Starguide - A Practical Guide to Astro-Photography

Bias Frames are used to capture bias calibration frames and belong to the group of calibration frames.

Bias Frames don't require use of a filter, but need to be taken in an environment that does not allow light to hit the camera sensor (if you have light leaks, you can wrap dark foil to wrap around the telescope parts). 

It is acceptable to take these frames and build a library (such that you don't need to take them every imaging session.

Important: 

• Cover your camera: your camera or scope must be covered to block all light off the sensor
• Set GAIN or ISO to the same setting as the light frames you want to calibrate 
• Temperature of the camera should be identical as during the light frame shooting (so that the sensor generates the same amount of noise). If the camera can be cooled, then just set the camera temperature to the same value used by the light frames. If it's a DSLR without cooling, then you should shoot the bias frames directly after or before capturing the light frames, assuming the outside temperature didn't change much.
• Exposure time must be set to the minimum possible time for the camera (or 0 if available).
• No. of frames: take a lot of bias frames, the more you have, the better the result (typically around 100)
• Bias frames can be taken any time as long the the above parameters match the requirements
• Re-usability: Bias frames can be reused as long as they match the same gain and temperature values
• Bias Master Library: you can combine (stack) all bias frames of a distinct gain and temperature value into Master calibration frames that can be reused during the picture post-processing steps or for ASIAIR Live stacking plans. Make sure the file name of such a master file includes the string "master" to be recognized by PixInsight as a master file.

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