User Manual
Doc. Number: ESO-323064
Doc. Version: 2
Released on:
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46 of 82
Document Classification: Public
It is extremely important that users check if their astronomical lines fall on top of one of
these spectral features as their data may be useless if this occurs (see Figure 31).
Figure 31: CRIRES wavelength setting coverage compared to the atmospheric
transmission. High-resolution version of this figure can be found online
(http://www.eso.org/sci/facilities/paranal/instruments/crires/img/CRIRES-wavelength-
settings2021.png)
The amount of telluric absorption varies non-linearly with zenith distance and PWV (see
Figure 32).
The transmission spectrum of the sky for a particular setting is an optional output provided
by the ETC, which allows one to select spectra based on di
ff
erent amounts of PWV (i.e.,
Figure 31). The available values range from a minimum of 0.05 mm to a maximum of 30
mm. The median value of the PWV on Paranal is 2.5 mm.
Note that the PWV is a mandatory user-definable constraint in p1 and p2. For L and M-band
observations, we strongly recommend setting the PWV constraint to a value equal or
smaller than 2.5.
For any given CRIRES frame, the corresponding PWV is store in the data-fits header
keywords TEL AMBI IWV START, while the Phase 2 requested value is available in the
keyword HIERARCH ESO OBS WATERVAPOUR.