CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
51 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
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 may affect the quality of the AO correction if the AO natural guide star is fainter
than R=14 mag. In this case, reducing the FLI constraint to 0.7 is recommended.
5.3.5 Sky Transparency
•
‘Variable, thick cirrus’ is only possible in NoAO mode.
•
‘Variable, thin cirrus’ usually do not affect CRIRES observations of bright objects
(H<10).
•
‘Clear’ 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. It is recommended to request ‘Clear’
conditions for AO observations with AO stars fainter than R=10.
•
'Photometric’ conditions should be only requested for observations requiring
accurate flux calibration of the spectra.
5.4 Observing Strategy
5.4.1 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. 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.
5.4.1.1 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
