Image calibration, also known as reduction, fixes tiny defects in the camera sensor and optical system. Image calibration may sound like a boring topic, but proper calibration is of the utmost importance in producing good quality images.
Despite many years of refinement, no electronic imaging device is perfect. Each sensor has a different bias level (zero point), dark current (sensitivity to temperature), and sensitivity to light. These effects don’t just vary from sensor to sensor; they vary from pixel to pixel in the same sensor. Each of these effects corrupts the intensity represented in every pixel of the image in a specific way. The effect of these pixel defects may be small on a daylight image, but for a low light level image such as one taken through a telescope, they become extremely important.
Fortunately, the majority of the problems caused by these variations can be removed by calibration. Performing basic calibrations on your images can result in a huge improvement in the signal-to-noise ratio, resulting in much greater sensitivity.
Some astrophotographers do not bother with calibrations because they are ”difficult.” This is a huge waste of the camera’s capabilities. At the very minimum, a basic dark frame should be subtracted.
The three basic calibration steps are bias, dark, and flat-field. Each of these will be discussed in turn.