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GSC-ACT Catalog: Catalog Help

Overview

The GSC-ACT is an astrometric recalibration of the Hubble Guide Star Catalog (GSC) version 1.1, using the ACT (Astrographic Catalog/Tycho) from the US Naval Observatory. A copy of the Tycho catalog was combined with the stellar proper motions from the ACT, and then each record was matched with corresponding record(s) in the GSC 1.1 catalog. The results were then stored on a per-GSC plate basis, so that for each given plate, there would be a file containing data for each ACT star on that plate. The data would consist of (full) Tycho data, the ACT proper motion (if any) and (full) GSC 1.1 data.

The data for each plate were then loaded, and the ACT positions were corrected for proper motions. The residuals between GSC and ACT positions were used as inputs to a least-squares fitting routine to calculate the intrinsic distortions in the plates, up to a fifth-order polynomial.

The result is that while the random errors in astrometry remain more or less the same as the original GSC catalog, the systematic errors are effectively reduced to zero. This can be particularly useful for applications which involve combining many astrometic observations (such as calculating asteroid orbits).

Parameters

GSC_ID:
Guide Star Catalogue number

RA:
Right Ascension of the star at the requested epoch.

Dec:
Declination of the star at the requested epoch.

Mag:
Since the GSC is drawn from two overlapping surveys, the quoted magnitude may be either a Quasi-V ("QV mag") or a Quasi-B magnitude ("QB mag"). The former comes mostly from the northern Palomar survey, and the latter from the southern UK SERC survey.

Class:
The classification flag uses the following codes:

------------------------------------------------------
     Code       Meaning
     ----       -------
       1        Star
       2        Galaxy
       3        Non-Star
       5        Artifact
------------------------------------------------------

Band:
HST bandpass. See table.

N:
Number of entries for this star.

Arcsec:
Distance from the search centre in arcsec.

Useful links

GSC-ACT at Project Pluto
 
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