Simple linear coordinates in yellow -
pixel coordinates in grey
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The first coordinate rules for FITS date from 1981 (Wells,
Greisen, & Harten).
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Simple linear world
coordinates:
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Basic mathematical requirement is ...
X = X0 + dX I,
Y = Y0 + dY J,
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... but the actual implementation was
X = X0 +
dX (I - I0),
Y = Y0 +
dY (J - J0).
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While not strictly necessary in a mathematical sense, introduction
of the reference point
(I0, J0) as a
convenience was to prove important later.
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Can only handle separable, linear systems, i.e.
X = X(I),
Y = Y(J).
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Additional FITS keywords:
CRPIXj (I0),
CDELTi (dX),
CRVALi (X0).
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