Gnomonic projection: celestial coordinates in
native coordinates in grey
Native spherical coordinates are not often of primary interest.
Need to define a spherical coordinate rotation from native
coordinates to the celestial spherical
coordinates of interest.
Final step in the algorithm chain:
Spherical rotation specified by three Euler angles.
For historic & practical reasons, these Euler angles are defined in
a way which relates them to the image - indirectly via
First-order approximation holds at the reference point (for most
projections, default LONPOLE):
Third Euler angle must be specified - need new keyword(s).
Additional FITS keywords: