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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
Accessing the Digitized Sky Survey
J. E. Morrison
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore
MD 21218
Abstract. The Digitized Sky Survey is now available on CD­ROMs and
will be on the ST ScI archive. This is a whole sky survey constructed from
the Palomar E and SERC J Schmidt surveys. We present an overview of
software produced by the ST ScI STSDAS group to access and manipulate
these images. We have developed portable code to use in the IRAF
environment, hopefully in the future this we be available as a stand alone
package.
1. Introduction
ST ScI has digitized Schmidt plates of the entire Sky. Due to the massive volume
of data involved (a total of about 600 Gbytes) the scans have been compressed
by a factor of 10 and are made available in the form of 100 CD­ROMs. Software
to give the astronomical community access to these scans is being developed by
the STSDAS and Guide Stars groups at ST ScI.
2. Background Information: Plates and Scans
The northern­hemisphere data are from scans of the E plates of the National
Geographic Society -- Palomar Observatory Sky Survey (NGS­POSS) obtained
using the Oschin Telescope on Palomar Mountain. These plates are deep 103a­
E exposures obtained through a red plexiglass filter. The southern­hemisphere
plates were obtained using the UK Schmidt Telescope while it was operated
by the Royal Observatory Edinburgh (ROE). These are deep IIIa­J exposures
obtained through a GG 395 filter. The scans were made at the Space Telescope
Science Institute using the scanning microdensitometers, which are described in
Lasker et al. (1990). A pixel size of 25 ¯m with a 50 ¯m apodized aperture was
used throughout.
3. Software
The original plate images were composed of 14,000 by 13,999 pixels (a pixel is
1.70 arcseconds square). To allow fast access to any portion of the plate, the
image was divided into blocks of 500 by 500 pixels that were separately com­
pressed by a factor of ten. Experiments to study the degradation of astrometry
on the compressed images compared to the original image were performed by
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White et al. (1992), who concluded that the astrometry is hardly affected by
the compression for modest compress factors (up to about a factor of 20).
The software to read the compressed images is provided with the set of CD
ROMs and is also a new task in the GASP/STSDAS package called getimage.
In the getimage software the user specifies the region of interest. The software
then accesses the data, decompresses the scans and returns the data in a form
of a photographic intensity as a function of x and y plate pixel coordinates.
New tasks are being added to the GASP package that will allow the user to
manipulate this data. At this time these tasks use the original plate parameters
produced by a single global model reduction of the plates. In the foreseeable
future the astrometry will use a more sophisticated method.
4. New Tasks in or to be Added to GASP/STSDAS
getimage accesses the Digitized Sky Survey (DSS) CD ROMs and decom­
pressed the data returning a photographic intensity as a function of x and
y pixel coordinates. This software package is available on the CD ROM
set and was written by Jesse Doggett and placed into GASP/STSDAS by
Allen Farris.
dss targets extracts Guide Star Catalog (GSC) stars from the CD­ROM Guide
Star Catalog in the field specified by the user and overlays the these stars
on an DSS image produced by getimage.
dss pxcoord given a table of GSC stars (or other table of equatorial coordi­
nates) finds the pixel coordinates corresponding to the DSS image.
dss xyeq procedure to calculate the equatorial coordinates from a set of (x,y)
pixels using the original plate parameters.
dss eqxy procedure to calculate the pixel coordinates from a set of equatorial
coordinates using the original plate parameters.
Acknowledgments. I would like to acknowledge Jeffrey Hayes, Brian
McLean, and Jesse Doggett for their scientific assistance.
References
Lasker, B., Sturch, C. R., McLean, B. J., Russell, J. L., & Jenkner, H. 1990,
AJ, 99, 2019
White, R. L., Postman, M., & Lattanzi, M. G. 1992, Digitized Optical Sky
Surveys, 167

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Figure 1. An image from the Digitized Sky Survey: center (0: h 0,
\Gamma89: ffi 0), and size: 0: ffi 2. Produced by getimage and dss targets tasks.
Getimage was used to extract the data, while dss targets was used
to display the image, search the Guide Star Catalog for stars in the
region, and then plot white circles around the GSC stars found in the
image.