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Guide to image data from the SHS

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.

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 with extensions. The initial (Oth) extension, eg file.fits[0], contains the image data, the second [1] contains tables of IAM parameters for detected objects in the extracted region and the third extension, [3], is a pixel quality map indicating if any regions of the extracted image contain sub-optimal data. This extension is further described here

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 (0.67 arcsec).

Pixel Units

The pixel value range depends on the chosen pixel units. The available choices are for density, transmission or intensity space. Details of these choices and how they are derived are given in here. Standard SuperCOSMOS surveys are only given in density space but scaled to be integers in the range 0 to 32767 and are 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 formats of object catalogues.

  • FITS tables
    There are two FITS tables of object parameters which are included in the same FITS file as the pixel data as a fits table extension. The first table contains a selection of useful object parameters such as co-ordinates (both equatorial and galactic), magnitudes, image classification criteria etc for the H-alpha and matching SR data.
    Note: Placeholders are also included in this first FITS table for object parameters that will eventually be included from the earlier Galactic Plane I/SR survey (taken between 1978-1985 on IV-N and IIIaF emulsions on standard 5-degree field centres). These I/SR parameters have NULL values at present. Apart from the co-ordinates the other object parameters are grouped in sets of 4, one for each band: H-alpha and SR2 and then SR1 and I.

    The second fits table just lists all 32 standard SuperCOSMOS image parameters for each object as measured on a single (the primary) waveband selection (usually H-alpha).

  • 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, i.e. those parameters thought to be of most use to the typical user of this type of data.
Object parameters were determined during SuperCOSMOS's Image Analysis Mode (IAM) of each scan. 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. However associated IAM parameters for matching objects in the other wavebands are also returned if a given object in the primary band pairs up with an object detected in the other wavebands.

NULL values
Null values occur in the FITS tables where there is missing or un-matched data. The Null values for each parameter are as follows:
Image Areas: 0
Ellipticities: 9.999
Magnitudes: 99.999
Profile Statistic N(0,1): 999.99
Blend and Quality parameters: 99999

A secondary magnitude of 99.999 indicates one of the following:

  • the exposure 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
  • pairing problems

Parameters in the FITS tables
Parameters in tab-separated listing

Description of FITS table object parameters

FITS table 1

The first FITS 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.

Each object is described 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)

1RADoubleCelestial Right Ascensionradians
2DECDoubleCelestial Declinationradians
3l2RealGalactic longitudedec.degrees
4b2RealGalactic latitudedec.degrees
5R_HaRealH-alpha equivalent R-magmags
6SR2Realmatching SR magnitude (SR2)mags
7SR1RealFirst epoch SR magnitude (SR1)mags
8IRealFirst epoch I magnitudemags
9AREA_HaIntegerTotal area H-alpha imagepixels
10AREA_SR2IntegerTotal area SR2 imagepixels
11AREA_SR1IntegerTotal area SR1 imagepixels
12AREA_IIntegerTotal area I imagepixels
13ELL_HaRealH-alpha image ellipticity
14ELL_SR2RealSR2 image ellipticity
15ELL_SR1RealSR1 image ellipticity
16ELL_IRealI image ellipticity
17PRFSTT_HaRealH-alpha N(0,1) profile classification statistic0.001 sigma
18PRFSTT_SR2RealSR2 N(0,1) profile classification statistic0.001 sigma
19PRFSTT_SR1RealSR1 N(0,1) profile classification statistic0.001 sigma
20PRFSTT_IRealI N(0,1) profile classification statistic0.001 sigma
21BLEND_HaIntegerH-alpha Deblending flag (0 if not deblended)
22BLEND_SR2IntegerSR2 Deblending flag (0 if not deblended)
23BLEND_SR1IntegerSR1 Deblending flag (0 if not deblended)
24BLEND_IIntegerI Deblending flag (0 if not deblended)
25QUAL_HaIntegerH-alpha image Quality flag
26QUAL_SR2IntegerSR2 image Quality flag
27QUAL_SR1IntegerSR1 image Quality flag
28QUAL_IIntegerI image Quality flag
29PAIntegerCelestial position angle (selected band)degrees
30CLASSIntegerClassification flag (selected band)

FITS table 2

The second table lists all 32 parameters as measured/extracted from the exposure 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)

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 the same 30 parameters (plus a running ID number) as for the first FITS table described above. These parameters include 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 H-alpha, matching SR2 exposure with placeholders containing null values for the SR1/I data that will eventually be matched in from an earlier survey of the Galactic Plane. The second table in the FITS file contains all 32 IAM parameters in the selected band.

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 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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