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GSC 2.3 Release Notes


These notes contain basic information about the GSC2.3 catalog content, format and data access methods. Several new fields are provided with this release including proper motions, additional magnitude columns, and identifiers. The file structure is identical to GSC2.2.

General Description
Source Counts
Catalog Fields
Data Access Software
Export Catalog File Format
FITS HDU 2, ASCII Table: HTM Level 10 lookup
FITS HDU 3, Export Binary Table Fields and Types
Future Release Enhancements
Classification and Source Status Codes
Known catalog limitations


General Description

GSC2.3 is an all-sky export of calibrated photographic survey plate source parameters from the COMPASS database.  All sources exported are from the second-generation plate-processing pipeline with the exception of Tycho-2 and Skymap sources in the case of very bright objects. The Skymap sources are exported when there is no matching GSC or Tycho sources.

Source Counts

The number of unique objects exported is approximately 945,592,683.  The catalog properties for this release represent a magnitude-selected subset of the full COMPASS database.  The storage capacity required for GSC 2.3 is approximately 200 Gigabytes.

Catalog Fields

Each GSC 2.3 entry contains only one position and one magnitude per bandpass for each unique sky object. In addition this version also contains proper motions for sources which satisfy specific constraints since the COMPASS database contains multiple epochs and observations for a given source. To maximize the quality of the exported data the catalog parameter selection was performed as follows:

  • Positions and magnitudes from plate observations were selected by choosing the observation which occurs closest to the plate center. Position selection was then prioritized by
  • If an observation occurred at different plate scanning resolutions, the highest resolution (15 um pixels) was selected over the lower resolution (25 um pixels) for each bandpass regardless of plate location.
  • Star/nonstar classifications were assigned by voting all available classifications from 15um scans, before applying the magnitude cutoff. Classifications from 25um scans were used only if no 15um data were available for the object. Ties were broken in favor of nonstar, and defect classifications were considered to be nonstellar for matched objects. Unmatched plate defects were excluded from the catalog.

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Data Access Software

For this release, the main public access supported is through the GSC-2.3 Web form interface. The underlying software is currently only for specialized distribution to the GSC-II consortium until the final 2.3 catalog calibrations are completed.  A C++ cgi script has been developed for integration into telescope observation tools consistent with the standard interfaces used with SkyCat.  There is a Beta version of ShowSky visualization tool , a JAVA application available to the public which query access to GSC-2.2, DSS, and various other catalogs .

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Export Catalog File Format

The export catalog is a distributed FITS file system based on the linear quad-tree Hierarchical Triangular Mesh (HTM) sky partitioning scheme (see HTM Reference ). GSC-2 catalog uses a level 6 HTM triangulation which results in each file containing roughly 1 square degree of sky data. The source numbers per file are not balanced at this time, but future options are available to extend this capability into the HTM access methods.

Each FITS file contains all calibrated celestial sources for a single triangular section of sky (HTM level 6 leafnode). The file names are the quad-tree representation of each triangular section, with the 1st character being either an 'N' or 'S' representing North or South.

A FITS file contains 1 primary HDU, a second HDU (1st extension) ACSII table, and a third HDU (2nd extension) binary table. HDU 2 is a fixed length 256 entry lookup table into a variable sized HDU 3. The HDU 3 is sorted into HTM level 10 cells, and thus the lookup points to the first record of each level 10 cell with the number of entries in that cell also included in the ASCII table. The level 10 provides an indexing resolution of approximately 18 square arc minutes.

