Most digital photographs are saved in JPEG compressed format. For astronomy, JPEG is not ideal because it is a "lossy" form of compression; low-contrast detail is discarded, and that may be exactly the detail you want to preserve and enhance. What's more, operations such as dark-frame subtraction rely on images that have not been disrupted by lossy compression.
For that reason, astrophotographers normally set the camera to produce raw images which record exactly the bits recorded by the image sensor (or nearly so; some in-camera corrections are performed before the raw image is saved).
The term raw is not an abbreviation and need not be written in all capital letters; it simply means "uncooked" (unprocessed). Filename extensions for raw images include .CRW and .CR2 (Canon Raw) and .NEF (Nikon Electronic Format). Adobe (the maker of Photoshop) has proposed a standard raw format called .DNG (Digital Negative) and has started distributing a free software tool to convert other raw formats into it.
Raw images are compressed - their size varies with the complexity of the image - but the compression is lossless or nearly so; the exact value of every pixel in the original image is recovered when the file is decoded in the computer. Canon and Nikon raw images occupy about 1 megabyte per megapixel, varying with the complexity of the image. Uncompressed, a 12-bit-deep color digital image would occupy 4.5 megabytes per megapixel.
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