"scheme, which can be used to add distortion to basic FITS-WCS projections. This scheme was first defined by the Spitzer Space Telescope and is described in the following document: http://irsa.ipac.caltech.edu/data/SPITZER/docs/files/spitzer/shupeADASS.pdf The default for SipOK is 1.
When using astRead to read a FITS-WCS encoded header, a suitable PolyMap will always be included in the returned FrameSet if the header contains SIP keywords, regardless of the value of the SipOK attribute. The PolyMap will be immediately before the MatrixMap that corresponds to the FITS-WCS PC or CD matrix.
When using astWrite to write a FrameSet to a FITS-WCS encoded header, suitable SIP keywords will be included in the header if the FrameSet contains a PolyMap immediately before the MatrixMap that corresponds to the FITS-WCS PC or CD matrix, but only if the SipOK attribute is non-zero. If the FrameSet contains a PolyMap but SipOK is zero, then an attempt will be made to write out the FrameSet without SIP keywords using a linear approximation to the pixel-to-IWC mapping. If this fails because the Mapping exceeds the linearity requirement specified by attribute FitsTol, astWrite will return zero, indicating that the FrameSet could not be written out. Note, SIP headers can only be produced for axes that form part of a SkyFrame.
Note, the SIP distortion scheme is independent of the TPV/TPN distortion schemes (see attribute PolyTan). A FITS-WCS header could in principle, contain keywords for both schemes although this is unlikely.