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
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.