Astronomical Spectroscopy - Physics - University of Cincinnati

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be obtained for each grating setting and filter combination you use during the night,
the same as with optical data. Furthermore, they should be obtained with exposure
levels that push to the linearity limits in order to maximize the signal-to-noise ratio.
This linearity limit can be as low as 10,000 e − for some IR detectors. However, unlike
the case for the optical, one must also obtain an equivalent set (in terms of exposure
time and number) of dome flats with the dome illumination turned off, as the dome
flat itself is radiating in the NIR. One can demonstrate the necessity for this by a
simple experiment: subtract an image obtained with the lamps turned off from an
image with the lamps turned on, and see how many counts there really are over the
entire wavelength range. One may be surprised how many counts were removed from
the original exposure, particularly in the longer wavelength K band. But it is this
subtracted dome flat that will be used to flatten the data frames, so there needs to
be a lot of counts in it in order not to reduce the signal-to-noise! For the K band,
where there is high background counts and because the dynamic range of the detector
is small, one may need 20 or more flat-on, flat-off pairs to get sufficient real signal.
• Quartz flats will be taken during the night, but daytime exposures will test the
exposure levels and reveal any structural differences between it and the dome and
twilight flats. Of course, dark frames with matching integration times will be needed.
• Comparison lamps should be taken throughout the night, but obtaining a few during
the day may also prove to be useful. Be sure to get dark frames to match the integration
time.
During the night, one will take:
• Telluric standard stars are (nearly) featureless stars that will be used to derive the
spectrum of the telluric absorption bands for removal from the program data. These
are not spectrophotometric stars. They must be chosen to lie at similar (within 0.1)
airmasses as the target stars for the reasons explained above, and observed within an
hour or two of the target (more often for observations beyond 3µm).
• Quartz Lamps may be useful or not, depending upon the specific instrument.
• Comparison lamp exposures are a good idea. In the infrared, one can almost always
use the night sky emission lines as the wavelength reference source. But separate
comparison lamp exposures may prove useful if there turns out not to be enough
counts in the sky lines, or if some regions are too void of sky lines, such as the long-K
region. It is also a good idea to take these if you are moving the grating often, the
program integration times are short, or if the dispersion is high.