Astronomical Spectroscopy - Physics - University of Cincinnati

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• Twilight flats may wind up not being used, but they could be handy. Many NIR
spectrographs will need some illumination correction (as in the optical) and the twilight
flats are the easiest means to make such corrections. Be sure to get dark frames of the
same integration time to remove dark current and bias levels later.
• Stellar flats are spectra of a very bright star which is moved by small amounts along
the slit. The images can be co-added to create a very rough point-source flat, and used
much like a twilight flat to check on illumination corrections or other irregularities
which can uniquely appear in point-source observations.
The basic two-dimensional reduction steps for NIR spectroscopic data are presented
here; one should review § 3.1 to compare and contrast these with what is involved in optical
reductions. The relevant IRAF tasks are mentioned for convenience, although again the goal
here is not to identify what buttons to press but rather to be clear about the steps needed.
1. Subtract one slit position frame from another. This step removes the bias and
all other uniform additive sources, such as evenly illuminated sky. (Lumpy, nebular
emission will subtract horribly.) In long-slit spectroscopy one has to observe the star
in two (or more) positions along the slit. This basic subtraction step needs to be
done while one is observing, and it behooves the observer to always look at the result.
There should be a zero-level background throughout, with two stars, one positive, one
negative, running along the dispersion axis. The primary thing to check is to see if
the OH night-sky emission lines are canceling. There may be residual positive and/or
negative OH lines running perpendicular to the dispersion axis. This can happen just
as a consequence of the temporal and spatial variations in the OH spectra, but certainly
will happen if light clouds move through the field. As there will be many frames, try
all combinations with unique slit positions to find the best OH removal. Nothing else
should be there, and if there is, there are some serious problems. The only IRAF tool
needed is imarith.
2. Construct and apply a master, normalized, featureless flat. The “lamp on”
flats and the “lamp off” flats are median averaged using a rejection algorithm (IRAF:
imcombine) and then the “lamps off” average is subtracted from the “lamps on” average
to produce a master flat. As with the optical, one must use one’s own judgement about
how to normalize. This flat is then divided into each of the subtracted frames. The
process is repeated for each grating/filter combination.
3. Construct and use an illumination correction. This step is identical to that of
the optical, in that twilight flats can be used to correct any vignetting left over from