The Third USNO CCD Astrograph Catalog
Data Format

WCSTools
Catalogs
num item   fmt unit        explanation                            remark
------------------------------------------------------------------------
 1  ra     I*4 mas         right ascension at  epoch J2000.0 (ICRS)  (1)
 2  spd    I*4 mas         south pole distance epoch J2000.0 (ICRS)  (1)
 3  im1    I*2 millimag    UCAC fit model magnitude                  (2)
 4  im2    I*2 millimag    UCAC aperture  magnitude                  (2)
 5  sigmag I*2 millimag    UCAC error on magnitude (larger of sc.mod)(3)
 6  objt   I*1             object type                               (4)   
 7  dsf    I*1             double star flag                          (5)   
         16
 8  sigra  I*2 mas         s.e. at central epoch in RA (*cos Dec)      
 9  sigdc  I*2 mas         s.e. at central epoch in Dec                 
10  na1    I*1             total # of CCD images of this star
11  nu1    I*1             # of CCD images used for this star        (6)
12  us1    I*1             # catalogs (epochs) used for proper motions
13  cn1    I*1             total numb. catalogs (epochs) initial match
          8
14  cepra  I*2 0.01 yr     central epoch for mean RA, minus 1900     
15  cepdc  I*2 0.01 yr     central epoch for mean Dec,minus 1900  
16  pmrac  I*4 0.1 mas/yr  proper motion in RA*cos(Dec)           
17  pmdc   I*4 0.1 mas/yr  proper motion in Dec                    
18  sigpmr I*2 0.1 mas/yr  s.e. of pmRA * cos Dec                   
19  sigpmd I*2 0.1 mas/yr  s.e. of pmDec                            
         16
20  id2m   I*4             2MASS pts_key star identifier          
21  jmag   I*2 millimag    2MASS J  magnitude                     
22  hmag   I*2 millimag    2MASS H  magnitude                       
23  kmag   I*2 millimag    2MASS K_s magnitude                     
24  icqflg I*1 * 3         2MASS cc_flg*10 + phot.qual.flag          (7)
25  e2mpho I*1 * 3         2MASS error photom. (1/100 mag)           (8)
         16
26  smB    I*2 millimag    SuperCosmos Bmag
27  smR2   I*2 millimag    SC R2mag                                  (9)
28  smI    I*2 millimag    SC Imag
29  clbl   I*1             SC star/galaxy classif./quality flag     (10)
30  qfB    I*1             SC quality flag Bmag                     (11)
31  qfR2   I*1             SC quality flag R2mag                    (11)
32  qfI    I*1             SC quality flag Imag                     (11)
         10
33  catflg I*1 * 10        mmf flag for 10 major catalogs matched   (12)
34  g1     I*1             Yale SPM object type (g-flag)            (13)
35  c1     I*1             Yale SPM input cat.  (c-flag)            (14)
36  leda   I*1             LEDA galaxy match flag                   (15)
37  x2m    I*1             2MASS extend.source flag                 (16)
38  rn     I*4             MPOS star number; identifies HPM stars   (17)
         18
------------------------------------------------------------------------
         84 = 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 UCAC3, 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 = 18 (18000 mmag entry in catalog).  Systematic errors
    are expected to be below 0.1 mag for im1, im2 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 -1 for error in magnitude means "unknown".
    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 (900 mmag entry in catalog data).

Note  (4):  The object type flag is used to identify possible problems 
    with a star and has the following meaning:

    -2 = warning: object could be from possible streak
    -1 = warning: object is near overexposed star
     0 = good star
     1 = good star (data copied from another entry)
     2 = warning: contains at least 1 overexposed image
     3 = warning: all images are overexposed or "bad"

Note  (5):  The dsf flag is a double star flag used to indicate the
    type/quality of double star fit with the following meaning:

     0 = single star
     1 = primary of pair with unreal secondary = single
     2 = forced separation, on same frame
     3 = blended image, some CCD frames show single star, some double
     4 = forced separation, 2 objects on same frame number
     5 = primary   component of real double
     6 = secondary component of real double
     7 = other "odd" case  

Note  (6):  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 <= na).

Note  (7):  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  (8):  The photometric errors from 2MASS were rounded by 1 digit
    here to fit into fewer bytes (1/100 mag instead of millimag).

Note  (9):  The R magnitude from the second epoch Schmidt plates (R2) is 
    used for UCAC3, as provided in the SuperCosmos source catalog.

Note (10):  clbl is a combination of the SuperCOSMOS "meanclass" and
    modified "blend" flag (meanclass + 10*blend) originally for each
    magnitude (B, R1, R2, I).  The flag provided here is the maximum
    value over all bands for each of the 2 flags. The SuperCOSMOS mean 
    class flag is an estimate of image class based on unit-weighted mean 
    of individual classes from (B, R1, R2, I).  The SuperCOSMOS modified 
    blend flag indicates if blending is detected.

