There are three different types of calibration frame that all TelescopeLive network telescopes acquire:
Dark calibration frame
This frame is generated to compensate for the dark current generated by the CCD sensor itself.
Dark frames can be taken by closing the shutter of the telescope using the same time and temperature settings as the light frames. This should remove the majority of hot pixels within the image.
Flat field frame
Flat calibration frames are to help remove the physical imperfections that may appear as a result of dust, along with any vignetting around the corners of the image. Vignetting is caused by the imaging of a circular source on a square detector, in this case the circular telescope and square CCD camera. The calibration frames are taken wither using a large, evenly illuminated flat light panel that will provide an even source of light to the entire frame, or imaging the sky during twilight.
Flat field frames need to be independently corrected for dark current and bias level using dark and bias calibration frames before they can be used.
The flat field calibration is then performed by dividing the raw image, already corrected for dark current and bias level,
When subtracted from the light frame this will remove any of the physical imperfections.
Bias calibration frame
Much like the dark calibration frame, bias calibration frames are taken with the telescope shutter closed, however there is no need to maintain temperature, instead these images are taken with a low exposure time at high sensitivity.