Representing Celestial Coordinate Systems - 7
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.