Calstis4, calstis11, calstis12: Wavecal Processing in the STIS Calibration Pipeline

Due to uncertainties in positioning by the mode select mechanism (MSM) and thermal drifts internal to STIS, an image may be shifted on the detector by several pixels from its nominal location. The purpose of wavecal processing is to determine the shift of the image on the detector in each axis. This is only used for spectroscopic data. It requires one or more wavecal (line lamp) observations, taken without moving the MSM from the setting used for the science data. This document updates and supersedes STIS ISR 96-19, and it describes the functionality that was implemented for calstis V1.7.