These routines is designed to simulate or fix the effects of A/D conversion errors in data obtained with the WF/PC1. The basic algorithm that simulates the generation of errors is a direct software duplication of the A/D conversion process. The pixels input are assumed to be analogue signals and must have a smooth and continuous distribution of floating values. The A/D converter will translate these values into output integer DN by successively comparing the input signal with a decrementing power of 2 series of voltages. At any stage, if the input signal is greater than the comparison voltage, the appropriate bit is set in the output DN, and the reference voltage is subtracted from the signal before the next stage. The WF/PC1 produces 12 bit DN, so the first bit tested will be the 2048 bit. The 1024 bit is then tested and so on.
The errors in the WF/PC1 digitization process appear to occur during the reference voltage comparison, but not during the subsequent subtraction of the reference from the signal once the (possibly erroneous) bit has been set. More specifically, the error model assumes that a bit-dependent error in the reference voltage occurs during the comparison that will cause improper setting of the corresponding bit in the output DN for signals within ``error value'' of the reference voltage. Once the bit decision is made, however, the actual voltage subtracted from the signal for subsequent bit-tests is free from error (even though the decision to subtract it or not may be erroneous). Errors in the reference voltages appear to be constant for a given WF/PC1 CCD at a given electronic bay temperature. Errors may be less than one DN, therefore, for proper simulation of the WF/PC1 A/D the input pixel MUST NOT BE PRESET TO INTEGER VALUES. Default operation is to simulate A/D conversion errors. The FIX keyword calls a routine that corrects the simulated image data for systematic errors introduced by the A/D converter. This routine generates a lookup table that translates the observed DN value into a best estimate "correct" value. The output value will be floating. The fix-up routine cannot correct for the variable-width DN bins produced by the A/D converter, thus histograms will still look funny.
Both the simulator and fix-up routines require a file containing the reference voltage error in each bit. The file is specified with the FILE=filename keyword. By default, the error files are assumed to be in the DATA directory with a .ATD extension. A given set of errors should be constant for a given WF/PC1 CCD at a given bay temperature. The start and end of an example file is given below.