Representing Celestial Coordinate Systems - 8

Gnomonic projection: celestial coordinates in red -
native coordinates in grey

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  • 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 CRVALi.

  • 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:   LONPOLE, LATPOLE.