UCAC4 Catalogue
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C2A
can read the native version of the UCAC4 catalogue (Fourth
U.S. Naval Observatory CCD Astrograph Catalogue). 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.
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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 statistics about UCAC4 stars are as follows:
- 113 780 093 total number of stars in UCAC4 (incl. supplement stars)
- 109 921 682 with 2MASS identification
- 106 689 821 with proper motions
- 81 897 551 with 2 epoch PM
- 27 245 403 with 3 or more epoch PM
- 80 806 744 with 2 or more images from "good fit" CCD observations
- 48 323 349 matched with UCAC2
- 54 690 matched with LEDA galaxies
- 7 6020 matched with 2MASS extended source catalog
- 8 925 supplemented stars (no CCD obs.)
- 121 350 UCAC4 entries with a matched Hipparcos star ID
- 104 681 UCAC4 entries with CCD obs. substituted by FK6/Hip/Tycho-2
You select the UCAC4 catalogue as the
current catalogue through the Stars tab of the Options
dialogue box, or by SHIFT-right-clicking on the map.
C2A takes into account the proper motions given
in the UCAC4 catalogue in determining star positions at a date specified by
the user (see Advanced Options in the Stars tab of the options
dialogue box). Furthermore, you specify in the advanced options which magnitude
is to be used in the filters and displays. The available magnitudes are as
follows: Fit Model (V-R), Aperture (V-R), 2MASS J,
2MASS H, 2MASS K_s, APASS B, APASS V,
APASS g, APASS r and APASS i.
UCAC4 star flags are used to display stars:
-
If the star has the flag "object could
be from possible streak", it is displayed within a blue circle
-
If the star is over exposed on one or more
images, or if it is near an over exposed star, it is displayed enclosed in
a red circle.
-
If the star is not a simple star (flag "dsf"),
it is displayed in white
In all other cases, it is displayed in the stars’ default colour.
Note that, unlike the UCAC2, the UCAC4 catalogue
does not need to be complied for it to be read by C2A. The structure of the
orginal catalogue must be preserved:
- Files z001 to z900 must be in a sub-directory
called u4b.
- The files zone_stats
and u4index.unf must be present in a sub-directory called u4i.
The information reported for each star is in the standard
format of the UCAC4 catalogue. The description of the fields given in the
UCAC4 documentation is provided below.
col byte item fmt unit explanation notes
---------------------------------------------------------------------------
1 1- 3 ra I*4 mas right ascension at epoch J2000.0 (ICRS) (1)
2 5- 8 spd I*4 mas south pole distance epoch J2000.0 (ICRS) (1)
3 9-10 magm I*2 millimag UCAC fit model magnitude (2)
4 11-12 maga I*2 millimag UCAC aperture magnitude (2)
5 13 sigmag I*1 1/100 mag error of UCAC magnitude (3)
6 14 objt I*1 object type (4)
7 15 cdf I*1 combined double star flag (5)
15 bytes
8 16 sigra I*1 mas s.e. at central epoch in RA (*cos Dec) (6)
9 17 sigdc I*1 mas s.e. at central epoch in Dec (6)
10 18 na1 I*1 total # of CCD images of this star
11 19 nu1 I*1 # of CCD images used for this star (7)
12 20 cu1 I*1 # catalogs (epochs) used for proper motions
5 bytes
13 21-22 cepra I*2 0.01 yr central epoch for mean RA, minus 1900
14 23-24 cepdc I*2 0.01 yr central epoch for mean Dec,minus 1900
15 25-26 pmrac I*2 0.1 mas/yr proper motion in RA*cos(Dec) (8)
16 27-28 pmdc I*2 0.1 mas/yr proper motion in Dec
17 29 sigpmr I*1 0.1 mas/yr s.e. of pmRA * cos Dec (9)
18 30 sigpmd I*1 0.1 mas/yr s.e. of pmDec (9)
10 bytes
19 31-34 pts_key I*4 2MASS unique star identifier (10)
20 35-36 j_m I*2 millimag 2MASS J magnitude
21 37-38 h_m I*2 millimag 2MASS H magnitude
22 39-40 k_m I*2 millimag 2MASS K_s magnitude
23 41 icqflg I*1 2MASS cc_flg*10 + ph_qual flag for J (11)
24 42 (2) I*1 2MASS cc_flg*10 + ph_qual flag for H (11)
25 43 (3) I*1 2MASS cc_flg*10 + ph_qual flag for K_s (11)
26 44 e2mpho I*1 1/100 mag error 2MASS J magnitude (12)
27 45 (2) I*1 1/100 mag error 2MASS H magnitude (12)
28 46 (3) I*1 1/100 mag error 2MASS K_s magnitude (12)
16 bytes
29 47-48 apasm I*2 millimag B magnitude from APASS (13)
30 49-50 (2) I*2 millimag V magnitude from APASS (13)
31 51-52 (3) I*2 millimag g magnitude from APASS (13)
32 53-54 (4) I*2 millimag r magnitude from APASS (13)
33 55-56 (5) I*2 millimag i magnitude from APASS (13)
34 57 apase I*1 1/100 mag error of B magnitude from APASS (14)
35 58 (2) I*1 1/100 mag error of V magnitude from APASS (14)
36 59 (3) I*1 1/100 mag error of g magnitude from APASS (14)
37 60 (4) I*1 1/100 mag error of r magnitude from APASS (14)
38 61 (5) I*1 1/100 mag error of i magnitude from APASS (14)
39 62 gcflg I*1 Yale SPM g-flag*10 c-flag (15)
16 bytes
40 63-66 icf(1) I*4 FK6-Hipparcos-Tycho source flag (16)
41 icf(2) .. AC2000 catalog match flag (17)
42 icf(3) .. AGK2 Bonn catalog match flag (17)
43 icf(4) .. AKG2 Hamburg catalog match flag (17)
44 icf(5) .. Zone Astrog. catalog match flag (17)
45 icf(6) .. Black Birch catalog match flag (17)
46 icf(7) .. Lick Astrog. catalog match flag (17)
47 icf(8) .. NPM Lick catalog match flag (17)
48 icf(9) .. SPM YSJ1 catalog match flag (17)
4 bytes
49 67 leda I*1 LEDA galaxy match flag (18)
50 68 x2m I*1 2MASS extend.source flag (19)
51 69-72 rnm I*4 unique star identification number (20)
52 73-74 zn2 I*2 zone number of UCAC2 (0 = no match) (21)
53 75-78 rn2 I*4 running record number along UCAC2 zone (21)
12 bytes
---------------------------------------------------------------------------
78 = total number of bytes per star record
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.
