Select a well-exposed, but unsaturated flat-field image from the sequence of pairs of images used to derive the transfer curve. For each bit of the digital dynamic range used, count the number of occurences of 1's and 0's in the image. If there are no problems with the ADC, the numbers of 1's and 0's in the lower-significant bits should be almost equal, up to bit-values near the level of illumination of the image. Figure 17 shows an example where there is a slight biasing towards values of 0.
Figure 17: Example of distribution of 1's and 0's in a bias and well-illuminated
flat field. The solid line indicates the fraction of bits which have
a 0 value, the dotted line is the fraction of 1's. Note that the numbers
of 1's and 0's are almost equal for the lower 7 bits in the well-illuminated flat (which are
receiving essentially random data), but there is a slight bias
towards more frequent 0's. The data were taken from the ESO, La Silla CCD test data set
09-94-10-23.
This is the last step in reducing a CCD test data set.