Non-linear photometric response and spatial sensitivity variations in the detectors are present in all raw data and must be taken into consideration before the data can be utilized for scientific purposes. In accounting for the detector response, the PHOTOM procedure converts raw data numbers (DN) into normalized flux numbers (FN) which are linearly related to the incident photon flux. PHOTOM is performed on a pixel-by-pixel basis in the two-dimensional raw image.
The PHOTOM procedure in NEWSIPS differs in two fundamental ways from the correction performed in IUESIPS. The Intensity Transfer Functions (ITFs) (i.e., graded ultraviolet floodlamp exposures used to linearize and normalize the camera response) are constructed in a manner which eliminates the necessity to resample, and consequently degrade, the data and preserves the inherent characteristics of the detector. In addition, a more precise image registration technique (Chapter 5) is used to align accurately the raw science data with the ITFs (Shaw 1990, De La Peña et al., 1990).