Astronomical Spectroscopy - Physics - University of Cincinnati

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as needed between sets. Finally, always check, as the data is coming in, that when you do
subtract one slit (or sky offset) position from another that you do get zero counts outside
of the star. What can go wrong here, as was mentioned in § 2.5.2, is that the strengths
of the OH bands vary with time. This is particularly true if clouds move in or the seeing
changes between slit positions, with the result that the OH lines will not cancel entirely and
reduction will be much more difficult. If this occurs, one is forced to use a shorter integration
per step to find a time frame over which the sky emission is sufficiently stable.
Note that even if one does have “perfect” sky subtraction of the OH spectra, the large
signal in the OH lines invariably adds noise to the final reduced spectra. This is unavoidable,
and makes the entire concept of defining the signal-to-noise-ratio in the NIR tricky to define,
as it is bound to vary depending upon the OH spectra within a particular region.
What could possibly go wrong A lot. The second author has never worked on an
infrared spectrometer that didn’t offer additional challenges which fell outside the standard
operation as listed above. To keep this brief, four of the most common problems that occur
will now be discussed. They include: flexure, fringing, wavelength calibration problems,
and poor telluric matching. Each of these can lead to greatly reduced signal-to-noise in the
spectra, far below what would be predicted by counts alone.
• Flexure. No spectrograph is absolutely rigid! Flexure can be a larger issue for infrared
spectrographs because the optics must be cooled to reduce the thermal background, and
it is challenging to minimize the thermal conduction to the internal spectrograph bench
while also minimizing the mechanical flexure between the bench and outer structure of
the instrument. Depending on how the instrument is mounted, when observing to the
east, versus looking to the west, for instance, or as the telescope passes meridian, the
internal light path will shift due to structure shifts. The amount of flexure depends
on the instrument and telescope set up. Previous users or the support astronomer for
the instrument can help the new user decide if this needs to be considered and how
to mitigate the effects. This is best addressed during the observing, keeping telluric
standards, lamps and objects on the same side of the meridian for instance.
• Fringing. Optical CCDs fringe in the red wavelength regions due to interference
within the surface layers of the detector; for IR spectrographs, fringing can occur due
to parallel optical surfaces, such as blocking filters, in the optical path. While in
principle, this should cancel out with the dome flats, due to flexure in the system and
light path differences, they typically do not cancel well. Common remedies include
obtaining quartz lamps at the exact same sky location as the observations. This might
work and should be included in the observing plan. However, the fringes are often not