User Manual
Doc. Number: ESO-323064
Doc. Version: 2
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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 in the light-path. A carriage stage (not depicted in
Fig.1) can then insert one of the following elements in 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; (ii) a pinhole used for calibration purposes;
(iii) an AO fiber for MACAO calibrations; (iv) an Uranium-Neon Lamp for 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 3 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 through a new dichroic window.
The optical light is reflected and used for the adaptive optics system, the infrared light (0.95
μm < λ < 5.2 μm)
will be 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 sec. 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 A), which comprises a movable mask with tw
o slits: 0.2” (resolving power
~100,000) 0.4” slit (resolving power ~50,000)
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 will not limit 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
will significantly enhance the SV camera performance when compared to oCRIRES.