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USG: CGS4 sloping spectra
Correcting sloping CGS4 spectra.
Most CGS4 users in semester 95A were affected by a mechanical
problem that developed with the new rotating slit mechanism which
was installed as part of the CGS4 upgrade. The result of this is
most clearly visible in arc observations, where the arc lines
are clearly tilted with respect to the direction perpendicular to the
dispersion. OH lines will also be obviously tilted, and you will
have to correct for the tilt before combining the two beams if you
nodded along the slit, and before trying to fit to and remove any
residual atmospheric lines.
Here are some suggestions for dealing with the above during data
reduction with various software packages.
 CGS4DR
It is not possible to corrrect the entire detector within CGS4DR. However
for point sources you can calibrate the detector using the arc line at
the row of your source. This will ensure your wavelength calibration is
correct for your object. If you are nodding on the detector, repeat the
process separately for the row with the nodded object.
Figaro
Use the ARC command to fit to a single spectrum which has been
extracted from a row close to the centre of the image. Then use the
IARC command to derive the fit across the detector, working from the
centre outwards. It is important to answer yes to the IARC prompt
"cross correlate to find shifts". Then use the ISCRUNCH command to
apply the fit to straighten out the arc lines and your data frames
(RO's or RG's).
IRAF
If you have been using IRAF to reduce your data, you have to use the
tasks noao.twodspec.longslit.identify and reidentify as usual along some dispersion line. A 2-D fit of wavelength as a function of pixel co-ordinate
can then be performed with the task fitcoords (also in the longslit
package). Finally the transform task will perform the geometrical correction.
For more information please see the User's Guide to Reducing Slit Spectra
with IRAF (Massey, Valdes & Barnes, 1992).
Last Modification Date 1995/09/13 - Last Modification Author: gsw
UKIRT Software Group (
uksg@jach.hawaii.edu)
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