FINDSHIFT: Compute Relative Shifts Between Two Spectra

Form: FINDSHIFT s1 s2 [RAD=r] [SHIFT=s] [XS=i] [XE=j] [ROW=k] [LOAD] [SILENT]
RAD=R
Search Radius for the Chi-squared minimum. The default is 1/4 of the number of columns.
SHIFT=
A guess of SHIFT, i.e. the column shift on the second spectrum required to match the template.
XS=I
First column, on template, of the spectral region for Chi-squared computation.
XE=J
Last column of the selected region.
ROW=k
Estimate shift in row k (when images are given).
LOAD
Loads variables SHIFT, DSHIFT, and SHIFTVAR with the relative shift, an estimation of its error and the unbiased mean variance of the spectra.
SILENT
Loads variables but does not print results.

FINDSHIFT computes the relative shift between two spectra. Given two spectra (1 row images), it computes SHIFT, the number pixels that the second (shifted) spectrum would have to be shifted to match (in the least squared sense) the first (template) spectrum. The user can selected a spectral region for the chi-squared computation. The algorithm solves the interpolation problem, so the command provides accurate fractional shifts.

Since the algorithm minimizes the difference in counts, results are more accurate if the spectra are normalized to have the same number of counts within the spectral region of interest. The algorithm looses sense if the spectra are very different.

If two images are given (more than one row), a single row will be considered for the shift calculation, the template's starting row is the default row, but can be changed with the 'ROW=' keyword.