Starguide - A Practical Guide to Astro-Photography

Dark Frames are used to capture dark calibration frames and belong to the group of calibration frames. 

Important: 

• Cover your camera: your camera or scope must be covered to block all light off the sensor
• GAIN must be set to an identical value as the light frames you want to calibrate
• Exposure time must be the same as for the light frames
• No. of Dark frames: they require a lot of time, so taking about 20 Dark Frames is typically enough
• Temperature of the camera should be identical as during the light frame shooting (so that the sensor generates the same amount of noise)
• Dark frames can be taken any time as long the the above parameters match the requirements
• Dark frames can be reused as long as they match the same gain, temperature and exposure time values

They don't require use of a filter, but need to be taken in an environment that does not allow light to hit the CCD (if you have light leaks, you can use foil to wrap around the nose). 

Furthermore, most dark frames are taken at the same exposure length and temperature as the corresponding light frames (some image processing applications can scale darks to any exposure length). It is acceptable to take these frames and build a library (such that you don't need to take

them every imaging session.

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