Astronomical Spectroscopy - Physics - University of Cincinnati

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similar between the quartz lamp and point sources. The second author has instead
turned to Fourier filtering methods to simply remove fringes outright. Software exists
within IRAF in the STSDAS package to lock in on the fringe and remove it. This is
easily done with flat fields, and virtually impossible for point source images. Stellar
flats (§ 3.1.2) can be used to create a very crude two-dimensional illumination of a
point source. If the fringe pattern is relatively strong, the Fourier filtering packages
should be able to lock in on the overarching pattern and create a fringe correction
which can be applied to all your frames before extraction.
• Wavelength Calibration. For many long slit spectrometers, the wavelength solution
is a function of position on the slit. It was already suggested that wavelength calibration
should be applied using comparison lamp solutions which were extracted at the exact
same location as the star was extracted. If this is not done, then there will be slight
variations in solution with slit position and the resolution will be inadvertently reduced
by co-adding such data. Moreover, this can lead to even more serious problems later
on when applying telluric corrections.
• Telluric Correction. The telluric standards need to have been observed at a very
similar airmass and hopefully fairly close in time to the program object. When observing,
one needs to be sure that the telluric standard observations extend to the same
range of airmasses as the program objects (both minimum and maximum). If the
match isn’t great, one can interpolate as needed by using hybrid spectra of two telluric
stars to get a better airmass correction. Also, IRAF has very useful software which
actually uses the telluric lines with a statistical minimization routine, to make the best
match. Finally, if one is working in an area of very strong telluric lines, there may be a
good match with the features only when the target and standard were observed in the
exact same location in the slit. This requires keeping all spectra separate until telluric
removal, then combining the final set of spectra as the last step.
Possibly the strongest recommendation is that one needs to talk to a previous user of
the instrument, preferably one who actually knows what they are doing. Make sure that the
answers make sense, though. (Better still would be to talk to several such previous users,
and average their responses, possibly using some strong rejection algorithm.) Maybe they
will even be willing to share some of their data before the observing run, so that one can
really get a sense of what things will look like.