RealSky is the name of a commercially available, digital photographic sky atlas. It is a subset of the Digitized Sky Survey

Digitized Sky Survey

The Digitized Sky Survey is a digital version of several photographic atlases of the night sky, and an ongoing project to produce more digital versions of photographic astronomical datasets.- Versions and source material :...

 (DSS) and was published in 1996 by the Astronomical Society of the Pacific

Astronomical Society of the Pacific

The Astronomical Society of the Pacific is a scientific and educational organization, founded in San Francisco on February 7, 1889. Its name derives from its origins on the Pacific Coast, but today it has members all over the country and the world...

.

RealSky is composed of the "RealSky" product, covering the northern sky, and "RealSky South," published slightly later and covering the southern sky. RealSky is on nine CD-ROM

CD-ROM

A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....

s (eight data discs and one software disc), and RealSky South uses 11 CD-ROMs (ten data discs and one software disc).

Major differences from DSS

1. DSS files were compressed by a factor of 7, but RealSky files are compressed by a factor of roughly 100. RealSky consequently takes up less space, but the additional compression makes it inappropriate for use in photometry
   Photometry (astronomy)
   Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
   
   , and fine detail in the images is degraded.
2. The DSS covers the whole sky in two colors, red and blue; but RealSky provides only red coverage of the northern hemisphere while RealSky South provides only blue coverage of the southern hemisphere.
3. The DSS includes both long and short exposures for the area around the Andromeda Galaxy
   Andromeda Galaxy
   The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
   
   , but RealSky provides only the short exposure plate.