Raw spectra are usually
not sampled at constant wavelength or frequency steps, and sometimes
even with gaps in between the bins. At some stage the
independent variable will have to be converted into linear or non-linear
functions of wavelength or frequency units and gaps will have to be filled with
interpolated values. Frequent cases are: wavelength calibration, redshift
correction, 
versus 
presentation, and the comparison of narrow-band
filter spectrophotometry with scanner
data. Related commands are REBIN/LONG, already described,
REBIN/LINEAR for linear rebinning, i.e. scale and offset change, REBIN/II
(IT,TI,TT) to do nonlinear rebin conserving flux (see below) and
CONVERT/TABLE to interpolate table data into image data.
Note that in our implementation we make a conceptual difference between straightforward interpolation and rebinning. REBIN/II (IT, TI, TT) redistributes intensity from one sampling domain into another. There is no interpolation across undefined gaps and no extrapolation at the extremities of the input data. If you need these, you will have to manipulate the input data first (generating non-existent information !). REBIN/II (IT, TI, TT) conserves flux locally and globally.