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skysub noao.imred.specred



skysub -- Sky subtract extracted multispec spectra


skysub input


List of input multispec spectra to sky subtract.
output = ""
List of output sky subtracted spectra. If no output is specified then the output replaces the input spectra.
objaps = "", objbeams = ""
Object aperture and beam numbers. An empty list selects all aperture or beam numbers. Only the selected apertures are sky subtracted. Other apertures are left unmodified. Note that it is valid to include the sky apertures in the object selection which results in residual sky spectra after subtraction by a mean sky.
skyaps = "", skybeams = ""
Sky aperture and beam numbers. An empty list selects all aperture or beam numbers.
skyedit = yes
Edit the sky spectra? If yes the sky spectra are graphed using the task specplot and the user may delete contaminated sky spectra with the d key and exit with q.
combine = "average" (average|median|sum)
Option for combining pixels at the same dispersion coordinate after any rejection operation. The options are to compute the "average", "median", or "sum" of the pixels. The median uses the average of the two central values when the number of pixels is even.
reject = "none" (none|minmax|avsigclip)
Type of rejection operation performed on the pixels which overlap at each dispersion coordinate. The algorithms are discussed in the DESCRIPTION section. The rejection choices are:

      none - No rejection
    minmax - Reject the nlow and nhigh pixels
 avsigclip - Reject pixels using an averaged sigma clipping algorithm

scale = no
Scale the sky spectra by the mode?
saveskys = yes
Save the sky spectra? If no then the mean sky spectra will be deleted after sky subtraction is completed. Otherwise a one dimensional image with the prefix "sky" and then the output name is created.
logfile = ""
Logfile for making a record of the sky subtraction operation.


This task selects a subset of aperture spectra from a multispec format image, called sky spectra though they could be anything, and combines them into a master spectrum which is subtracted from another subset of spectra called the objects. Options include saving the master sky spectrum and reviewing the selected sky spectra graphically and deleting some of them.

The sky apertures are selected using the aperture and beam numbers defined during extraction (see the apextract package). In some applications the beam numbers are used to code object and sky apertures and selection by beam number is quite easy. Otherwise one must list the aperture numbers explicitly.

The object apertures are also selected using an aperture and beam number list. Spectra not selected to be objects are not modified by the sky subtraction. Note that it is perfectly valid to include the sky spectra in the object list to produce residual sky spectra.

When interactively editing the sky spectra the task specplot is used. To delete a spectrum type d. To undelete the last deleted spectrum type e. When finished type e.

The sky spectra are combined using one of combining and rejection options from the task scombine except for the option "none".


1. To median and subtract apertures 1,10,15,20 from all apertures:

    ms> skysub skyaps="1,10,15,20"


specplot, scombine

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