SCROSS-Cross-correlate two spectra & get relative shift

Description:

SCROSS cross-correlates two spectra and reports a relative shift in pixels between the two.

Parameters:

SPECTRUM

The name of the spectrum whose shift relative to a template spectrum is to be determined.

TEMPLATE

The name of the spectrum to be used as a template - the spectrum relative to which the shift of SPECTRUM is to be determined.

XSTART

The shift determination can be carried out over a restricted number of pixels. XSTART is the X-value of the first pixel to be used. If SPECTRUM and TEMPLATE have different X arrays then that of SPECTRUM is used. Ie, XSTART is converted into a pixel number using the calibration for SPECTRUM, ditto XEND, and this then gives the pixel range used from both SPECTRUM and TEMPLATE.

XEND

The end of the range used. Note that XSTART and XEND effectively specify a spectrum which is a subset of the original spectrum, and it is these subset spectra that are used - i.e. the cosine bell is applied to these subsets, not the whole spectrum, and similarly for the other operations performed.

FITCONT

SCROSS - unlike some other cross-correlators - attempts a continuum fit to the spectra prior to applying a cosine bell and taking the fourier transform. This replaces the reduction to zero mean sometime applied. For some data, this may not be a suitable thing to do, and it can be inhibited by setting FITCONT=NO. However, if you do so, you should have applied your own reduction to zero mean to the data prior to using SCROSS. FITCONT is the recommended option.

CBPC

By default, a cosine bell covering 10% of the specturm is applied. This is normally OK, but it can be changed by specifying a different value for CBPC.

RECORD

As an option, SCROSS will create a file containing the cross-correlation function. This can be used to see how good the cross-correlation was, or can be used to re- calculate the shift, by applying a different centering algorithm to the central spike. (You can use it as input to ARC, for example.)

CROSS

If RECORD has been set, CROSS gives the name of the cross-correlation spectrum to be created.

Source comments:

 S C R O S S

 Main body of the Figaro SCROSS function.  This computes
 the cross-correlation of two spectra and the location of the
 central peak of the cross-correlation.  It can be used to
 determine a relative shift between two spectra.  The cross
 correlation function can also be saved in a disk structure.

 SCROSS uses the Fourier cross-correlation technique, which is
 described in, for example:

   R. W. Hunstead, 1980, Proc. Astron. Soc. Australia, vol. 4, no. 1,
   pp. 77-80.

   J. Tonry and M. Davis, 1979, Astron. J, vol. 84, pp.1511-1525.

 Command parameters -

 SPECTRUM    (Character) The spectrum to be compared with
             a template spectrum.
 TEMPLATE    (Character) The template spectrum to be used.
             The two spectra should be the same length.
 XSTART      (Numeric) Data with an axis data value less than XSTART
             will be ignored in the cross-correlation.
 XEND        (Numeric) Data with an axis data value greater than XEND
             will also be ignored.  Note that these values are
             used to determine the channel numbers to be used
             for SPECTRUM, and the same ones will be used for
             TEMPLATE, even if TEMPLATE has a  different axis
             structure.
 CBPC        (Numeric) Percentage of spectrum covered by a cosine
             bell prior to application of the FFT.
 CROSS       (Character) the name of the data structure to hold
             the cross-correlation, if it is to be saved.
             The file created will be cross.dst, and will look
             like an ordinary spectrum - i.e. can be plotted by
             SPLOT, etc.  CROSS is ignored if RECORD is not
             set.

 Command keywords -

 FITCONT     If set, a continuum fit is performed on the two
             spectra prior to application of the cosine bell.
 RECORD      If set, the cross-correlation of the two spectra
             will be recorded as a new data structure.

 User variables used -

 SHIFT       (Numeric) The relative shift of the two spectra.

                                         KS / CIT 3rd Oct 1983