"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
" greater than 4, then the
distorted TAN convention is used. Otherwise, the standard convention is used.