Use PVi_m keywords to define distorted TAN projection? PolyTan
"
TAN"
projections should be
treated when reading a FrameSet from a foreign encoded FITS header. If zero, the
projection is assumed to conform to the published FITS-WCS standard. If positive, the
convention for a distorted TAN projection included in an early draft version of
FITS-WCS paper II are assumed. In this convention the coefficients of a polynomial
distortion to be applied to intermediate world coordinates are specified by the PVi_m
keywords. This convention was removed from the paper before publication and so does
not form part of the standard. Indeed, it is incompatible with the published
standard because it re-defines the meaning of the first five PVi_m keywords on the
longitude axis, which are reserved by the published standard for other purposes.
However, this scheme has now been added to the registry of FITS conventions
(http://fits.gsfc.nasa.gov/registry/tpvwcs.html) and headers that use this convention
are created by the SCAMP utility (http://www.astromatic.net/software/scamp) and the
Dark Energy Camera at NOAO.
The default value for the PolyTan attribute is -1. A negative values causes the
used convention to depend on the contents of the FitsChan. If the FitsChan
contains any PVi_m keywords for the latitude axis, or if it contains PVi_m
keywords for the longitude axis with "
m"
greater than 4, then the distorted
TAN convention is used. Otherwise, the standard convention is used.