TSR 2012-03: NPOL Reference File

Abstract: Authors: Nadezhda Dencheva, Warren Hack Date: 12 Oct 2010

The HST pipeline originally used two types of reference files to correct for distortion: IDCTAB files which contain the coefficients for a polynomial correction and DGEO files which are images with the residual distortion. A new format for the residual distortion, called NPOL files (extension _npl.fits), is presented in this document. The conversion from DGEO files to NPOL files is described and an example of the format is given using ACS/WFC F606W filter. Tests of the conversion procedure show that the differences between the DGEO files and NPOL files are of the order of 10^{-4} pixels.