User Manual
Doc. Number: ESO-323064
Doc. Version: 2
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Beyond 2200 nm, the thermal background dominates with contributions from both
atmospheric and telescope emission.
Detailed sky spectra with OH line identifications are available at:
http://www.eso.org/sci/facilities/paranal/decommissioned/isaac/tools.html
To this end, spectroscopic observations in the near-IR regime are performed by using the
following techniques, which mostly rely on splitting the total integration on source in multiple
sub-exposures taken at different positions.
5.2.5.1 Nodding
The classical technique in spectroscopy consists in observing a given target at two positions
along the slit (i.e., nodding along the slit), with the specific purpose of removing the sky
emission lines, the detector dark current, glow and eventually some ghosts. Indeed, the sky
background is effectively removed by subtracting one frame from the other and vice versa.
This process is sometimes called
double subtraction
.
Specifically, the total on-source integration time is split in N pairs of exposures. The
exposures pair (i.e., usually referred to as AB pair) are taken always along the slit but at a
given separation. The telescope nods between the two positions, A and B. The number of
nodding cycles (i.e., number N of AB pair spectra) and the nodding throw (i.e., separation
between the A and B positions) are free observing parameters. However, in spectro-
polarimetry mode the nodding throw is fixed at 2.5”.
The number of defined AB pair affects
the total amount of overheads, therefore already at Phase 1 we strongly encourage the user
to carefully plan the observing strategy.
5.2.5.2 Jittering
The purpose of jittering is to correct for bad pixels and decrease systematics originating
from the detector. This is particularly important for CRIRES observations, as the detectors
suffer from a relatively large number of bad pixels. In addition, the spatial extent of a
spectrum is at most a few pixels in nominal conditions with AO. Jittering is obtained by
adding a small, random o
ff
set to the telescope in addition to the nodding offset. The
recommended maximum size of the jitter offset is a free parameter. It must be smaller than
half the nodding offset but larger than the spatial extent of the minimum feature that one
hopes to detect; for point sources, it should be larger than the width of the spatial profile.
The jitter offset is always along the slit.
5.2.5.3 M2 Chopping
No M2 chopping can be done with CRIRES.
5.2.6 Flux calibration and telluric correction
5.2.6.1 General Procedure
Flux calibration and telluric correction are generally carried out in three steps: