Modes of Measurement

There are essentially 3 modes of measurement on SuperCOSMOS:-
"Little Image Mode", "Mapping Mode" and "Image Analysis Mode".

Little Image Mode (LIM):-

The purpose of Little Image Mode is to allow several small areas of the plate to be scanned (minimum size 12.8mm x 12.8 mm), producing a Starlink NDF 2-D image for each small area. This mode is useful if there are only 1 or 2 small areas of interest on the plate (e.g. a comet or galaxy cluster) and data for the entire plate are not generally required. However, if there are several areas to be extracted, then it is typically more efficient to scan the plate in Mapping Mode and to use the software utilities "bigimtondf" and "blocktondf" (see section on Processing software) to extract the areas of interest.

Mapping Mode (MM):-

Mapping mode is the main backbone of the SuperCOSMOS scanning methodology. In this mode, all pixels are stored on the output disk (up to 2 Gbytes for each Schmidt Telescope plate). This mode is a necessary prerequisite for Image Analysis Mode. While the data are output by the transputers to the offline disks, the transputers are also computing the sky background in squares of 128 x 128 pixels over the plate.

The Mapping Mode pixel data for each plate are routinely backed up onto high capacity tape (exabyte or DLT) for long-term archival storage. The entire survey plate data will eventually be stored in this manner. Standard software packages (e.g. "bigimtondf" and "blocktondf") are available to run on these data and to provide 2-D images in Starlink NDF format which can be viewed by any of the standard Starlink software (e.g. Kappa).

Image Analysis Mode (IAM):-

This is the mode which is of most use to 90% of astronomy projects. In this mode the pixel data from Mapping Mode are processed using the COSMOS crowded-field analysis package (Beard et al. 1990, MNRAS, 247, p311) producing as a result a catalogue of all the images detected above a threshold set for detection (a % above the local sky intensity level). Some 32 parameters are stored for each object, and the object list includes "parent" objects as well as "daughter" objects. TRansformation of x,y on the plate to celestial coordinates on the sky (Right Ascension and Declination) is performed through cross-referencing with the PPM reference star calogue. The resulting celestial coordinates are output in the FK5 system with equinox J2000 adopted as standard.