The Fifth USNO CCD Astrograph Catalog

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From the Readme of UCAC5 project files
Version 2.0  170224 NZ, CF,  incl. zone files description
See also  https://arxiv.org/abs/1702.05621 .
Contact e-mail:  nz@usno.navy.mil or nzIAUc8@gmail.com .

1) Overview, goals
2) Procedure
3) Results
4) File formats
5) Some statistics

UCAC5 Overview and Goals

The US Naval Observatory (USNO) has a long history of providing accurate astrometric data for millions of stars from their own observations plus other data. The USNO CCD Astrograph Catalog (UCAC) project utiized the "redlens" 20 cm aperture astrograph in an all-sky observing program between 1997 and 2004 (CTIO in the south, NOFS in the north) with a limiting magnitude of about R = 16.5. The previous release, UCAC4, became available in 2012.

The 1st Gaia data release provides proper motions for only about 2 million stars (TGAS subset of the Tycho-2 stars) in the mainly 6 to 11.5 magnitude range. Gaia DR2 which will contain proper motions of about a billion stars is scheduled for release in April 2018. In the meantime the astronomical community would benefit from proper motions of millions of stars fainter than the Tycho-2 limit, if a substantial improvement in precision and accuraccy could be made beyond what was available in the pre-Gaia era.

Such improvements are achieved here with 2 new products:

  1. Re-reduction of UCAC + combine with Gaia DR1 provides proper motions for over 107 million stars on the 1 to 5 mas/yr level, strongly depending on magnitude. UCAC observations (mean epoch 2001) provide positions with 10 to 70 mas precision, and about 14 years of epoch difference to Gaia DR1. These data will be made available through CDS / Vizier.
  2. For over 500 million additional stars fainter than UCAC new proper motions were obtained from the combination of NOMAD with Gaia DR1. NOMAD is the Naval Observatory Merged Astrometric Data, based on Hipparcos, Tycho-2, UCAC2 and Schmidt survey plate scans with a limiting magnitude of about R = 20. These data can be obtained upon request.

Please see also the upcoming paper (Zacharias, Finch, Frouard, AJ), which contains several usful figures and validation results for the UCAC5 data.

UCAC5 Procedure

Step 1: NOMAD - Gaia match

The NOMAD positions were updated to epoch 2015 using its proper motions. These positions were then sorted by Dec and matched to the full Gaia DR1 catalog, sorted by Dec as well. A "match" was assumed if the position difference in RA*cosDec and Dec were less equal to 1 arcsec:
number of stars from Gaia DR1 = 1142679769
number of stars from NOMAD = 1117612349
number of accepted matches = 638935130

The number of matches appears low at first glance but is similar to what was shown at the Madrid ESA Gaia meeting for a match between Gaia DR1 and the PPMXL catalog, which contains a similar amount of stars as NOMAD, both based on the same large photographic sky surveys of the last century. The small difference between the Gaia system and the ICRF were neglected at this time because of the relatively large position errors of NOMAD of about 100 to 200 mas per coordinate.

A catalog was generated which contains an entry for every Gaia DR1 star. For the TGAS stars the Gaia data was copied into the new catalog. For other stars matched with NOMAD new proper motions were calculated based on the about 1970 NOMAD and 2015 Gaia epoch positions. Proper motions of the remaining not matched stars were set to zero.

The NOMAD file also contains photographic plate magnitudes and 2MASS photometry. These data were propagated into the NOMAD-Gaia output file as well. A Gaia G-mag vs. 2MASS J color index for example is output on the UCAC-TGAS file (see below).

Step 2: UCAC re-reduction

No new pixel reductions were performed. Instead, the object detection results (x,y,mag) of the UCAC4 data were used in new "plate" reductions after matching with the TGAS stars. New frame-by-frame solutions were obtained with TGAS instead of Tycho-2 reference stars (as used in UCAC4). Thus these UCAC5 positions are automatically on the Gaia reference system. Systematic errors were re-visited and new field-distortion patterns generated and applied separately for the northern and southern operations. The pixel-phase error corrections were improved as well.

No changes in the systematic error correction model as function of magnitude were made. These were obtained from earlier dedicated reductions with 2MASS stars to allow complete coverage of the UCAC magnitude range. The TGAS data only reaches to about magnitude 11.5 and is thus not suitable for a re-evaluation of the systematic errors as a function of magnitude (the UCAC limiting magnitude is about 16.5). Nevertheless, residuals of the TGAS solutions were plotted as a function of magnitude and systematic errors in the accessible magnitude range are small (order 5 mas or less) for the global averages of all data. Local sub-sets of the data show somewhat larger systematic errors in the order of up to 10 mas, as expected from previous UCAC4 results.

