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The CTYPEn and CUNITn values provide information about the axis to which this group of values applies. The rest of the keywords can then be used to calculate points of interest upon that axis. For instance, the center of an axis in axes longer than 1 is given by, we have:
Minimum: CRVALn + CRDELn*(1-CRPIXn)
Center: CRVALn + CRDELn*(NAXISn/2 - CRPIXn)
Maximum: CRVALn + CRDELn*(NAXISn - CRPIXn)
There are similar formulae for the minimum and maximum values of an axis.
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