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:

(x',y') &= DET2IM(x,y)

\binom{u'}{v'} &= \binom{x' - CRPIX1}{y' - CRPIX2}

\left( \begin{array}{ll}
   \alpha \\
   \delta \\
   \end{array} \right) &=
\left( \begin{array}{ll}
\end{array} \right) +
\left( \begin{array}{cc}
CD11 & CD12 \\
CD21 & CD22\\
\end{array} \right)
\left( \begin{array}{ll}
u' + f(u',v') + LT_x(x',y') \\
v' + g(u',v') + LT_y(x',y') \\
\end{array} \right)

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

f(u',v') = \sum_{p+q=2}^{AORDER} A_{pq} {u'}^{p} {v'}^{q}
g(u',v')  = \sum_{p+q=2}^{BORDER} B_{pq} {u'}^{p} {v'}^{q}


  • 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