This
attribute is a boolean value which specifies how SIP keywords should be
handled when reading a FITS-WCS encoded header using the
astRead function. See
http://irsa.ipac.caltech.edu/data/SPITZER/docs/files/spitzer/shupeADASS.pdf for more
information about SIP headers. If SipReplace is non-zero, then any SIP keywords
describing the inverse transformation (i.e. from WCS to pixel coordinates) are ignored.
Instead a new inverse transformation is found by performing a fit to the forward
transformation. The SipReplace attribute can be set to zero to prevent this happening.
If SipReplace is zero, any SIP keywords describing the inverse transformation are used
as supplied, rather than being replaced using a new fit. The default value is 1.