A new catalog of UCAC mean positions from these reductions with TGAS was constructed containing over 180 million entries covering all-sky. These positions have mean epochs around 1998-2000 in the south, and around 2001 to 2003 in the north. Formal position errors are about 8 mas around magnitude 11, 20 mas at magnitude 14, and 60 mas at magnitude 16 (see Journal paper).

Step 3: UCAC5 with new proper motions

The Gaia DR1 catalog supplemented by NOMAD proper motions as described in Step 1 was propagated to epoch 2000, sorted by Dec and then matched with the TGAS reduced UCAC mean positions as described in Step 2. Again, a match was assumed if the position difference is less equal to 1 arcsec in each coordinate. This resulted in 107.7 million stars in common between the UCAC and Gaia data. New proper motions were calculated for all those stars based only on the 2 epoch positions with a mean epoch difference of about 14 years.

For several millions of stars up to about magnitude 14 the errors on these new proper motions are on the same level as the TGAS proper motions (1 to 2 mas/yr, see below). This set of UCAC - Gaia matched stars with new positions and proper motions is called "UCAC5".

Formal errors on the new proper motions were calculated by root-square-sum of a) formal position error of UCAC, b) formal position error of TGAS (both at their respective mean epochs), and c) a systematic error contribution of 10 mas at the UCAC epoch data. This RSS value was then divided by the epoch difference to arrive at the proper motion errors given in the UCAC5 data files.

UCAC5 Results

There are 6 data product files. (RECL = record length (byte), F = formatted, B = binary, direct access)
file name    RECL    n.records  description
----------------------------------------------------------------------
u5tgas.dat   133 F     2054491  TGAS + UCAC5 PM, diff.PM
u5cat.dat     40 B   107758513  UCAC5 PM (Gaia + UCAC) Dec sorted
z001..900     52 B   107758513  UCAC5 Dec zone files, sorted by RA
u5index.asc            1296000  ASCII table of index data for zone files
u5index.unf  5184000 B       2  same as binary data 
u5gnom.dat    28 B   540130390  other stars: NOMAD + Gaia PM
----------------------------------------------------------------------
The files u5cat.dat and the zone files contain the same stars with the same data, except that on the zone files the Gaia 2015 epoch positions and errors were added for convenience (ready to be used as reference star catalog).

Although contained in the main UCAC5 catalog, a separate file was output listing only the UCAC5-TGAS stars. Because of its small size it is a plain ASCII text file with fixed columns (see below) also containing the original TGAS proper motions and the differences between the UCAC5 and TGAS proper motions.

Of course there is some correlation between the 2 sets of proper motions because the new UCAC reductions used TGAS as reference star catalog. However, only 6 (linear) parameters need to be solved for in each of the UCAC exposure reductions and on average about 50 TGAS stars were used for that with 2 coordinates each. Furthermore, the UCAC results are based on typically 4 individual exposures with different telescope pointings. This large overdetermination in those least-squares reductions lead to a largely independent observation of the TGAS star positions while k

eeping the UCAC results strictly on the Gaia reference frame. The following histograms were obtained from the UCAC-TGAS file (for description of columns, see below). The bin size is given on the 2nd row of these histograms, while the 1st column gives the bin number. The rest of the table gives the number of stars per bin with respect to the data item of a particular column. The first and last bin (+-40) contain also stars beyond that bin.

