User Manual
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Figure 32: Effect of changes in airmass on PWV absorption produced by the CRIRES ETC.
5.2.3 The influence of the Moon
The Moon illumination (i.e., Fraction of Lunar Illumination, FLI) is defined as the fraction of
the lunar disk illuminated at local (Chilean) civil midnight, where 1.0 is fully illuminated. By
definition, Moon FLI equals 0 when the Moon is below the horizon on Paranal.
Moonlight does not noticeably increase the background in any of the CRIRES modes, so
there is no need to request dark (i.e., FLI <0.4) or grey time (i.e., 0.4
≤
FLI
≤
0.7). However,
it should be avoided to observe targets closer than 30
to the Moon, as this can lead to
problems linked with the telescope guiding and active optics correction system.
The Moon, however, may affect the quality of the AO correction if the source used for
wavefront sensing is fainter than R=14 mag. In this case, reducing the FLI constraint to 0.7
and increasing the distance to the Moon to approximately 50 degrees is recommended.
5.2.4 Sky Transparency
Thin clouds (THN) usually do not hamper CRIRES observations for bright objects. Clear
(CLR) conditions are justified for observations requiring stable AO corrections, for example
to study the close environment of the target, as clouds could affect the quality of the adaptive
optics correction. For the same reason, CLR conditions should be requested for AO
observations using AO stars fainter than R=14. Please note that AO observations cannot
be performed under THK condition. Observations requiring accurate flux calibration of the
spectra should be done under PHO conditions.
5.2.5 Background removal
Background removal in NIR spectra consists of dealing with sky emission lines and sky
brightness variability, as well as with detector cosmetics and instabilities.
The sky background emission can be divided in two regimes depending on the wavelength.
Below 2200 nm, the sky emission is dominated by OH lines, formed at an altitude of 80 km.
