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

Overview

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The unpublished plate measure data obtained by StarScan from the AGK2, the Hamburg Zone Astrograph, the USNO Black Birch Astrograph, and the Lick Astrograph have contributed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 nz@usno.navy.mil 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

UCAC1

initial results covering small sky area in the south (obsolete)

UCAC2

-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)

UCAC3

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)

UCAC4

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 brenda.hicks@navy.mil, 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 nz@usno.navy.mil (Norbert Zacharias) or finch@usno.navy.mil (Charlie Finch). The latest update on astrometry related projects at USNO can be found at: http://www.usno.navy.mil/usno/astrometry .

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:

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).

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.

Completeness

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:

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.

Magnitudes

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 http://www.aavso.org/apass and http://www.ipac.caltech.edu/2mass/releases/allsky/ .

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:

UCAC4-zzz-nnnnnn

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).