CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
12 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
3. The instrument
The optical layout of CRIRES after the upgrade is shown in Figure 1. Light enters from the
direction of the telescope Nasmyth focus, either via the telescope or from the calibration
unit after insertion of a calibration mirror into the light-path. A carriage stage (not depicted
in Fig.1) can then insert one of the following elements into the light path: (i) The new
polarimetry unit;; (ii) a gas-cell either for wavelength calibrations when used with the halogen
lamp (which creates an absorption spectrum), or for accurate radial-velocity measurements,
similar to the way for the iodine cell technique;; (iii) a pinhole used for calibration purposes;;
(iv) an AO fiber for MACAO calibrations;; (v) a Uranium-Neon Lamp for absolute wavelength
calibration. This carriage has also a free position, with no optical element (see a detailed
description of the Calibration Unit in Section 3.2.3).
Light then goes through a three mirror de-rotator which can be used to counteract the
telescope field rotation for observations with a slit fixed relative to the sky. On the other
hand, for point sources, it can also maintain the slit aligned along the parallactic angle to
accommodate the differential atmospheric refraction between the R band used by the
adaptive optics system and the IR band used for observations and slit viewer guiding. The
light enters the cold dewar of the spectrograph through a dichroic window.
The optical light is reflected and used for the adaptive optics system, the infrared light (0.95
μm < λ < 5.3 μm) is transmitted to the cold optics of CRIRES. The AO system concentrates
the light on the spectrograph’s entrance slit. Further details of the AO system can be found
in Section 3.2.1
of this manual. CRIRES can be used without adaptive optics, in which case
the AO module just acts as relay optics and the spatial resolution is given by the natural
seeing. Under normal conditions this leads to higher slit losses than when AO is used.
3.1 The Cold Part: Opto-mechanics
After the dichroic window, the infrared light passes through a new entrance slit unit (see
Figure 4 box "A"), which comprises a movable mask with two slits: 0.2” (minimum resolving
power 80,000) 0.4” slit (minimum resolving power 40,000) preserving the spectral resolution
of CRIRES. The mask can also be positioned so that neither slit is in the optical path and
the spectrograph is closed to light from the telescope. The reproducibility and stability are
significantly enhanced compared to the old slit mechanism.
In addition, the CRIRES entrance slit mechanism includes a decker for polarimetric
observations allowing for the left and right-hand polarised beams at two nodding positions.
To cover the additional orders the spatial extent of the two main slits was reduced from 40
to 10 arcseconds, providing a balanced compromise (based on an analysis of the past and
future scientific requirements and science cases) between cross-dispersion implementation
and the old CRIRES long slit usage. The 10 arcsec long slit allows observations of extended
sources and allow nodding for precise background subtraction observing methods.
The light reflected by the slit mask is used by the slit viewer camera to assist the adaptive
optics system in centring and keeping the targets PSF on the slit as for the oCRIRES.
However, the CRIRES slit viewer subsystem has been substantially modified: it is
composed of two folding mirrors, a camera to image the entrance slit on a detector and a
filter wheel to select the filter for guiding. The SV detector is now a H2RG detector, which