col        3       4       5       6       7       8       9      10
bins    0.50    0.10   10.00   10.00   10.00   10.00    1.00    1.00
 -40       0    7205     180     302     181     299     178    1667
 -39       0       0      17      23      16      19      22     224
 -38       0       0      16      24      14      22      17     248
 -37       0       1      11      19      13      20      18     271
 -36       0       1      27      36      22      36      39     281
 -35       0       0      17      39      23      37      36     287
 -34       0       1      18      37      18      41      20     322
 -33       0       0      34      42      37      37      32     340
 -32       0       0      35      47      32      41      41     375
 -31       0       2      35      61      35      61      48     399
 -30       0       0      43      57      43      59      48     404
 -29       0       1      56      63      54      61      63     476
 -28       0       0      51      77      51      75      64     455
 -27       0       1      74     105      72     100      74     458
 -26       0       0      75      92      80      90      82     482
 -25       0       1      91     112      89     114      82     494
 -24       0       0     108     144     104     138      90     534
 -23       0       1     112     161     120     154      98     513
 -22       0       0     138     211     132     198     113     524
 -21       0       1     156     226     155     217     126     464
 -20       0       2     165     265     155     272     153     517
 -19       0       3     215     304     230     305     151     497
 -18       0       2     253     375     248     359     175     558
 -17       0       3     306     480     303     463     204     592
 -16       0       0     359     561     349     566     236     648
 -15       0       2     459     674     470     669     284     709
 -14       0       2     604     849     606     821     326     773
 -13       0       2     731    1021     731    1013     422     882
 -12       0       2     978    1356     979    1308     533     946
 -11       0       1    1268    1732    1223    1682     704    1001
 -10       0       1    1628    2401    1651    2289     966    1212
  -9       0       6    2297    3258    2291    3124    1363    1589
  -8       0       5    3215    4471    3177    4346    2045    2101
  -7       0       2    4689    6617    4649    6242    3144    2906
  -6       0       9    6977   10262    7010    9771    5209    4247
  -5       0       4   11342   15962   11377   15148    9807    7041
  -4       0      17   19367   27259   19271   26317   19532   13630
  -3       0      61   36575   49334   36585   47977   43912   31628
  -2       0     218   78712  103386   79215  101948  109370   89223
  -1       0     819  228717  269007  232773  271642  280984  275747
   0       0   10958 1368621 1367485 1361344 1369721 1093433 1200110
   1       0   18507  146435  108655  148709  110855  287759  266353
   2       0   30678   59828   37368   60395   37376  108701   80301
   3       0   43499   29295   16473   29346   16224   41650   28033
   4       0   57762   16644    8599   16583    8260   18350   11719
   5       0   75284    9894    4799    9938    4581    8731    5922
   6       0  106078    6381    2915    6324    2817    4854    3375
   7       0  178874    4168    1789    4226    1744    2911    2011
   8       1  258543    3032    1222    3006    1148    1805    1426
   9      57  244880    2068     896    2082     867    1195    1037
  10     280  160039    1516     647    1492     621     845     784
  11    1061   86469    1208     437    1206     435     587     612
  12    3914   64282     890     332     886     318     454     489
  13    7602   89391     688     236     700     241     324     451
  14   13267  123862     567     206     563     227     277     391
  15   23291  120531     498     140     499     137     228     340
  16   38961   91810     386     130     377     135     183     307
  17   64829   67395     295     114     307     112     155     281
  18  105470   51517     257      80     252      82     132     257
  19  165937   38997     221      82     225      76     117     234
  20  249813   30301     177      65     185      65      79     244
  21  324964   23149     158      45     150      44      77     206
  22  418591   17747     141      47     143      45      54     212
  23  401725   13864     119      31     114      35      63     182
  24  200782   10949     103      29     103      29      67     164
  25   31986    8406      76      29      76      27      44     147
  26    1672    6465      68      27      67      23      48     129
  27     178    4860      57      13      58      15      50     143
  28      52    3699      54      21      56      22      45     132
  29      34    2645      39      12      36      16      46     119
  30      13    1923      36      14      40      11      34     124
  31       9    1313      35      11      30      12      32     123
  32       1     823      30       7      33       8      36     144
  33       1     328      34       4      32       4      24     141
  34       0     110      35       9      36       9      29     143
  35       0      60      23       7      24       6      21     119
  36       0      29      20       8      20       6      10     137
  37       0      18      20       6      19       3      18     120
  38       0      15      24       6      28       8      19     105
  39       0       9      21       3      16       5      14     122
  40       0      46     178      40     181      40     179    1437

col       11      12      13      14
bins    0.10    0.10    0.10    0.10
   0       0       0   76960   82501
   1       0       0   13647    9156
   2       0       0   10766   10434
   3       0       0   36991   63046
   4       0       0   71194  126986
   5       0       0  104172  175529
   6    2805    2805  127204  214072
   7   77900   79029  143305  218864
   8  260493  263601  144521  197908
   9  379555  384762  138639  164546
  10  389890  393335  126617  125406
  11  320180  320746  115043   99207
  12  200119  194090  101302   78431
  13  120603  117170   88455   64172
  14   61428   59285   76703   54194
  15   31859   31115   68243   46826
  16   37886   37292   60296   40167
  17   27519   27557   54719   34362
  18   34650   35051   49538   29586
  19   23336   22893   44474   24934
  20   19544   19225   39224   21356
  21   15851   15576   35700   17638
  22   14879   15070   31178   15118
  23    9636    9511   28154   12936
  24    6730    6732   25206   11439
  25    5426    5326   22841    9603
  26    4171    4247   20521    8702
  27    2315    2325   18411    7705
  28    2285    2214   16793    6886
  29    1614    1693   15340    6250
  30     836     842   13933    5636
  31    1124    1043   12988    4969
  32     702     793   11879    4579
  33     294     306   10985    4183
  34     156     158    9870    3946
  35     129     119    9133    3556
  36      96     106    7785    3201
  37      71      65    6309    2952
  38      77      86    5643    2810
  39      51      45    4919    2611
  40     281     278   54890   38088
The distribution of the new UCAC-Gaia proper motions are very similar to that of the TGAS data. The formal errors of the UCAC-Gaia proper motions (columns 11, 12) peak at 1.0 mas/yr, comparable to the accuracy of the TGAS proper motions. Fewer stars have errors of 1.5 mas/yr or larger in UCAC-Gaia proper motions than in TGAS.

