UCAC5

UCAC5

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:

  • a) 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.
  • b) 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 Zacharias et al which contains several useful figures and validation results for the UCAC5 data.

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

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).

Results

There are 6 data product files. (RECL = record length (byte), F = formatted, B = binary, direct access)

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.

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 keeping the UCAC results strictly on the Gaia reference frame.

UCAC5 tables on Gaia@AIP

The UCAC5 main data, crossmatched with GDR1.gaia_source by source_id and random_index:
UCAC5.zones
NOMAD data, additional starts, not in the above catalogs, also crossmatched with GDR1.gaia_source by source_id and random_index:
UCAC5.NOMAD

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