The Catalogs and Surveys Branch of the Space
Telescope Science Institute has been digitizing the photographic Sky Survey
plates from the Palomar and UK Schmidt telescopes in order to support HST operations
and provide a service to the astronomical community.
These plates cover 6.5 x 6.5
degrees of the sky and have been digitized using a modified PDS
microdensitometer with a pixel size of either 25 or 15 microns (1.7 or 1.0
arcsec respectively) and the resultant images are stored on optical discs.
These images are 14000x14000
(0.4GB) or 23040x23040 pixels (1.1GB) in size and are difficult to access
quickly.
In order to provide convenient access to these data, the
images have been compressed using a technique based on the H-transform to reduce
the data volume. Although the technique is lossy, it is adaptive so that
it preserves the signal very well. We typically compress the data by a factor
of 10 but much higher compression ratios are possible. These compressed data
are then placed on CDROM and placed into a jukebox for rapid access. Users can
then easily retrieve image data for any part of the sky.
The first epoch images have already been published on CDROM, and the second epoch POSS-II and SES surveys (DSS-II) will be distributed widely to the community once they are completed. At the current time, CDROMs of the prepublication data are only being distributed to a small number of institutions who are supporting the CASB effort to digitize and compress these survey plates.
All data are available on CD-ROM(102 CD-ROM). Documentation is available on disk, on-line, and as hard copy. Software (FORTRAN and C) programs for data retrieval are provided on CD-ROM.Data Volume 60GB