UCAC5Statistics


a) u5tgas.dat    (TGAS stars, formatted file)

total number of records = 2054491
column number, min, max, average, number of stars used for average:
  1       0.000     360.000     187.166  2054491  RA  [degree]
  2     -89.890      89.832      -3.220  2054491  Dec [degree]
  3       4.395      16.571      10.815  2054491  Gaia magnitude
  4      -9.000       7.871       1.134  2047284  G-J color index
  5   -1274.500    1134.800      -1.757  2044105  U5 PM RAc   [mas/yr]
  6   -2101.100     802.800      -4.806  2045127  U5 PM Dec   [mas/yr]
  7   -1271.000    1138.000      -1.740  2044088  TGAS PM RAc [mas/yr]
  8   -2092.400     822.900      -4.680  2045320  TGAS PM Dec [mas/yr]
  9    -178.000      79.900      -0.009  2048083  PM diff. RAc U-G  [mas/yr]
 10     -85.900      92.600      -0.048  2032327  PM diff. Dec U-G  [mas/yr]
 11       0.700      70.200       1.215  2054436  error U5 PM RAc   [mas/yr]
 12       0.700      70.200       1.212  2054436  error U5 PM Dec   [mas/yr]
 13       0.010      18.340       1.375  2044466  error TGAS PM RAc [mas/yr]
 14       0.010      18.850       1.069  2045546  error TGAS PM Dec [mas/yr]
 15       7627862074752 6917521537218608640      Gaia source ID number range
 16       1.000      52.000       5.598  2054491  numb. all images 
 17       0.000      47.000       4.945  2054491  numb. used images

b) u5cat.dats   (main catalog file)

total number of records = 107758513
column number, min, max, average, number of stars used for average:
  1               34359896320       6917528649684202496  Gaia ID number range
  2         1.000         3.000         2.064  107758513  flg
  3         0.000        59.000         2.769  107758513  nu
  4         1.121         7.381         3.311  107758513  epu
  5         0.000       360.000       209.151  107758513  RA  [degree]
  6       -89.984        89.975       -10.144  107758513  Dec [degree]
  7     -2867.000      2552.300        -2.263  107708547  PM RAc [mas/yr]
  8     -2210.700      1647.600        -3.189  107710634  PM Dec [mas/yr]
  9         0.700       103.100         4.083   96852222  error PM RAc [mas/yr]
 10         0.700        99.600         4.108   98241877  error PM Dec [mas/yr]
 11         3.240        23.554        15.156  107758513  Gaia mag
 12         3.872        20.000        15.549  107758513  UCAC mag
 13         0.000        30.000        15.763   98803442  R mag
 14        -0.676        30.000        13.906   98803441  J mag
 15        -1.739        30.000        13.445   98803434  H mag
 16        -2.099        30.000        13.324   98803426  K mag

c) Gaia-NOMAD file (upon request to nziauc8@gmail.com)

total number of records = 503640468 
column number, min, max, average, number of stars used for average:
  1                 65408   6917528993283204480  Gaia source ID number range
  2          0.00        360.00        212.14  503640468  RA  [degree]
  3        -89.99         89.90        -10.15  503640468  Dec [degree]
  4          5.24         28.61         18.53  503640468  Gaia magnitude
  5          4.32         30.00         18.63  503640468  R mag
  6          1.77         30.00         23.34  503640468  J mag
  7      -3271.50       3247.40         -1.67  502676536  PM RAc [mas/yr]
  8      -3082.10       3258.80         -4.62  502545795  PM Dec [mas/yr]
  9          0.80         32.77          3.33  485856173  error PM RAc [mas/yr]
 10          0.81         32.77          3.28  485561998  error PM Dec [mas/yr]
 11         60.10         96.99         80.32  503640468  epoch RA  [year-1900]
 12         60.10         96.99         80.32  503640468  epoch Dec [year-1900]
 13          9.00  553726830.00     480549.62  503589212  NOMAD flag

======== end of UCAC5 readme file