# Reference Files¶

The straylight step uses the REGIONS reference file, which stores locations of the slice regions on the detectors. This reference file provides 2-D detector images in which each pixel is set to the number of the corresponding slice (or 0 for inter-slice pixels) at each of 9 different throughput levels ranging from 10% - 90%. While assign_wcs uses a fairly exclusive slice mask (selecting only pixels with high throughput for science analysis), the straylight step uses a very inclusive 20% threshhold to define slice pixels and obtain a cleaner sample of inter-slice pixels.