    The "meanclass" has the following meaning from SuperCOSMOS:
    1 = galaxy
    2 = star
    3 = unclassifiable
    4 = noise

    The modified "blend" flag has the following meaning:
    0 = no blending
    1 = possible blending detected

Note (11):  This is a modified quality flag from the "qualB", "qualR2",
    and "qualI" quality flag from SuperCOSMOS, which gives an indication
    of the quality of the image from the three bands (B, R2, I).  The
    modified quality flag qfB, qfR2, and qfI have the following meaning:

    -1 = qual blank    in SuperCOSMOS, no flag given
     0 = qual zero     in SuperCOSMOS, no problems detected
     1 = qual < 128    in SuperCOSMOS, reliable image
     2 = qual < 65535  in SuperCOSMOS, problems detected
     3 = qual >= 65535 in SuperCOSMOS, spurious detection

Note (12):  The catflg provides reference to 10 major catalogs used
     in the computation of the proper motions and catalog matching.
     Each of the 10 numbers range from 0 to 6 and are the "mmf" 
     (multiple match flag) with respect to each of the 10 following
     catalogs:

           Hip, Tycho, AC2000, AGK2B, AGK2H, ZA, BY, Lick, SC, SPM
     catflg 1     2       3      4      5     6   7   8     9   10

     The value for each byte, the mmf flag, has the following meaning:

     0 = star not matched with this catalog
     1 = unique match,  not involving a double star
     2 = closest match, not involving a double, likely o.k. 
     3 = unique match,  and involving a double star
     4 = closest match, and involving a double, likely o.k.
     5 = maybe o.k. smallest sep. match in both directions
     6 = same as 5, but involving a double star

Note (13):  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

Note (14):  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 (15):  This flag is either 0 (no match) or contains the log of
    the apparent total diameter for I-band (object size) information
    copied from the LEDA catalog (galaxies).  A size value of less
    than 1 has been rounded up to 1.

Note (16):  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.  If the size is larger than 127, the flag was set to 127.

Note (17):  MPOS running star numbers over 140 million indicate high
    proper motion stars which were identified in UCAC pixel data from
    matches with known HPM stars.  The position given for those HPM
    stars is the unweighted mean of the CCD observations and the
    proper motion is copied from the literature catalog.

The following table gives the total range of values present these data.

 # explanation                               unit   minimum    maximum
----------------------------------------------------------------------
 1 right ascension     epoch J2000.0 (ICRS) [mas]         2 1295999990
 2 south pole distance epoch J2000.0 (ICRS) [mas]     57990  647908484
 3 UCAC fit model magnitude                [mmag]      2329      18000
 4 UCAC aperture  magnitude                [mmag]      2797      18000
 5 error on magnitude (larger of sc.model) [mmag]        -1        900
 6 object type                                           -2          3
 7 double star flag                                       0          7
 8 s.e. at central epoch in RA (*cos Dec)   [mas]         5        500
 9 s.e. at central epoch in Dec             [mas]         5        500
10 total numb. of CCD images per star                     1         70
11 number of CCD images used per star                     0         68
12 numb. catalogs (epochs) used for proper motion         1         31
13 total numb. of catalogs (epochs) initial match         0        113
14 central epoch - 1900 for mean RA    [1/100 yr]      -477      10438
15 central epoch - 1900 for mean Dec   [1/100 yr]      -938      10438
16 proper motion in RA*cos(Dec)      [0.1 mas/yr]    -37060      67670
17 proper motion in Dec              [0.1 mas/yr]    -58130     102770
18 s.e. of pmRA * cos Dec            [0.1 mas/yr]       -10        500
19 s.e. of pmDE                      [0.1 mas/yr]       -10        500
20 2MASS pts_key star identifier                          0 1340037129
21 2MASS J  magnitude                      [mmag]     -2652      30000
22 2MASS H  magnitude                      [mmag]     -3732      30000
23 2MASS K_s magnitude                     [mmag]     -4227      30000
24 2MASS cc_flg*10 + phot.qual.flag J band                0         58
25 2MASS cc_flg*10 + phot.qual.flag H band                0         58
26 2MASS cc_flg*10 + phot.qual.flag K band                0         58
27 2MASS error photom. J band         [1/100 mag]         0        127
28 2MASS error photom. H band         [1/100 mag]         0        127
29 2MASS error photom. K band         [1/100 mag]         0        127
30 SC Bmag                                 [mmag]         0      30000
31 SC R2mag                                [mmag]         0      30000
32 SC Imag                                 [mmag]         0      30000
33 SC star/galaxy classif./quality flag                   0         14
34 SC quality flag Bmag                                  -1          2
35 SC quality flag R2mag                                  0          2
36 SC quality flag Imag                                  -1          2
37 mmf flag Hipparcos catalog                             0          1
38 mmf flag Tycho-2 catalog                               0          1
39 mmf flag AC2000 catalog                                0          1
40 mmf flag AGK Bonn catalog                              0          6
41 mmf flag AGK Hamburg catalog                           0          6
42 mmf flag ZA catalog                                    0          6
43 mmf flag BY catalog                                    0          6
44 mmf flag Lick catalog                                  0          6
45 mmf flag SuperCosmos catalog                           0          6
46 mmf flag SPM catalog                                   0          6
47 Yale SPM object type (g-flag)                          0          3
48 Yale SPM input cat.  (c-flag)                          0          7
49 LEDA galaxy match flag    (size info)                  0        118
50 2MASS extend.source flag  (size info)                  0        127
51 MPOS star number                                       3  140051297