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Guide to image data

The sections linked below provide basic information on the data returned by the small area image extraction form, Get an Image, and on the GAIA software. More detailed discussions are planned.

Updates to the on-line data and associated access software are listed in the Release History.

For the complete beginner a short step-by-step description of a typical extraction and the examination of the data returned using GAIA is given here.

The pixel data

The pixel data is returned as a (gzipped) standard FITS file. The original pixel data have been compressed using the H-transform by a factor of 20 without loss of resolution i.e. the pixel size is still 10 microns (0.7 arcsec). Pixel values range from 0 to 32767 and are representative of the plate density (derived from the measured transmission).

A world coordinate system (WCS) has been written into the FITS file so that astronomical coordinates (as opposed to x and y) are readily available. The WCS system is defined for each extraction using the positions of objects taken from at least a 10 x 10 armin area in the object-catalogue. The objects in this catalogue were detected during the SuperCOSMOS Image Analysis Mode (IAM). Typically some 1000 objects define the xy-to-RA/Dec transformation and the coordinates of a given pixel are thought to be accurate to a few tenths of an arcsec.

The object-catalogue data

The image extraction form returns two fomats of object catalogues.

  • FITS tables - these are included in the same FITS file as the pixel data. One table contains the most useful parameters, colours and proper motions, the other lists all 32 image parameters for each object as measured on the primary waveband plate.
  • Tab listing - a tab-separated ascii file formatted to be compatible with the GAIA software. This listing contains the same information as the first FITS table (see previous bullet point) i.e. those parameters thought to be of most use to the typical user.
Object parameters were determined during SuperCOSMOS's Image Analysis Mode (IAM). For a discussion of IAM go here (some of the parameters i.e. the COSMOS magnitude and parameters 31 & 32 have been re-defined since this document was written and have been replaced by a calibrated SuperCOSMOS magnitude, a N(0,1) profile classification statistic, and a SuperCOSMOS profile magnitude).

Only objects detected in the primary/selected waveband are returned. Magnitudes in any/all of the other wavebands are also returned if a given object in the primary colour pairs up with an object detected in the other wavebands.

A secondary magnitude of 99.999 indicates one of the following:

  • the plate in the seondary waveband does not exist or has not been measured, if this is the case all objects in this waveband will be entered as 99.999
  • the object is too faint or has a colour that prevents it from being detected on the secondary plate
  • proper motion/pairing problems

Parameters in the FITS tables
Parameters in tab-separated listing

Description of FITS table object parameters

FITS table 1

The first table lists the most useful parameters and contains magnitudes of the objects in the other wavebands that have been paired with those detected in the primary (i.e. the one requested in the form) waveband/survey.

Each object is desribed by the parameters listed below. Some of the parameters have links pointing to more detailed information. FITS tables can be examined using various software (eg using xcatview under CURSA)

No.NameTypeDescriptionUnits
1RADoubleCelestial Right Ascensionradians
2DECDoubleCelestial Declinationradians
3MU_ACOSDRealProper motion in RAmas/yr
4MU_DRealProper motion in Decmas/yr
5SIGMU_ARealError on proper motion in RAmas/yr
6SIGMU_DRealError on proper motion in Decmas/yr
7B_JRealUK-J (Bj) magnitudemags
8R_1RealESO-R or POSS-I E magnitudemags
9R_2RealUK-R magnitudemags
10IRealUK-I magnitudemags
11AREAIntegerTotal areapixels
12A_IIntegerWeighted semi-major axis0.01 microns
13B_IIntegerWeighted semi-minor axis0.01 microns
14P_AIntegerCelestial position angledegrees
15CLASSIntegerClassification flag
16N(0,1)IntegerN(0,1) profile classification statistic0.001 sigma
17BLENDIntegerDeblending flag (0 if not deblended)
18QUALITYIntegerQuality flag

FITS table 2

The second table lists all 32 parameters as measured/extracted from plate in the primary (selected) waveband. Some of the parameters have links pointing to more detailed information. FITS tables can be examined using various software (eg using xcatview under CURSA)

