Information on Service 'Corrections between UCAC3 and PPMXL'

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Differences between UCAC3 and PPMXL in positions and proper motions, on an all-sky one-degree grid. At each gridpoint we give the differences X(PPMXL)- X(UCAC3) averaged over all stars in a sqrt(2)/2-degrees environment around the gridpoint given. The corrections given here should bring UCAC3 based astrometry to the ICRS.

For a list of all services and tables belonging to this service's resource, see Information on resource 'UCAC3 nocross'

Service Documentation

Observers using UCAC3 for the reduction of their CCD frames get positions of their targets which are not on the ICRS. Such positions can neither be used to derive inertial stellar proper motions, nor should they be used for in orbit determinations in the case of solar system bodies.

To aid in reducing these observations to ICRS, we present systematic correction tables from UCAC3 to PPMXL for positions at epoch 2000.0 and for proper motions. The tables give means of the differences in positions and proper motions between PPMXL and UCAC3 in circles of radius sqrt(2)/2 around the centers of 360 by 180 spherical squares covering the sky. The application of these tables is straighforward. Suppose you have an observation (α,δ) based on UCAC3 at epoch T (in years). To locate the corrections in downloaded tables (see below), compute floor(α)+0.5, floor(δ)+0.5 or use the web service.

The conversion to ICRS is given by

./static/simplecorr.png

In applying these formulae note that they are not rigorous, with the approximations used breaking down close to the poles. Also, be careful when crossing α=0 during the correction.

The full correction table can be downloaded as VOTable or plain text.

Here are maps of the corrections over the entire sky for RA, Declination and the proper motions:

./static/icrscorrplots.png

Overview

You can access this service using:

This service is published as follows:

local means it is listed on our front page, ivo_managed means it has a record in the VO registry.

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
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 Ignored Ignored N/A N/A
RA Alpha (ICRS) Right Ascension (ICRS decimal) deg pos.eq.ra
SR Search Radius Search radius deg 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
alpha Center RA Center for field, RA deg pos.eq.ra;meta.main
d_alpha PPMXL-UCAC3, RA Correction PPMXL-UCAC3 in alpha (Epoch 2000.0) deg pos.eq.ra;arith.diff
d_delta PPMXL-UCAC3, Dec Correction PPMXL-UCAC3 in delta (Epoch 2000.0) deg pos.eq.dec;arith.diff
d_pmalpha PPMXL-UCAC3, PM RA Correction PPMXL-UCAC3 in proper motion alpha*cos(delta) deg/yr pos.pm;pos.eq.ra;arith.diff
d_pmdelta PPMXL-UCAC3, PM Dec Correction PPMXL-UCAC3 in delta deg/yr pos.pm;pos.eq.dec;arith.diff
delta Center Dec Center for field, Dec deg pos.eq.dec;meta.main
nob N Number of objects the correction is based on N/A meta.number

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
alpha Center RA Center for field, RA deg pos.eq.ra;meta.main 1
delta Center Dec Center for field, Dec deg pos.eq.dec;meta.main 1
d_alpha PPMXL-UCAC3, RA Correction PPMXL-UCAC3 in alpha (Epoch 2000.0) deg pos.eq.ra;arith.diff 1
d_delta PPMXL-UCAC3, Dec Correction PPMXL-UCAC3 in delta (Epoch 2000.0) deg pos.eq.dec;arith.diff 1
d_pmalpha PPMXL-UCAC3, PM RA Correction PPMXL-UCAC3 in proper motion alpha*cos(delta) deg/yr pos.pm;pos.eq.ra;arith.diff 1
d_pmdelta PPMXL-UCAC3, PM Dec Correction PPMXL-UCAC3 in delta deg/yr pos.pm;pos.eq.dec;arith.diff 1
nob N Number of objects the correction is based on N/A meta.number 18

VOResource XML (that's something exclusively for VO nerds)

The Data Center is currently running in emergency mode after our database server failed catastrophically (see our blog for infos and updates).

This means, in particular, that services built on top of large tables may currently be broken. Since our replacement server is much slower than our usual machine, restoring them will take quite some time.

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