Getting Started

The Digitized Sky Survey (DSS) is a ground-based imaging survey of the entire sky in several colors. The survey, performed with Palomar and UK Schmidt telescopes, produced photographic plates that were later digitized at STScI to produce the Hubble Guide Stars Catalog (GSC).

Each plate produced by the Survey covers 6.5 x 6.5 degrees of the sky, and the plates have been digitized using a modified PDS microdensitometer. The digital images have a pixel size of either 25 microns (1.7 arcsec per pixel) or 15 microns (1.0 arcsec per pixel), and are 14000 x 14000 or 23040 x 23040 npixels perl side. The images are stored on 12-inch optical media and are difficult to access quickly.

In order to provide convenient access to this valuable resource, 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 written to CDROM which are then placed into jukeboxes for rapid access. Users can then easily retrieve image data for any part of the sky.

DSS images may be retrieved through the Web. To retrieve an image, you supply either a position (RA and Dec) or a target name, for which the position will be looked up in either the SIMBAD or NED online databases. You also specify the format and size of the image and the survey from which to extract, and the image will be delivered directly back though your browser. There is also an interface that will show you which plates your image could be extracted from, and allows you to select images from a particular plate.

Images are also accessible through StarView, and other interfaces such as the Visual Target Tuner (VTT), Aladin, and xephem access online versions of the DSS. The DSS is also available through the web from other web sites as well as at STScI.

Some information is available at this site for computing astrometry and photometry of DSS images.