A further variation is that numerical values in subscript expressions may be
specified either as integers, in which case they refer to pixel indices, or as
floating-point numbers, in which case they refer to axis coordinates
(for a description of axis coordinates, see §
).
Both linear and non-linear axis coordinates are supported, the values
supplied being automatically converted into the corresponding pixel indices
before use. For instance:
could be used to select the appropriate region of a spectrum calibrated in Angstroms, while:
21)
would select a 21-pixel wide region of the spectrum centred on 5500.0 Angstroms.