Digitisation and archiving

The photographic material is digitised to 15 bits (32768 grey levels) at a resolution of 10 microns (0.67 arcsecs on UKST plates). The central 320 x 320 mm (6 x 6 square degrees) of a UKST plate is scanned in a timescale of 2 hours, producing over 2 Gbytes of pixel data. These data are processed in a routine manner using the COSMOS crowded-field algorithm (Beard et al 1990) to produce a catalogue of the sources (stars, nebulae, etc) detected above a pre-selected isophotal threshold level.

Thirty two parameters are determined for each source, including coordinates on the plate, size, shape, orientation and integrated brightness. Transformation of plate positions to coordinates on the sky is undertaken using the PPM catalogue at present. The Tycho/Hipparcos catalogues will be used once fully interfaced into the SuperCOSMOS system. Typically, plate solutions with residuals in Right Ascension and Declination of order 0.20-0.25 arcsecs are obtained currently, from cross-referencing with PPM.

The survey itself will consist of 200 films with field centres separated by 4 degrees. The central 5 x 5 square degrees on each film will be digitised, thereby allowing overlap from field to field, and resulting in 1.37 Gbytes of data from each. The total pixel data from the 200 field set will occupy some 275 Gbytes. All data (both pixels and point sources) will be archived on high capacity (20 Gbyte) exabyte tape. For safekeeping the "master" backup of the H-alpha survey scan data will be held at the ROE, while a copy will be shipped to the AAO.

© Copyright Astronomical Society of Australia 1997