Next Contents Previous

3.14. Scanned Data

3.14.1. The Digital Sky Survey

As part of the activities leading to the Guide Star Catalog (GSC) photographic plates covering the whole sky where scanned. For the northern hemisphere, a new set of shorter exposure plates (the Quick-V plates) were obtained. For the south the ESO/SERC Southern J plates were used. In addition some regions were covered by special plates. In parallel with the scanning process, the original Palomar Observatory Sky Survey (POSS-I) XE plates were scanned. The last and the southern plates constitute the Digitized Sky Survey (DSS). A detailed description of the GSC, its contents and the plates can be found in Lasker et al. (1990), Russell et al. (1990) and Jenkner et al. (1990).

Once these scans were finished the two PDS machines at STScI were completely refurbished. After an agreement with the Palomar Observatory, the plates that constitute the POSS-II are being scanned. All these scans are available on-line at the following URL

The original scans were made with a pixel size of 25µm, resulting in a 14,000 x 14,000 pixel image with a scale of 1.67"/pix. A 512 x 512 image thus covers an area of 14.3' x 14.3'. The new scans are done with a pixel size of just 15µm resulting in a raster of 23,000 x 23,000 pixels.

Please note that to provide coordinates for targets to be observed with the Hubble Space Telescope it is necessary to use only the plates used to create the GSC and not the plates from the POSS-I.

The Digitized Sky Survey is a set of CD-ROMs containing the POSS-I XE and SERC J scanned plates compressed 10X. It covers the whole sky. A 100X compressed version, called RealSky is presently available only for the northern sky, with the southern part to be released soon. Both sets are sold by the Astronomical Society of the Pacific.

Table 3, which is based on Lasker et al 1996, describes some characteristics of these scans.

Table 3. Scanned plates at STScI available on-line.

Survey /
Epoch Band Dec range Limit
pixel size

Pal QV 1983-85 V +90:+06 19.5 25 GSC
SERC J 1975-87 J -20:-90 23.0 25 GSC
SERC EJ 1979-88 J -00:-15 23.0 25
POSS-I 1950-58 E +90:-30 20.0 25 up to dec -18.0
POSS-I 1950-58 O +90:-30 21.0 25 selected plates
POSS-II 1987-96 J +90:+00 22.5 15
POSS-II 1987-97 R +90:+00 20.8 15
POSS-II 1987-97 N +90:+00 19.5 15
SES 1990-96 R -20:-90 22.0 15
SERC ER 1990-95 R -00:-15 22.0 15
SERC 1986-88 V 14.0 25 galactic plane; GSC
SERC 1979 V 14.0 25 LMC; GSC
SERC 1985 V 14.0 25 SMC; GSC
SERC 1985 V 14.0 25 M31; GSC

These new scans will be used to construct the GSC-II. This updated version will include 2 x109 objects (100 times more than GSC-I), at least 2 epochs and 2 bands; proper motions will be calculated with an error less than 4.0 mas/yr and a magnitude error between 0.1 and 0.2mag (GSC-I did not contain proper motion information and the magnitude error is 0.4mag).

3.14.2 Automatic Plate Scanner

The University of Minnesota created the Automatic Plate Scanner Catalog (APS) from the scans of the POSS-I O and E plates with |b| > 20°. The catalog consists of a list of classified objects found on both plates using a neural network algorithm to separate the stellar and non-stellar objects. The two catalogs (one for each band) are available on-line (, they contain coordinates, magnitudes, colors and other parameters. They provide information on 108 stars and around 107 galaxies up to a limiting magnitude of less than 21 in the blue.

3.14.3 Plate database

The Institute of Astronomy of the Bulgarian Academy of Sciences maintains a database of Wide-Field Plates ( The information is standardize and lists the object name, emulsion and filter, J2000 coordinates and the UT of the exposure.

Next Contents Previous