The declination is given in south pole distance (spd) and can be
converted back to a true declination by subtracting 324000000 mas.
Note (2): Unknown, or unrealistic photometric results are set to
magnitude = 20 (20000 mmag entry in catalog). 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 (3): A value of 99 for error in magnitude means "no data".
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 (= value 90 in catalog data, unit = 10 mmag).
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:
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 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".
The following number of stars have cdf flag values:
0 110299935
1 262162
2 355769
3 57103
4 2408 14 19590 24 23967 34 1053229
5 7884 15 78344 25 209822 35 1226761
6 23787 16 13096 26 20945 36 125291
Note (6): The range of values here is 1 to 255 which is represented
as a signed 1-byte integer (range -127 to 127); thus add 128 to the
integer number found in the data file. There is no 0 mas value;
data less than 1 mas have been set to 1 mas. Original data larger
than 255 mas have been set to 255.
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 (nu smaller
than or equal to na).
Note (8): A value of 32767 for either proper motion component means
the real PM of that star is larger and found in the extra table
file u4hpm.dat (32 stars, ASCII). The cross reference is established
by the unique, modified MPOS number (column 51 of main data file),
which is also given on the HPM supplement stars file.
For stars without valid proper motion the proper motion data are set
to 0. However, valid proper motions can also be 0. The "no data"
case is indicated by the sigma proper motion columns (see below).
Note (9): Values in the binary data files are represented as signed,
1-byte integer (range -128 to 127). Add 128 to bring those values
to the range of 0 to 255, which is the error in proper motion in
unit of 0.1 mas/yr, with the following exception.
Data entries above 250 indicate larger errors as follows:
251 --> 275 = 27.5 mas/yr
252 --> 325 = 32.5 mas/yr
253 --> 375 = 37.5 mas/yr
254 --> 450 = 45.0 mas/yr
255 --> "no data" = set to 500 for output tables
For astrometric data copied from the FK6, Hipparcos and Tycho-2
catalogs a mean error in positions was adopted depending on input
catalog and the brightness of the star rather than giving the
individual star's error quoted in those catalogs.
Note (10): The 2MASS items copied into UCAC4 are described at
pegasus.astro.umass.edu/ipac_wget/releases/allsky/doc/sec2_2a.html
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
greater than10 AND [jhk]cmsig<0.10857
6 = ph_qual 2MASS B, valid measurement, [jhk]snr
greater than 7 AND [jhk]cmsig<0.15510
7 = ph_qual 2MASS C, valid measurement, [jhk]snr
greater than 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 to fit into fewer bytes (1/100 mag instead of millimag).
These data were taken from the j_msigcom, h_msigcom, and k_msigcom columns
of the 2MASS point source catalog. See note (10).
Note (13): Data are from the AAVSO Photometric all-sky survey (APASS)
DR6 plus single observation stars kindly provided by A.Henden.
A magnitude entry of 20000 indicates "no data". For bright stars
the apasm(1) = B mag and apasm(2) = V mag 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 apasm(1) smaller
than 20000 and apase(1) = 0 for B mags,
and similarly for apasm(2) smaller
than 20000 and apase(2) = 0 for V mags.
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): 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.
The valid range for each APASS magnitude error is +-90 = +-0.90 mag.
For "no data" (i.e. magnitude = 20000 = 20.0 mag) the error is set to 99.
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
3 = UCAC2
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, 17) binary data: a single 4-byte integer is used to store
the 10 flags of "icf". That 4-byte integer has the value:
icf = icf(1)*10^8 + icf(2)*10^7 + ... + icf(8)*10 + icf(9)
Note (16): The FK6-Hipparcos-Tycho-source-flag has the following meaning:
(= icf(1))
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
Note (17): The catflg match flag is 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. The flag has the following meaning:
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 (18): 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.
Note (19): 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.
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 0 if the UCAC4 star could not be found in UCAC2.
Further information on the UCAC4 catalogue can be found
on the UCAC homepage at: http://www.usno.navy.mil/USNO/astrometry/optical-IR-prod/ucac.
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