Astronomical Spectroscopy - Physics - University of Cincinnati

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The steps involved in this reduction are straightforward, and when observing stars using
a long slit it is hard to imagine why an astronomer would not have fully reduced data at
the end of the night. Carrying out these reductions in real time allows one to see that the
signal-to-noise ratio is as expected, and avoids there being any unpleasant surprises after
the the observing run. The IRAF task doslit was specifically designed with this in mind,
to allow a quick-look capability that was in fact done sufficiently correctly so that the data
reduced at the telescope could serve as the final reduction.
3.1.1. Multi-object techniques
Most of what has just been discussed will also apply to multi-object data, be it fibers
or slitlets. There are a few differences which should be emphasized.
For fibers, sky subtraction is accomplished by having some fibers assigned to blank
sky. However, how well sky subtraction works is entirely dependent upon how well the flatfielding
removes fiber-to-fiber sensitivity variations (which will be wavelength dependent) as
well as how well the flat fields match the vignetting of a particular fiber configuration. Since
the vignetting is likely to depend upon the exact placement of a fiber within the field, the
best solution is to either model the vignetting (as is done in the Hectospec pipeline) or to
make sure that flat fields are observed with each configuration. For some telescopes and
instruments (such as CTIO/Hydra) it is more time efficient to simply observe some blank
sky, making a few dithered exposures near the observed field.
For multi-slits one of the complications is that the wavelength coverage of each slitlet
will depend upon its placement in the field, as discussed above in § 2.4.1. This results in
some very challenging reductions issues, particularly if one plans to flux-calibrate. Consider
again the IMACS instrument (§ 2.4.1). There are eight chips, each of which will have its own
sensitivity curve, and even worse, the wavelength coverage reaching each chip will depend
upon the location of each slitlet within the field. The perfect solution would be to observe
a spectrophotometric standard down each of the slits, but that would hardly be practical.
Flux calibrating multislit data is hard, and even obtaining normalized spectra can be a bit
of a challenge.
3.1.2. NIR techniques
When reducing near-infrared spectra, the basic philosophy outlined in § 3.1 for the
optical generally applies. But there are two significant differences between the infrared and