Guide to image data from the SHS

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

• pixel data (included in FITS file)
• object catalogues (included as tables in the FITS file & as a tab-separated list)
• GAIA software

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.

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

• Object parameters in table 1 in the FITS file
• Object parameters in table 2 in the FITS file

FITS table 1

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)

No.	Name	Type	Description	Units
1	RA	Double	Celestial Right Ascension	radians
2	DEC	Double	Celestial Declination	radians
3	l2	Real	Galactic longitude	dec.degrees
4	b2	Real	Galactic latitude	dec.degrees
5	R_Ha	Real	H-alpha equivalent R-mag	mags
6	SR2	Real	matching SR magnitude (SR2)	mags
7	SR1	Real	First epoch SR magnitude (SR1)	mags
8	I	Real	First epoch I magnitude	mags
9	AREA_Ha	Integer	Total area H-alpha image	pixels
10	AREA_SR2	Integer	Total area SR2 image	pixels
11	AREA_SR1	Integer	Total area SR1 image	pixels
12	AREA_I	Integer	Total area I image	pixels
13	ELL_Ha	Real	H-alpha image ellipticity
14	ELL_SR2	Real	SR2 image ellipticity
15	ELL_SR1	Real	SR1 image ellipticity
16	ELL_I	Real	I image ellipticity
17	PRFSTT_Ha	Real	H-alpha N(0,1) profile classification statistic	0.001 sigma
18	PRFSTT_SR2	Real	SR2 N(0,1) profile classification statistic	0.001 sigma
19	PRFSTT_SR1	Real	SR1 N(0,1) profile classification statistic	0.001 sigma
20	PRFSTT_I	Real	I N(0,1) profile classification statistic	0.001 sigma
21	BLEND_Ha	Integer	H-alpha Deblending flag (0 if not deblended)
22	BLEND_SR2	Integer	SR2 Deblending flag (0 if not deblended)
23	BLEND_SR1	Integer	SR1 Deblending flag (0 if not deblended)
24	BLEND_I	Integer	I Deblending flag (0 if not deblended)
25	QUAL_Ha	Integer	H-alpha image Quality flag
26	QUAL_SR2	Integer	SR2 image Quality flag
27	QUAL_SR1	Integer	SR1 image Quality flag
28	QUAL_I	Integer	I image Quality flag
29	PA	Integer	Celestial position angle (selected band)	degrees
30	CLASS	Integer	Classification flag (selected band)

FITS table 2

No.	Name	Type	Description	Units
1	RA	Double	Celestial Right Ascension	radians
2	DEC	Double	Celestial Declination	radians
3	XMIN	Integer	Left extent	0.01 microns
4	XMAX	Integer	Right extent	0.01 microns
5	YMIN	Integer	Bottom extent	0.01 microns
6	YMAX	Integer	Top extent	0.01 microns
7	AREA	Integer	Total area	pixels
8	IPEAK	Integer	Brightest intensity above sky
9	MAG	Real	COSMOS magnitude	magnitudes
10	ISKY	Integer	Sky intensity at XCEN_I, YCEN_I
11	XCEN_I	Integer	Intensity weighted X centroid	0.01 microns
12	YCEN_I	Integer	Intensity weighted Y centroid	0.01 microns
13	A_U	Integer	Unweighted semi-major axis	0.01 microns
14	B_U	Integer	Unweighted semi-minor axis	0.01 microns
15	THETA_U	Integer	Unweighted orientation	degrees
16	A_I	Integer	Weighted semi-major axis	0.01 microns
17	B_I	Integer	Weighted semi-minor axis	0.01 microns
18	THETA_I	Integer	Weighted orientation	degrees
19	CLASS	Integer	Classification flag
20	P_A	Integer	Celestial position angle	degrees
21	AP(1)	Integer	Area above areal profile level 1	pixels
22	AP(2)	Integer	Area above areal profile level 2	pixels
23	AP(3)	Integer	Area above areal profile level 3	pixels
24	AP(4)	Integer	Area above areal profile level 4	pixels
25	AP(5)	Integer	Area above areal profile level 5	pixels
26	AP(6)	Integer	Area above areal profile level 6	pixels
27	AP(7)	Integer	Area above areal profile level 7	pixels
28	AP(8)	Integer	Area above areal profile level 8	pixels
29	BLEND	Integer	Deblending flag (0 if not deblended)
30	QUALITY	Integer	Quality flag
31	N(0,1)	Integer	N(0,1) profile classification statistic	0.001 sigma
32	PRFMAG	Integer	SuperCOSMOS profile magnitude	millimags

Description of object parameters in tab-separated list

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 .