SCRUNCH-Rebin a spectrum to a linear wavelength range

Description:

SCRUNCH rebins a spectrum or a set of spectra so that the resulting data have either a linear wavelength scale or a wavelength scale that is logarithmic (i.e. has a constant velocity step). Note that SCRUNCH is not suitable for data in which the bins contain data that covers discontinuous or overlapping wavelength ranges (FIGS data, for example).

Parameters:

SPECTRUM

The spectrum (or spectra) to be scrunched. If SPECTRUM is an image (i.e. 2-D data) it will be treated as a set of spectra all of which will be scrunched individually. In this case, if the wavelength data is a 2-D array each of the data spectra will be scrunched according to the corresponding cross-section of the wavelength array. If the wavelength array is 1-D, this single array will be used for all the spectra.

WSTART

The wavelength of the center of the first bin of the resulting output data.

WEND

Normally, represents the wavelength of the center of the last bin of the resulting output data. Because this is not always the most convenient value to supply, SCRUNCH will allow WEND to be used to specify the wavelength increment value (for linear data, this is the constant wavelength difference between bins; for logarithmic data it is the constant velocity step in Km/sec). The way WEND is interpreted may be controlled explicitly by the FINAL and INCREMENT hidden keywords, but by default SCRUNCH will assume WEND is an incremental value if it is less than WSTART, and a final value if it is greater than WSTART.

BINS

The number of elements to be used for the resulting spectrum.

OUTPUT

The name of the resulting data file containing the scrunched spectrum or set of spectra. The structure of OUTPUT will be the same as that of the input file, except that the data array will generally be a different size, and any error array will have been deleted. Eventually SCRUNCH will be able to calculate the errors in the new spectrum..)

INCREMENT

If set, SCRUNCH will assume WEND is an incremental value (a velocity or wavelength step), even if it is greater than WSTART.

FINAL

If set, SCRUNCH will assume that WEND is the wavelength of the final element of the resulting spectrum, even if it is less than WSTART.

LOG

Controls whether or not the data is binned to a linear or a logarithmic wavelength scale.

LINEAR

Specifies that linear rather than quadratic interpolation is to be used during the scrunching. Usually, better results are obtained with quadratic interpolation. Note that LINEAR is the opposite of QUAD, and NOT of LOG.

QUAD

Specifies that quadratic rather than linear interpolation is to be used during the scrunching. Usually, better results are obtained with quadratic interpolation.

FLUX

If the input data represents flux (photons, for example) rather than flux density (AB magnitudes, Janskys, etc) then it should be scrunched so as to conserve flux, and FLUX should be set. See HELP FIGARO TECHNIQUES WAVELENGTH fo more details on this point.

MEAN

If the input data represents flux density (AB magnitudes, Janskys, etc) rather than flux (photons, for example) then it should be scrunched so as to conserve the mean value of the data, and MEAN should be set. See HELP FIGARO TECHNIQUES WAVELENGTH fo more details on this point.

Source comments:

 S C R U N C H

 Figaro routine to scrunch a spectrum or set of spectra.  Can scrunch
 either into a linear wavelength scale, where the wavelength
 increment from bin to bin is constant across the spectrum, or
 into a logarithmic scale, where the increment of the log of
 the wavelength from bin to bin is constant.  The operation
 is performed by the routine FIG_REBIN.

 If the input file is 2-D data, then it is treated as a set of
 1-D spectra and each is scrunched individually.   If the wavelength
 array is 1-D, then this single array will be used for all
 the spectra.  If it is 2-D, then each spectrum will be scrunched
 according to the corresponding cross-section of the wavelength
 array.

 The routine can either conserve flux or the mean value
 of the data.  Conserving flux is appropriate where the data is
 actually in flux units, but not when the data is in magnitude
 units.  (Consider the case where each input bin maps to two
 output bins; if the data is in flux units - photon counts, for
 example - then flux should be conserved and the mean data level
 should drop by a factor 2; if the data is in magnitudes, then
 the rebinning should not change the mean level.)  The program
 does not try to determine for itself whether the data is in flux
 or in magnitudes - it uses a command keyword.

 Command parameters -

 SPECTRUM     (Character) The name of the spectrum to be scrunched.
 WSTART       (Numeric) The wavelength of the CENTER of the first
              bin of the resulting scrunched spectrum.
 WEND         (Numeric) The wavelength of the CENTER of the final
              bin of the resulting scrunched spectrum.  If WEND is
              less than WSTART, then SCRUNCH assumes that it is the
              increment rather than the final value that is being
              specified.  If the scrunch is logarithmic and WSTART
              is greater than WEND, SCRUNCH assumes that the WEND
              value represents a velocity in km/sec.  These
              assumptions can be controlled directly by the keywords
              INCREMENT and FINAL, if they will not give the desired
              effect.
 BINS         (Numeric) The number of bins for the resulting spectrum.
 OUTPUT       (Character) The name of the resulting spectrum.
              Note that SCRUNCH cannot rebin a spectrum into itself
              and so will always create a new output file.

 Command keywords -

 LOG          Bin into logarithmic wavelength bins.
 MEAN         Conserve mean data level rather than flux.
 FLUX         Conserve flux rather than mean data level.
 LINEAR       Use linear interpolation when rebinning.
 QUAD         Use quadratic interpolation when rebinning.
 INCREMENT    WEND is an increment value, even though it is > WSTART.
 FINAL        WEND is a final value, even though it is < WSTART.

 User variables -

 SCRUNCH_INC  Is set to the wavelength increment if linear
              rebinning is used, and to the velocity increment if
              log rebinning is used.
 SCRUNCH_END  Is set to the final wavelength value.  (This is for
              those cases where the WEND value represents an
              increment.)

                                          KS / CIT 7th Feb 1984