Digitized Sky Survey Information

Who controls the datarights for these images?

What format are the images?

  • The original digitized images are archived as FITS files
  • Most data centers have tile-compressed binary files + software to decompress and create image cutouts dynamically.
  • The different services can download these images as FITS files and optionally as GIF or JPG files

What is the image pixel size?

  • The original DSS-I plates were scanned at a resolution of 1.7 arcsec per pixel.
  • The later DSS-II plates were scanned at a resolution of 1.0 arcsec per pixel.

What do the pixel values represent?

  • The DSS images are digitized photographic plates so the images are 'negatives'. The pixel values are a measure of the photographic density of the original plate. The photographic density is a non-linear function of the incident intensity (unlike modern digital detectors). Special calibrations are needed to quantify density-intensity functions.

What are typical problems in the images?

  • Photographic plates consist of a thin emulsion of light-sensitive material layered onto a glass substrate (14x14 inches) that is then placed into a holder which bends it to the telescope focal plane. This makes it susceptible to several problems:
    • Emulsion scratches
    • Dust laying on emulsion (either during exposure OR when being scanned)
    • Fingerprints!
    • Internal telescope reflections (if close to a very bright star)
  • Satellite trails (straight line sometime varying in brightness if the sattelite is rotating/tumbling)
  • Aircraft trails (typically 2 or 3 parallel lines - body/wing lights)

Are these images available elsewhere?

  • Yes - a number of astronomical data centers have copies of these data and provide services to access them.  We are unable to provide a complete list but here is a partial list of 'stable' sites.
  • Color versions (using red/blue/nir plates) are also available


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