Astronomical CCD cameras are intended for imaging very faint objects.  However, all CCD cameras generate heat which can be detected by the sensitive CCD chip.  This appears as noise in an image which can quickly overwhelm the image of a very faint object.  To minimize this noise CCD cameras are cooled.  In a typical CCD camera, for every 5°C (9°F) change in temperature the noise is halved.  Astronomical CCD cameras typically cooled to about 30°C (80°F) below ambient temperature.  Colder is always better, especially in a climate like the southwest US where summer nighttime temperatures might still be around 85°F!

Even after being cooled, very sensitive CCDs will still detect what little noise remains.  To remove this remaining noise, dark frames are used.  A dark frame is an image taken with the camera's shutter closed.  This detects only the noise.  When the dark frame is subtracted from the image of an object, the noise is subtracted while the rest of the image is unaffected.

Newer CCD cameras use chips that have much less inherent noise.  Many modern CCDs do not require active cooling systems, and some can be used without requiring dark frames.

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