OPTEXTRACT-Extracts a long-slit spectrum using Horne's algorithm

Description:

OPTEXTRACT performs optimal extraction of a star spectrum from a 2-D long slit spectrum using the algorithm of Horne 1986 (PASP 98,609). A spatial profile image should have previously been determined using the PROFILE command, and the 2-D spectrum to be extracted must be sky subtracted (e.g. using the POLYSKY command).

The input IMAGE array may have error and quality information. An error or variance array is required to perform true optimal extraction as the points should be weighted using the errors on each point as well as the spatial profile information. If an error array is not present (or not all the errors in a column are known) the extraction is performed assuming equal errors on every point in the column. The output spectrum has an error array only if the input image has an error array. The WEIGHT keyword may be set to NO to suppress error weighting even when errors are available.

Parameters:

IMAGE

The name of a 2-D long slit spectrum.

PROFILE

The name of the normalized spatial profile image for the object to be extracted. This can be obtained using the PROFILE command.

SPECTRUM

The name of the resulting single spectrum.

WEIGHT

If WEIGHT is set, the error or variance information associated with the data is used to weight the data. Each point is weighted as 1/variance. If not set, or if no error information is available all points are given equal weight.

Source comments:

 O P T E X T R A C T

 OPTEXTRACT performs optimal extraction of a star spectrum from a
 2-D long slit spectrum using the algorithm of Horne 1986 (PASP 98,609).
 A spatial profile image should have previously been determined using
 the PROFILE command, and the 2-D spectrum to be extracted must be
 sky subtracted (e.g. using the POLYSKY command).

 The input IMAGE array may have error and quality information. An error
 or variance array is required to perform true optimal extraction as
 the points should be weighted using the errors on each point as well
 as the spatial profile information. If an error array is not present (or
 not all the errors in a column are known) the extraction is performed
 assuming equal errors on every point in the column. The output spectrum
 has an error array only if the input image has an error array. The
 WEIGHT keyword may be set to NO to suppress error weighting even
 when errors are available.

 The spatial profile array used by OPTEXTRACT must satisfy the following
 requirements.
  (i)  Its quality array must specify a window (of arbitrary shape)
       in which the extraction will be performed. Points inside the window
       must have zero quality values, other points must have non zero
       quality values
  (ii) The data must be normalized. i.e. The sum along each column
       must be one.
  (iii) The data values may not be negative.

 Command parameters -

 'IMAGE'    The name of the input 2-D file. This should be a
            sky subtracted 2-D spectrum from which the spectrum
            of an object is to be extracted. It may have error and
            quality arrays
 'PROFILE'  The normalized spatial profile image used to perform the
            extraction. The quality array should specify a window
            in which the profile is determined. The data within
            this window is the spatial profile to be used.
 'SPECTRUM' The name of the resulting 1-D spectrum. If the input image
            has errors, so will the output spectrum. The quality
            will be set to bad only if there is no data in the
            relevant column.

 Command keywords -

 'WEIGHT'    Use the error or variance array to weight the
             pixels during extraction.


                                 JAB / JAC 8th Feb 1991