Guide to image data

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. 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.

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

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

FITS table 1

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.	Name	Type	Description	Units
1	RA	Double	Celestial Right Ascension	radians
2	DEC	Double	Celestial Declination	radians
3	MU_ACOSD	Real	Proper motion in RA	mas/yr
4	MU_D	Real	Proper motion in Dec	mas/yr
5	SIGMU_A	Real	Error on proper motion in RA	mas/yr
6	SIGMU_D	Real	Error on proper motion in Dec	mas/yr
7	B_J	Real	UK-J (Bj) magnitude	mags
8	R_1	Real	ESO-R or POSS-I E magnitude	mags
9	R_2	Real	UK-R magnitude	mags
10	I	Real	UK-I magnitude	mags
11	AREA	Integer	Total area	pixels
12	A_I	Integer	Weighted semi-major axis	0.01 microns
13	B_I	Integer	Weighted semi-minor axis	0.01 microns
14	P_A	Integer	Celestial position angle	degrees
15	CLASS	Integer	Classification flag
16	N(0,1)	Integer	N(0,1) profile classification statistic	0.001 sigma
17	BLEND	Integer	Deblending flag (0 if not deblended)
18	QUALITY	Integer	Quality flag

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 parameters returned are given below:

No.	Name	Type	Description	Units
1	RA		Celestial Right Ascension	degrees
2	DEC		Celestial Declination	degrees
3	MU_ACOSD	Real	Proper motion in RA	mas/yr
4	MU_D	Real	Proper motion in Dec	mas/yr
5	SIGMU_A	Real	Error on proper motion in RA	mas/yr
6	SIGMU_D	Real	Error on proper motion in Dec	mas/yr
7	B_J	Real	UK-J (Bj) magnitude	mags
8	R_1	Real	ESO-R or POSS-I E magnitude	mags
9	R_2	Real	UK-R magnitude	mags
10	I	Real	UK-I magnitude	mags
11	AREA	Integer	Total area	pixels
12	A_I	Integer	Weighted semi-major axis	0.01 microns
13	B_I	Integer	Weighted semi-minor axis	0.01 microns
14	PA	Integer	Celestial position angle	degrees
15	CLASS	Integer	Classification flag
16	N(0,1)	Integer	N(0,1) profile classification statistic	0.001 sigma
17	BLEND	Integer	Deblending flag (0 if not deblended)
18	QUALITY	Integer	Quality 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 .