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.. _updatewcs:
**************************************
UPDATEWCS
**************************************
UPDATEWCS applies corrections to the WCS of a 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:
.. math:: (x',y') &= DET2IM(x,y)
.. math:: \binom{u'}{v'} &= \binom{x' - CRPIX1}{y' - CRPIX2}
.. math:: \left( \begin{array}{ll}
\alpha \\
\delta \\
\end{array} \right) &=
\left( \begin{array}{ll}
CRVAL1 \\
CRVAL2\\
\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
.. math:: 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}
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 Paper IV lookup table convention
* A, B are the SIP coefficients specified using the SIP convention
.. toctree::
:maxdepth: 2
updatewcs_ui
wcs_corrections
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