Description of the FITS filesAs well as the parameterised data stored in the SQL database tables each 6dFGS target object has a multi-extension FITS file (named targetname.fits)
The first 5 extensions contain thumbnail images with built in WCS. The optical B & R images come from SuperCOSMOS scans of blue and red survey plates. The 2MASS J, H & K images were extracted from datacubes supplied by IPAC. NB Although some objects in TARGET do not have 2MASS images the corresponding extensions still exist in the FITS file but contain small placeholder images . The remaining extensions contain the spectra. Each 6dFGS observation will usually result in a further 3 extensions, the V band spectrum, the R band spectrum and the combined/spliced VR spectrum. If an object is observed more than once further spectral extensions are added e.g. if an object is observed 3 times its FITS file will hold the 5 image extensions and 9 spectral extensions.
In cases of orphan fields, which contain only one of either V or R, the other extensions (R or V and VR) still exist but are just copies of the single passband frames.
The V and R extensions are images with 3 rows. The 1st row is the observed reduced SPECTRUM, the 2nd row is the associated variance and the 3rd row stores the SKY spectrum as recorded for each data frame. Wavelength information is provided in the header keywords: CRVAL1, CDELT1, CRPIX1. In that
wavelength(Angstroms) = CRVAL1 - (CRPIX1 - pixel no) x CDELT1
The VR extension also has an additional 4th row that represents the wavelength axis explicitly (as well as via WCS keywords), which has a continuous dispersion, achieved through the continuation of the V dispersion into the R half from rescrunching.
Additional WCS keywords are also included to ensure the wavelength information is displayed correctly when using image browsers such as Starlink's GAIA or SAOimage DS9.
Home | Intro | Schema | Access | FITS files
WFAU, Institute for Astronomy,