This attribute is a boolean value which specifies how FITS "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, headers that
use this convention are still to be found, for instance the SCAMP
utility (http://www.astromatic.net/software/scamp) creates them.
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.