Information on Service 'The fifth U.S. Naval Observatory CCD Astrograph Catalog (UCAC5)'

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New astrometric reductions of the US Naval Observatory CCD Astrograph Catalog (UCAC) all-sky observations were performed from first principles using the TGAS stars in the 8 to 11 magnitude range as reference star catalog. Significant improvements in the astrometric solutions were obtained and the UCAC5 catalog of mean positions at a mean epoch near 2001 was generated. By combining UCAC5 with Gaia DR1 data new proper motions on the Gaia coordinate system for over 107 million stars were obtained with typical accuracies of 1 to 2 mas/yr (R = 11 to 15 mag), and about 5 mas/yr at 16th mag. Proper motions of most TGAS stars are improved over their Gaia data and the precision level of TGAS proper motions is extended to many millions more, fainter stars. The database table uses actual NULLs for missing photometry, and all angular coordinates have been homogenised to degrees.

For a list of all services and tables belonging to this service's resource, see Information on resource 'The fifth U.S. Naval Observatory CCD Astrograph Catalog (UCAC5)'

Service Documentation

On Catalog Construction

UCAC5 Positions

For the UCAC positions given here, 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 2017arXiv170205621Z).

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.

Full source

The catalog source can be obtained at http://vo.ari.uni-heidelberg.de/ucac5.

Overview

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Input Fields

The following fields are available to provide input to the service (with some renderers, some of these fields may be unavailable):

NameTable Head DescriptionUnitUCD
DEC Delta (ICRS) Declination (ICRS decimal) deg pos.eq.dec
gmag m_G Gaia DR1 G magnitude mag phot.mag;em.opt
hscs_pos Position/Name Coordinates (as h m s, d m s or decimal degrees), or SIMBAD-resolvable object N/A N/A
hscs_sr Search radius Search radius in arcminutes N/A N/A
MAXREC Match limit Maximum number of records returned. Pass 0 to retrieve service parameters. N/A N/A
RA Alpha (ICRS) Right Ascension (ICRS decimal) deg pos.eq.ra
RESPONSEFORMAT Output Format File format requested for output. N/A meta.code.mime
SR Search Radius Search radius deg N/A
umag m_V Mean UCAC model magnitude (20 in the source is mapped to NULL) mag phot.mag;em.opt.V
VERB Verbosity Exhaustiveness of column selection. VERB=1 only returns the most important columns, VERB=2 selects the columns deemed useful to the average user, VERB=3 returns a table with all available columns. N/A N/A

Default Output Fields

The following fields are contained in the output by default. More fields may be available for selection; these would be given below in the VOTable output fields.

NameTable Head DescriptionUnitUCD
de_ucac UCAC DEC mean UCAC Dec at epoch epu on Gaia reference frame deg pos.eq.dec
dec Dec Gaia DR1 Declination at epoch J2015.0 deg pos.eq.dec;meta.main
dec_error Err. Dec Standard error of dec; from Gaia DR1. deg stat.error;pos.eq.dec
epu Epoch Mean UCAC epoch (i.e. epoch for ra_ucac and de_ucac) yr time.epoch
gmag m_G Gaia DR1 G magnitude mag phot.mag;em.opt
hmag m_H 2MASS H magnitude (0 and 30 in the source are mapped to NULL) mag phot.mag;em.IR.H
jmag m_J 2MASS J magnitude (0 and 30 in the source are mapped to NULL) mag phot.mag;em.IR.J
kmag m_K 2MASS K_s magnitude (0 and 30 in the source are mapped to NULL) mag phot.mag;em.IR.K
pmde PM Dec PM(Dec), computed from UCAC positions and Gaia DR1 positions. deg/yr pos.pm;pos.eq.dec
pmra PM RA PM(RA), cos(δ) applied, computed from UCAC and Gaia DR1 positions. deg/yr pos.pm;pos.eq.ra
ra RA Gaia DR1 Right Ascension at epoch J2015.0 deg pos.eq.ra;meta.main
ra_error Err. RA Standard error of ra (with cos δ applied); from Gaia DR1. deg stat.error;pos.eq.ra
ra_ucac UCAC RA Mean UCAC RA at epoch epu on Gaia reference frame deg pos.eq.ra
rmag m_R NOMAD photographic R mag (0 and 30 in the source are mapped to NULL) mag phot.mag;em.opt.R
sigpmd Err. PM(Dec) Standard error of proper motion in Dec deg/yr stat.error;pos.pm;pos.eq.dec
sigpmr Err. PM(RA) Standard error of proper motion in RA*cos(Dec) deg/yr stat.error;pos.pm;pos.eq.ra
source_id Source Id Gaia source ID N/A meta.id;meta.main
umag m_V Mean UCAC model magnitude (20 in the source is mapped to NULL) mag phot.mag;em.opt.V

VOTable Output Fields

The following fields are available in VOTable output. The verbosity level is a number intended to represent the relative importance of the field on a scale of 1 to 30. The services take a VERB argument. A field is included in the output if their verbosity level is less or equal VERB*10.

NameTable Head DescriptionUnitUCD Verb. Level
source_id Source Id Gaia source ID N/A meta.id;meta.main 1
ra RA Gaia DR1 Right Ascension at epoch J2015.0 deg pos.eq.ra;meta.main 1
dec Dec Gaia DR1 Declination at epoch J2015.0 deg pos.eq.dec;meta.main 1
pmra PM RA PM(RA), cos(δ) applied, computed from UCAC and Gaia DR1 positions. deg/yr pos.pm;pos.eq.ra 5
pmde PM Dec PM(Dec), computed from UCAC positions and Gaia DR1 positions. deg/yr pos.pm;pos.eq.dec 5
gmag m_G Gaia DR1 G magnitude mag phot.mag;em.opt 5
umag m_V Mean UCAC model magnitude (20 in the source is mapped to NULL) mag phot.mag;em.opt.V 5
ra_error Err. RA Standard error of ra (with cos δ applied); from Gaia DR1. deg stat.error;pos.eq.ra 15
dec_error Err. Dec Standard error of dec; from Gaia DR1. deg stat.error;pos.eq.dec 15
epu Epoch Mean UCAC epoch (i.e. epoch for ra_ucac and de_ucac) yr time.epoch 15
ra_ucac UCAC RA Mean UCAC RA at epoch epu on Gaia reference frame deg pos.eq.ra 15
de_ucac UCAC DEC mean UCAC Dec at epoch epu on Gaia reference frame deg pos.eq.dec 15
sigpmr Err. PM(RA) Standard error of proper motion in RA*cos(Dec) deg/yr stat.error;pos.pm;pos.eq.ra 15
sigpmd Err. PM(Dec) Standard error of proper motion in Dec deg/yr stat.error;pos.pm;pos.eq.dec 15
rmag m_R NOMAD photographic R mag (0 and 30 in the source are mapped to NULL) mag phot.mag;em.opt.R 15
jmag m_J 2MASS J magnitude (0 and 30 in the source are mapped to NULL) mag phot.mag;em.IR.J 15
hmag m_H 2MASS H magnitude (0 and 30 in the source are mapped to NULL) mag phot.mag;em.IR.H 15
kmag m_K 2MASS K_s magnitude (0 and 30 in the source are mapped to NULL) mag phot.mag;em.IR.K 15
srcflags Src flags Source for the star: 1 = TGAS, 2 = other UCAC-Gaia star, 3 = other NOMAD N/A meta.code;meta.bib 25
nu1 #Img used Number of images used for UCAC mean position N/A meta.number 25

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