Representing Celestial Coordinate Systems - 5

Prev 1 2 3 4 5 6 7 8 9 10 11 Next

• The first coordinate rules for FITS date from 1981 (Wells, Greisen, & Harten).



• Simple linear world coordinates:
  
  
  • Basic mathematical requirement is ...
    X = X₀ + d_X I,
    Y = Y₀ + d_Y J,
  
  
  
  • ... but the actual implementation was
    X = X₀ + d_X (I - I₀),
    Y = Y₀ + d_Y (J - J₀).


• While not strictly necessary in a mathematical sense, introduction of the reference point (I₀, J₀) as a convenience was to prove important later.



• Can only handle separable, linear systems, i.e.
  
  X = X(I),
  Y = Y(J).



• Additional FITS keywords:   CRPIXj (I₀),   CDELTi (d_X),   CRVALi (X₀).