Astronomical Spectroscopy - Physics - University of Cincinnati

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3.3.3. Observing with a NIR Spectrometer
With near-infrared observations, virtually all the steps taken in preparing and undertaking
a run in the optical are included and will not be repeated here. What will be done
here is a review of the few additional steps required for near-infrared spectroscopy. These
steps center around the need to remove OH sky emission lines (numerous and strong, particularly
in the H-band) and to correct for the absorption lines from the Earth’s atmosphere.
It will not be possible to do a very thorough reduction during the night like one can with
the optical, but one can still perform various checks to ensure the data will have sufficient
signal-to-noise and check if most of the sky and thermal emission is being removed.
The near-infrared spectroscopist is far more obsessed with airmass than the optical
spectroscopist. It is advised that the observer plot out the airmass of all of the objects
(targets and standards) well in advance for a run. This can be done using online software,
http://catserver.ing.iac.es/staralt/index.php. The output of this program is given in Figure
21. For this run, the authors were extraordinarily lucky that two of the telluric standards
from the Hanson et al. (2005) catalog were well suited to be observed during a recent SOAR
run: HD 146624 during the first half, and HD 171149 during the second half. Looking at
this diagram, one can make the best choices about when to observe the telluric standard
relative to any observations made of a target object. If the target observations take about
30 minutes (this includes total real time, such as acquisition and integration) then observing
Telluric Object 1 just before the program target during the early part of the night will mean
the target star will pass through the exact same airmass during its observations, optimizing
a telluric match. Later in the night, Object 3 can also be used, though observed after the
target. As hour angle increases, airmass increases quickly. Note the non-linear values of the
ordinate on the right of Figure 21. One must be ready to move quickly to a telluric standard
or the final spectra may be quite disappointing. This observer has been known to trace in
red pen in real time on such a diagram, the sources being observed as time progresses, to
know when it is time to move between object and telluric and vice versa. The goal should
be to observe the telluric standard when its airmass is within 0.1 of that of the observation
of the program object.
How to select telluric standards As was mentioned in § 2.5.2, early-A dwarfs or solar
analogues are typically used. Ideally, one should seek telluric standards which are bright (for
shorter integration times), have normal spectral types (no anomalies), and are not binaries
(visible or spectroscopic). But also, location in the sky is important. Referring back to
Figure 21 again, stars passing close to zenith at meridian have a different functional form
to their airmass curves than do stars that remain low even during transit. This can make it
hard to catch both target and telluric at the same airmass if their curves are very different.