Calibration for most visible imaging systems involves a combination of dark and flat-field measurements (to derive the gain and offset of each pixel, as discussed in Sec. 3.3), perhaps with additional measurements of spectral response and MTF. The techniques involved are essentially the same as for individual arrays, and the reader is referred to Sec. 1.2. Not all the techniques will be suitable for a complete system, however, depending on accessibility to the appropriate electronic signals and whether the collection optics are removable. In cases where a complete system (including optics) requires calibration, it should be borne in mind that re-calibration (especially for absolute intensity response) may be needed for each configuration of the optics (e.g., after focus, aperture, or filter changes).
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