DISTORTION.TXT is read by distortion. This file contains initial guesses at the values for the camera model (K matrix and distortion), various sigmas, and a list of .REG files that will be used to compute the distortion model. The first five lines contain information about the distortion model, the last line is an END, and the intervening lines are a list of .REG files you wish to use.
An annotated DISTORTION.TXT
8 --> this number determines which distortion parameters will be solved for. See [[DISTOTION]] for details. 2625.36 512.5 512.5 --> initial guesses for the mm focal length (mmfl), central pixel (P0), and central line, L0 (also in make_sumfiles.txt) 76.923 0.000 0.000 -76.923 --> initial guesses for KXX, KXY, KYX, KYY (also in make_sumfiles.txt) 0.0 0.0 0.0 0.0 --> initial guesses for the distortion parameters, E2, E4, E5, and E6. 1.E-10 1.E-2 1.E-4 --> sigmas on the central pixel/line location (SIGPLO), KXY (SIGKXY), and pointing (SIGMA_PTG) D692423600G0.REG --> the name of a .REG file you wish to use (no leading space) D692423601G0.REG etc. --> continue until you've listed all the .REG files you wish to use END --> end with END