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HDU 2, ASCII Table

Variable Type Variable Name Description
tfields=2 "Table fields" The table will contains 2 columns.
nrows=256 "Number of Rows"  Table will have 256 rows.
extname[]="HTMLevel10Idx" "Extension name" Name of the extention.
*ttype[]={"Freq", "L10Pointer"}; "Table Type" 
*tform[]={"I10","I10"}; "Table Form"
*tunit[]={" "," "}; "Table Unit"

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HDU 3, Export Binary Table Fields and Types

Field Type Form Unit Description
1 gscID2 J id internal id#
2 gsc1ID 11A id GSC1 name
3 hstID 11A id GSC2 name for HST ops
4 RightAsc D rad RA (J2000)
5 Declination D rad Dec (J2000)
6 PositionEpoch E year Epoch of position
7 RaEpsilon E arcsec RA error
8 DecEpsilon E arcsec Dec error
9 RaProperMotion E mas/yr Proper motion
10 DecProperMotion E mas/yr Proper motion
11 RaProperMotionErr E  mas/yr PM error
12 DecProperMotionErr E  mas/yr PM error
13 deltaEpoch E year time baseline used for pm calculation
14 FpgMag E mag IIIaF magnitude
15 FpgMagErr E  mag mag err
16 FpgMagCode I   bandpass code
17 JpgMag E mag IIIaJ magnitude
18 JpgMagErr E  mag mag err
19 JpgMagCode I   bandpass code
20 Vmag E mag 'V' band magnitude
21 VmagErr E  mag mag error
22 VmagCode I   bandpass code
23 Nmag E mag IV-N magnitude
24 NpgMagErr E mag mag error
25 NpgMagCode I   bandpass code
26 UMag E mag 'U' magnitude
27 UMagErr E mag mag error
28 UMagCode I   bandpass code
29 BMag E mag 'B' magnitude
30 BMagErr E mag mag error
31 BMageCode I   bandpass code
32 RMag E mag 'R' magnitude
33 RMagErr E mag mag error
34 RMagCode I   bandpass code
35 IMag E mag 'I' magnitude
36 IMagErr E mag mag error
37 IMagCode I   bandpass code
38** JMag E mag 2MASS J magnitude
39** JMagErr E mag mag error
40** JMagCode I   bandpass code
41** HMag E mag 2MASS H magnitude
42** HMagErr E mag mag error
43** HMagCode I   bandpass code
44** KMag E mag 2MASS K magnitude
45** KMagErr E mag mag error
46** KMagCode I   bandpass code
47 Classification J   classification code
48 SemiMajorAxis E pixels Object size
49 Eccentricity E   Object eccentricity
50 Positionangle E deg Object Position Angle
51 Source Status Flag J   Object Flags
52 Variable Flag J   Flag for known variable
53 MultipleFlag J   Flag for known multiple

** 2MASS magnitudes not yet filled in - will be needed for JWST so placeholder fields were allocated.

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Future Release Enhancements

  • TBA

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Classification Codes

0 Star
1 Galaxy
2 Blend
3 Non-Star
4 Unclassified
5 Defect

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Source Status Flag Codes

The source status flag is a 10 digit field used to encode more detailed information about the properties of the catalog object and its calibrated values. Since no GSC-II processed data is included for Tycho or SkyMap objects, the value of the Source Status Flag for a Tycho object is 99999900, Skymap is 88888800.

Digit (counting from the right)
Number of plates on which this object appeared.
This may include plate observations that were excluded from the GSC2.3.
These two digits are read together as an ordinary base ten number, eg, 04 or 12.
Centroider type associated with the exported position
0 Barycenter 4 multicircular
1 Circular Gaussian 5 multielliptical
2 Elliptical Gaussian 6 FPA + circular
3 FPA-applied barycenter 7 FPA + elliptical
Quality of exported J magnitude
not present
Quality of exported F magnitude
not present
Quality of exported V magnitude
not present
Classification unanimity
mixed vote
unanimous vote
unanimous defect
Classification voters
More than one 15um scan
One 15um scan
More than one 25um scan
One 25um scan
Incomplete processing
Completed processing on all plates
Object was too big to be cut out (> 256x256 pixels) on at least one plate
Deblended object
Single object on all plates
Child (deblended) object on at least one plate



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Last Modified: 2016-06-20 14:42