Digitized Sky Survey
Digitized photographic surveys of the night sky for astronomy.
The Digitized Sky Survey (DSS) is a set of digital scans taken from photographic surveys of the entire night sky, created by the Space Telescope Science Institute. Scanning began around 1988–1989, with the first digital release in 1994 used to generate the first Guide Star Catalog. Later, a second generation of scans was added, drawn from newer photographic surveys.
For the first generation, the northern sky was covered almost entirely by the red-sensitive E-band plates from the National Geographic Society – Palomar Observatory Sky Survey. The southern sky used the blue J-band plates from the ESO/SERC Southern Sky Atlas, along with the blue J-band plates from the SERC-J Equatorial Extension, both taken at the UK Schmidt Telescope in Australia. Three additional V-band plates with shorter exposures were included for the Andromeda Galaxy and the Magellanic Clouds. This first digital collection is now called the First Generation DSS.
The second generation, or DSS2, covers three spectral bands: blue, red, and near-infrared. The blue and red bands were released together in 2006. For the northern sky, it used the J (blue), F (red), and N (near-infrared) plates from the Palomar Observatory Sky Survey II, with the F-band providing the red coverage. For the southern sky, the red band combined several surveys from the UK Schmidt Telescope: the short red plates of the SERC I/SR Survey, the Equatorial Red survey, and the F-band Second Epoch Survey.
The survey was produced by the Catalogs and Survey Branch at the Space Telescope Science Institute. Plates were scanned with Perkin-Elmer PDS 2020G microdensitometers. First-generation scans used a pixel size of 25 micrometres (1.7 arcseconds), producing images of 14,000 by 14,000 pixels (about 0.4 gigabytes each). Second-generation scans used 15-micrometre pixels (1.0 arcseconds), yielding 23,040 by 23,040 pixels (about 1.1 gigabytes each). A single first-generation scan took just under seven hours. Because the images were so large, they were compressed with a lossy but adaptive H-transform algorithm, which preserved most of the original information and shrank first-generation files by a factor of seven. For the second generation, the microdensitometers were modified for multi-channel operation to keep scan times under 12 hours per plate.
The branch also released companion scientific products, most notably a photometric c
- producer
- Space Telescope Science Institute (Catalogs and Survey Branch)
- first_release
- 1994
- second_release
- 2006
- field
- Astronomical imaging and cataloging
- known_for
- Digitized all-sky photographic atlas for the Guide Star Catalog
- scanning_resolution_dss1
- 25 micrometers (1.7 arcseconds)
- scanning_resolution_dss2
- 15 micrometers (1.0 arcseconds)
Lore & Background
The Digitized Sky Survey was produced by the Catalogs and Survey Branch of the Space Telescope Science Institute. They scanned photographic plates using one of two Perkin-Elmer PDS 2020G microdensitometers. For the First Generation (DSS1), pixel size was 25 micrometers, resulting in images 14,000 x 14,000 pixels in size, scanned in under seven hours per plate. For the Second Generation (DSS2), pixel size was 15 micrometers, yielding images 23,040 x 23,040 pixels, with scan times kept under 12 hours per plate using modified multi-channel microdensitometers.
The source material varied by hemisphere and generation. For DSS1, the northern sky used the National Geographic Society – Palomar Observatory Sky Survey E-band (red) plates; the southern sky used the J-band (blue) of the ESO/SERC Southern Sky Atlas and the quick V-band SERC-J Equatorial Extension from the UK Schmidt Telescope. DSS2 added three spectral bands (blue, red, near infrared), using F-band plates from Palomar Observatory Sky Survey II for the north and several red surveys from the UK Schmidt Telescope for the south.
Due to large file sizes (about 0.4 GB for DSS1 and 1.1 GB for DSS2 per image), images were compressed using an adaptive, lossy H-transform algorithm. DSS1 files were typically shrunk by a factor of seven. In 1996, a more highly compressed version called RealSky was published, with compression by a factor of roughly 100, though this degraded fine detail and made it unsuitable for photometry.
Reader's Guide
The Digitized Sky Survey holds significance as a foundational digital resource for astronomy, providing wide-field, all-sky coverage from photographic plates that predate modern digital surveys. Its primary legacy is enabling the creation of the Guide Star Catalog, which was essential for pointing the Hubble Space Telescope. By digitizing multiple photographic surveys across different spectral bands and epochs, the DSS allowed astronomers to access historical sky data electronically, facilitating cross-matching with newer observations and enabling studies of variable objects, proper motions, and large-scale structures.
The survey's two generations reflect technological progress: DSS1 offered a first digital all-sky atlas at moderate resolution, while DSS2 improved resolution and added multiple color bands. The accompanying photometric calibration for parts of DSS1 allowed quantitative measurements from the digital images. Although the lossy compression limited some scientific uses, the DSS remains widely used for archival research, education, and as a background for sky visualization tools. Its online availability through multiple facilities ensures continued access for the astronomical community.
Did You Know?
- The Digitized Sky Survey was produced by the Catalogs and Survey Branch of the Space Telescope Science Institute between 1983 and 2006.
- First Generation DSS images were 14,000 x 14,000 pixels, while Second Generation images were 23,040 x 23,040 pixels.
- A more highly compressed version called RealSky was published in 1996, with compression by a factor of roughly 100.
- The scanning of First Generation DSS took a little under seven hours per plate, while Second Generation scans were kept under 12 hours using modified multi-channel microdensitometers.
Frequently Asked Questions
Who is Digitized Sky Survey?
The Digitized Sky Survey is a digital all-sky imaging product created by the Space Telescope Science Institute's Catalogs and Survey Branch. It consists of high-resolution scans of photographic plates that covered the entire night sky, first released to the public in 1994.
What are Digitized Sky Survey's powers/role?
Its core function is to convert old photographic sky surveys into digital format at a scanning resolution of 25 micrometers, roughly 1.7 arcseconds per pixel. This made it possible to build the Guide Star Catalog and gave astronomers a uniform digital atlas of the whole sky.
How does Digitized Sky Survey's story end?
The narrative closes with the second-generation release in 2006, which incorporated scans drawn from more recent photographic surveys. That update broadened the coverage and kept the digital atlas relevant as instrumentation continued to evolve.
Why is Digitized Sky Survey important?
It was the first project to digitize complete photographic sky surveys, turning glass plates into a searchable digital resource. The 1994 first release directly fed the initial Guide Star Catalog, which became a foundational reference for Hubble and other space-based observations.
What is Digitized Sky Survey's origin story?
Scanning work kicked off around 1988–1989 at the Space Telescope Science Institute, using red-sensitive E-band Palomar plates for the northern sky and blue J-band UK Schmidt plates for the south. The effort was driven by the need for a reliable digital star catalogue to support the Hubble Space Telescope's pointing system.
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