When reading a FrameSetFrameSet from a FitsChanFitsChan, projection parameters
PVi_0, PVi_1 and PVi_2 (for longitude axis
“i”) are used to indicate a user-defined fiducial point as
described in section 2.5 of paper II. This results in a shift of IWC
origin being applied before the WcsMapWcsMap which converts
IWC into
native spherical coordinates. The values of these projection parameters,
if supplied, are stored as the corresponding PVi_mPVi_m attributes
of the WcsMap.
When writing a FrameSet to a FitsChan, the PV attributes of the WcsMap
determine the native coordinates of the fiducial point (the fixed
defaults for each projection described in paper II are used if the PV
attributes of the WcsMap have not been assigned a value). The
corresponding celestial coordinates are used as the CRVALi
keywords and the corresponding pixel coordinates as the CRPIXj
keywords.