UCAC4

The Fourth U.S. Naval Observatory CCD Astrograph Catalog Definitions

Overview

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The unpublished plate measure data obtained by StarScan from the AGK2, the Hamburg Zone Astrograph, the USNO Black Birch Astrograph, and the Lick Astrograph have contributed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

The UCAC4 documentation can be found here. Note: in order to place uncertainties immediately following the corresponding quantities, some columns have been reordered relative to what is described in the documentation.

Name

Intype

Units

Description

double

deg

Right Ascension (J2000)

dec

double

deg

Declination (J2000)

rac

int

mas

right ascension at epoch J2000.0 (ICRS)

spd

int

mas

south pole distance epoch J2000.0 (ICRS)
To convert to Declination, subtract 324000000 mas.

momag

double

mag

UCAC fit model magnitude

apmag

double

mag

UCAC aperture magnitude

sigmag

double

mag

error of UCAC magnitude

ojt

int

object type

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

dsf

character

combined double star flag

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

x0 = no no double star, not sufficient #pixels or elongation to even call double star fit subroutine
x1 = elongated image but no more than 1 peak detected
x2 = 2 separate peaks detected, try double star fit
x3 = secondary peak found on each side of primary
x4 = case 1 after successful double fit (small separ. blended image)
x5 = case 2 after successful double fit (most likely real double)
x6 = case 3 after successful double fit (brighter secondary picked)

sigra

int

mas

RA uncertainty at central epoch

sigdc

int

mas

Dec uncertainty at central epoch

int

total number of CCD images of this star

int

number of CCD images used for this star

int

number of catalogs used for proper motions

cepra

int

0.01 yr

central epoch for mean RA, minus 1900

cepdc

int

0.01 yr

central epoch for mean Dec,minus 1900

pmrac

int

0.1 mas/yr

proper motion in RA*cos(Dec)

sigpmr

int

0.1 mas/yr

uncertainty in pmrac

pmdc

int

0.1 mas/yr

proper motion in Dec

sigpmd

int

0.1 mas/yr

uncertainty in pmdc

tmassid

int

2MASS unique star identifier

jmag

double

mag

2MASS J magnitude

double

mag

error 2MASS J magnitude

hmag

double

mag

2MASS H magnitude

double

mag

error 2MASS H magnitude

kmag

double

mag

2MASS K_s magnitude

double

mag

error 2MASS K_s magnitude

character

2MASS cc_flg*10 + ph_qual flag for J

0x = no contamination
1x = possible latent
2x = photometric confusion
3x = diffraction spike
4x = electronic stripe
5x = bandmerge confusion

x0 = no ph_qual flag
x1 = no valid brightness est.
x2 = upper limit
x3 = unreliable error
x4 = poor profile fit
x5 = ph_qual A valid measurement
x6 = ph_qual B valid measurement
x7 = ph_qual C valid measurement
x8 = ph_qual D valid measurement

character

2MASS cc_flg*10 + ph_qual flag for H

0x = no contamination
1x = possible latent
2x = photometric confusion
3x = diffraction spike
4x = electronic stripe
5x = bandmerge confusion

character

2MASS cc_flg*10 + ph_qual flag for K_s

0x = no contamination
1x = possible latent
2x = photometric confusion
3x = diffraction spike
4x = electronic stripe
5x = bandmerge confusion

bmag

double

mag

B magnitude from AAVSO Photometric All-Sky Survey

ber

double

mag

error of B magnitude from AAVSO Photometric All-Sky Survey
Positive errors are from the official release data error estimates (at least 2 observations per star). Formal, S/N estimated errors for single observations are multiplied by -1 for this column.

vmag

double

mag

V magnitude from AAVSO Photometric All-Sky Survey

ver

double

mag

error of V magnitude from AAVSO Photometric All-Sky Survey
Positive errors are from the official release data error estimates (at least 2 observations per star). Formal, S/N estimated errors for single observations are multiplied by -1 for this column.

gmag

double

mag

g magnitude from AAVSO Photometric All-Sky Survey

ger

double

mag

error of g magnitude from AAVSO Photometric All-Sky Survey
Positive errors are from the official release data error estimates (at least 2 observations per star). Formal, S/N estimated errors for single observations are multiplied by -1 for this column.

rmag

double

mag

r magnitude from AAVSO Photometric All-Sky Survey

rer

double

mag

error of r magnitude from AAVSO Photometric All-Sky Survey
Positive errors are from the official release data error estimates (at least 2 observations per star). Formal, S/N estimated errors for single observations are multiplied by -1 for this column.

imag

double

mag

i magnitude from AAVSO Photometric All-Sky Survey

ier

double

mag

error of i magnitude from AAVSO Photometric All-Sky Survey
Positive errors are from the official release data error estimates (at least 2 observations per star). Formal, S/N estimated errors for single observations are multiplied by -1 for this column.

character

Yale San Juan first epoch Southern Proper Motion data g-flag*10 c-flag

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

x1 = Hipparcos
x2 = Tycho2
x3 = UCAC2
x4 = 2MASS PSC
x5 = 2MASS XSC
x6 = LEDA (confirmed galaxies, Paturel et al. (2005))
x7 = QSO (Veron-Cetty & Veron (2006))

int

FK6-Hipparcos-Tycho source flag

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

int

AC2000 catalog match flag

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

int

AGK2 Bonn catalog match flag

int

AKG2 Hamburg catalog match flag

int

Zone Astrog. catalog match flag

int

Black Birch catalog match flag

int

Lick Astrog. catalog match flag

int

NPM Lick catalog match flag

int

SPM YSJ1 catalog match flag

int

LEDA galaxy match flag
This flag is either 0 (no match) or contains the log10 of the apparent total diameter for I-band (object size) information (unit = 0.1 arcmin) copied from the LEDA catalog (galaxies). A size value of less than 1 has been rounded up to 1.

int

2MASS extend.source flag
This flag is either 0 (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.

uniqueid

int

unique star identification number
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.

u2z

int

zone number of UCAC2 (0 = no match)

u2rsn

int

running record number along UCAC2 zone

ucac4_id

int

UCAC4 star ID number