# updatewcs

UPDATEWCS applies corrections to the WCS of an HST science file and adds reference information as header keywords and fits file extensions so that a science file contains all necessary information to represent astrometrically precise positions. The order in which the corrections are applied is important and is as follows:

Detector to Image Correction

Apply Time dependent distortion (if applciable)

Recomputing the basic WCS

Apply Velocity Aberration Correction

Apply polynomial distortion through the SIP coefficients

Apply non-polynomial distortion

Mathematically the entire transformation from detector to sky coordinates is described by:

where f(u’,v’) and g(u’,v’) represent the polynomial distortion correction specified as

where

x’, y’ are the initial coordinates x,y with the 68th column correction applied through the DET2IM convention

u’,v’ are the DET2IM-corrected coordinates relative to CRPIX1,CRPIX2

LT<sub>x</sub>, LT<sub>y</sub> is the residual distortion in the lookup tables written to the header using the FITS Distortion Paper lookup table convention

A, B are the SIP coefficients specified using the SIP convention