USG: CGS4 optical distortion

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USG: CGS4 optical distortion

Previous: CGS4DR Docs | Up: USG Main Page | Next: Sloping Lines

Correcting curvature in CGS4 spectra.

Users who have CGS4 data since the upgrade to the 256x256 array will have noticed their spectra having a small amount of curvature which causes them to stray into neighbouring rows. This is caused by an optical disortion in the CGS4 optics which has now become obvious because of the increased wavelength coveraged and diminished spatial pixel size. Here are some suggestions for dealing with the above during data reduction with various software packages.

CGS4DR

It is not possible to do curved extraction of spectra within CGS4DR.

Figaro

These routines can be used for both point and extended sources. Commands to be used:

ICUR to mark the centre of the spectrum.
SDIST which will trace out the indicated spectrum
CDIST to correct the data using the SDIST output.

You may wish to consult the extensive documentation written by Keith Shortridge on straightening spectra with Figaro, a copy of which is locally available .

IRAF

If you have been using the task noao.twodspec.apextract.appall (or just apfit) to extract your spectra, you should be setting pfit=fit2d in order to use the Marsh algorithm (PASP 101:1032, 1989) for optimal extraction of curved spectra. For additional information consult the APEXTRACT documentation (especially the revision summary dated May 1992).

Twodspec

The program COMB in the Starlink package TWODSPEC allows correction with the same functionality as the Figaro comman SDIST. For more information consult the TWODSPEC documentation (SUN 16).

Previous: CGS4DR Docs | Up: USG Main Page | Next: Sloping Lines

Last Modification Date 1995/07/17 - Last Modification Author: frossie
UKIRT Software Group (uksg@jach.hawaii.edu)

Contact: Tom Kerr. Updated: Wed Oct 6 12:07:27 HST 2004

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