No.NameTypeDescriptionUnits
1RADoubleCelestial Right Ascensionradians
2DECDoubleCelestial Declinationradians
3XMINIntegerLeft extent0.01 microns
4XMAXIntegerRight extent0.01 microns
5YMINIntegerBottom extent0.01 microns
6YMAXIntegerTop extent0.01 microns
7AREAIntegerTotal areapixels
8IPEAKIntegerBrightest intensity above sky
9MAGRealCOSMOS magnitudemagnitudes
10ISKYIntegerSky intensity at XCEN_I, YCEN_I
11XCEN_IIntegerIntensity weighted X centroid0.01 microns
12YCEN_IIntegerIntensity weighted Y centroid0.01 microns
13A_UIntegerUnweighted semi-major axis0.01 microns
14B_UIntegerUnweighted semi-minor axis0.01 microns
15THETA_UIntegerUnweighted orientationdegrees
16A_IIntegerWeighted semi-major axis0.01 microns
17B_IIntegerWeighted semi-minor axis0.01 microns
18THETA_IIntegerWeighted orientationdegrees
19CLASSIntegerClassification flag
20P_AIntegerCelestial position angledegrees
21AP(1)IntegerArea above areal profile level 1pixels
22AP(2)IntegerArea above areal profile level 2pixels
23AP(3)IntegerArea above areal profile level 3pixels
24AP(4)IntegerArea above areal profile level 4pixels
25AP(5)IntegerArea above areal profile level 5pixels
26AP(6)IntegerArea above areal profile level 6pixels
27AP(7)IntegerArea above areal profile level 7pixels
28AP(8)IntegerArea above areal profile level 8pixels
29BLENDIntegerDeblending flag (0 if not deblended)
30QUALITYIntegerQuality flag
31N(0,1)IntegerN(0,1) profile classification statistic0.001 sigma
32PRFMAGIntegerSuperCOSMOS profile magnitudemillimags

Description of object parameters in tab-separated list

The image extraction form also returns a tab-separated ascii listing. This file contains 16 parameters (plus a running ID number) for each object. These parameters include proper motions, magnitudes in all wavebands (where available/detected) and a subset of the full 32 parameters recorded by SuperCOSMOS during its Image Analyis Mode (IAM) of the primary plate. The second table in the FITS file contains all 32.

The tab-separated listing is formatted in the way it is so that it is directly readable by GAIA. At the top of the listing are a few lines of information (not garbage!) describing how GAIA should plot the object symbols, in this case ellipses, when the file is read in.

The parameters returned are given below:
No.NameTypeDescriptionUnits
1RACelestial Right Ascensiondegrees
2DECCelestial Declinationdegrees
3MU_ACOSDRealProper motion in RAmas/yr
4MU_DRealProper motion in Decmas/yr
5SIGMU_ARealError on proper motion in RAmas/yr
6SIGMU_DRealError on proper motion in Decmas/yr
7B_JRealUK-J (Bj) magnitudemags
8R_1RealESO-R or POSS-I E magnitudemags
9R_2RealUK-R magnitudemags
10IRealUK-I magnitudemags
11AREAIntegerTotal areapixels
12A_IIntegerWeighted semi-major axis0.01 microns
13B_IIntegerWeighted semi-minor axis0.01 microns
14PAIntegerCelestial position angledegrees
15CLASSIntegerClassification flag
16N(0,1)IntegerN(0,1) profile classification statistic0.001 sigma
17BLENDIntegerDeblending flag (0 if not deblended)
18QUALITYIntegerQuality flag

The GAIA software

GAIA is one of many applications that can read the standard FITS images produced during the extraction. More than just an advanced image display tool, GAIA also provides for the plotting of object-catalogues and is therefore well-suited to the data being discussed (of course users may prefer to use another package: SAOIMAGE, SAOTNG, XV, etc).

Currently GAIA requires object catalogues to be in a similar format to the tab-separated listing produced by the form. Future releases of GAIA will be able to read the catalogue already inlcluded as part of the FITS file.

For more detailed information on GAIA (including how to get and install the package) go here .



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WFAU, Institute for Astronomy,
Royal Observatory, Blackford Hill
Edinburgh, EH9 3HJ, UK

M.Read@roe.ac.uk
30/7/2008