hipparcos data)
7 Hippparcos 2007 solution position and proper motion
8 FK6 only PM substit. (not in UCAC4 data)
9 Hipparcos 2007, only proper motion substituted

The remaining flags are provided for major catalogs used in the computation of the proper motions. Each match is analyzed for multiple matches of entries of the 1st catalog to 2nd catalog entries, and the other way around. Matches are also classified by separation and difference in magnitude to arrive at a confidence level group. In each case the codes mean:

0 star not matched with this catalog
1 unique-unique match, not involving a double star
2 ... same, but involving a flagged double star
3 multiple match but unique in high confidence level group, no double
4 ... same, but involving a flagged double star
5 closest match, not involving a double, likely o.k.
6 ... same, but involving a flagged double star
7 maybe o.k. smallest sep. match in both directions, no double
8 ... same, but involving a flagged double star
Note 19

This flag is either NULL (no match) or contains the length of the semi-major axis of the fiducial ellipse at the K-band (object size) information copied from the 2MASS extended source catalog.

Note 2

Systematic errors are expected to be below 0.1 mag for magm, maga photometric results obtained from the UCAC CCD pixel data. The aperture photometry is considered more robust, particularly for "odd" cases, while the model fit magnitude is expected to be more accurate for "well behaved" stars.

Note 20

This unique star identification number is between 200001 and 321640 for Hipparcos stars, and between 1 and 9430 for non- Hipparcos stars supplemented to the UCAC4 catalog (no CCD observ.). For all other stars this unique star identification number is the internal mean-position-file (MPOS) number + 1 million. For both the Hipparcos and the supplement stars there is an entry on the u4supl.dat file providing more information, including the original Hipparcos star number. Note, there are several thousand cases where different UCAC4 stars link to the same Hipparcos star number due to resolved binary stars with each component being a separate star entry in UCAC4.

Note 21

This info provides a cross reference to UCAC2. Both zn2 and rn2 are NULL if the UCAC4 star could not be found in UCAC2.

Note 3

For many stars a photometric error based on the scatter from individual observations of that star on different CCD frames could be obtained. A model error was also attempted to be assigned, based on the S/N ratio. The error quoted here is the larger of the 2. If that error exceeds 0.9 mag the error was set to 0.9 mag

Note 4

The object type flag is used to identify possible problems with a star or the source of data. Of the individual image flags the one with the largest value (worst problem case) is propagated into this object type flag, unless it is superseded by an overriding flag at the combined image stage.

The object type flag has the following meaning:

good, clean star (from MPOS), no known problem
largest flag of any image = near overexposed star (from MPOS)
largest flag of any image = possible streak object (from MPOS)
high proper motion (HPM) star, match with external PM file (MPOS)
actually use external HPM data instead of UCAC4 (accuracy of positions varies between catalogs)
poor proper motion solution, report only CCD epoch position
substitute poor astrometric results by FK6/Hip/Tycho-2 data
added supplement star (no CCD data) from FK6/Hip/Tycho-2 data, and 2 stars added from high proper motion surveys
high proper motion solution in UCAC4, star not matched with PPMXL
high proper motion solution in UCAC4, discrepant PM to PPMXL (see discussion of flags 8,9 in redcution section 2e above)

The number of stars with those flags is:

0 103080317
1 2785787
2 15519
3 97042
4 35073
5 5004704
6 34473
7 8925
8 2028600
9 689653
Note 5

The cdf flag is a combined double star flag used to indicate the type/quality of double star fit. It is a combination of 2 flags,

cdf = 10 * dsf + dst

with the following meaning:

dsf = double star flag, overall classification

0 single star
1 component #1 of "good" double star
2 component #2 of "good" double star
3 blended image

dst = double star type, from pixel data image profile fits, largest value of all images used for this star

0 no double star, not sufficient #pixels or elongation to even call double star fit subroutine
1 elongated image but no more than 1 peak detected
2 2 separate peaks detected -> try double star fit
3 secondary peak found on each side of primary
4 case 1 after successful double fit (small separ. blended image)
5 case 2 after successful double fit (most likely real double)
6 case 3 after successful double fit (brighter secondary picked)

A word of caution: often a dsf= 1 or 2 image is paired with a dsf= 3. If for a star any of the several images reveals a "blended image", that higher dsf=3 flag is carried into the output file. This can happen for a regular double star with unique components 1 and 2. A flag dsf=3 means this could be component 1 or 2 but at least on one CCD frame a blended image was detected. This blend could be with the other component, or a spurious image or artifact. The double star flags need to be interpreted with caution; anything but a zero means "likely some double star component or blended image".

Note 6

If the astrometric data for a star was substituted from an external catalog like Hipparcos, Tycho or high proper motion data, a mean error in position and proper motion depending on the catalog and magnitude of the star was adopted.

Note 7

A zero for the number of used images indicates that all images have some "problem" (such as overexposure). In that case an unweighted mean over all available images (na) is taken to derive the mean position, while normally a weighted mean was calculated based on the "good" images, excluding possible problem images (u <= na).