Gnomonic projection: native coordinates in
orange 
skew linear coordinates in grey

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Rules to define coordinate systems for spherical
projections date from
1983 and
1986
(Greisen).

Originally included the use of keyword
CROTAn (associated with the
latitude axis) to define a bulk image rotation  no longer to
be used.

New mathematical method proceeds in steps:

Cartesian (X, Y) coordinates in the image plane 
projection plane
coordinates.

Express the projection equations in the coordinate system which
is most natural  native spherical
coordinates:
E.g. the gnomonic projection:

Reference point, (X, Y) = (0, 0), now has a special meaning
which depends on the projection.

Additional FITS keywords:
CTYPEi, PVi_m.
