5.1. Digitized Sky Survey Images
Digitizations of the POSS E plates in the northern sky and UKST BJ plates in the southern sky are available from SkyView at URL skyview.gsfc.nasa.gov. For the vast majority of astronomical institutions without a CD-ROM juke box, this web service allows much easier access to the DSS than working locally with the set of 102 CD-ROMs of the ASP. Batch requests for large lists of DSS extractions can be formulated from the command line using server URLs based on perl scripts (see skyview.gsfc.nasa.gov/batchpage.html or www.ast.cam.ac.uk/~rgm/first/collab/first_batch.html). Several other sites offer the standard DSS through their servers, e.g. the CADC at cadcwww.dao.nrc.ca/dss/dss.html, ESO at archive.eso.org/dss/dss, or Skyeye at Bologna (www.ira.bo.cnr.it/skyeye/gb), the University of Leicester in UK (ledas-www.star.le.ac.uk/DSSimage), and NAO in Japan at dss.mtk.nao.ac.jp. Each of these offer a slightly different ``look and feel'', e.g. CADC offers absolute coordinates labels around the charts (but only in J2000), SkyEye offers simultaneous extraction of objects from DIRA2 catalogues (Section 4.2) and an easy batch request for charts via email, etc. A comparison of performance and speed between these DSS servers can be found at star-www.rl.ac.uk/~acc/archives/archives.html (see [Charles & Shayler (1998)]).
Meanwhile almost all of the ``second-epoch survey'' (SES) plates have been taken: the northern POSS-II (ecf.hq.eso.org/iraf/ftp/iraf/conf/web/adass_proc/adass_95/laskerb/laskerb.html) at Palomar, and the southern UKST SES-R survey at AAO (www.roe.ac.uk/ukstu/ukst.html). Most of these have been digitized. Early in 1998 the STScI server (archive.stsci.edu/dss/dss_form.html) was the only one offering the second-epoch surveys (POSS-II or UKST R), if available, and otherwise POSSI or UKST BJ surveys.
A digitization of the POSS-I E- and O-survey plates was also performed with the Automated Plate Scanner (APS) at the Astronomy Department of the University of Minnesota. Only those plates (E and O) with |b| > 20° have been scanned. The APS uses a flying laser spot to record the transmission of each plate only above ~ 65% of the background transmittance as determined in an initial low-resolution scan. This compromise was needed to achieve the small pixel size of 0.3'' for the final images which contain only signal above background, i.e. their background is black. They can be retrieved in FITS format from aps.umn.edu/homepage.aps.html.