ESO CRIRES+ User manual

European Organisation for Astronomical Research in the Southern Hemisphere
European Southern Observatory
Headquarters Garching
Karl-Schwarzschild-Straße 2
85748 Garching bei München
www.eso.org
Programme: PIP
Project/WP: CRIRES+
CRIRES+ User Manual
Document Number: ESO-323064
Document Version: 2
Document Type: Manual (MAN)
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Prepared by:
Valenti, Elena
Validated by:
Bristow, Paul
Approved by:
Dorn, Reinhold
Name
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by ESO
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by Released by
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Date:
2021.02.26
09:05:09 +01'00'

CRIRES+ User Manual
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Authors
Name
Affiliation
Elena Valenti
ESO
Anna Brucalassi
INAF
Florian Rodler
ESO
Change Record from previous Version
Affected
Section(s)
Changes / Reason / Remarks
All
First version after on sky commissioning, to be used for operation

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Contents
Table of Contents
1. INTRODUCTION ........................................................................................................................ 5
1.1 SCOPE ................................................................................................................................. 5
1.2 DEFINITIONS,ACRONYMS AND ABBREVIATIONS ...................................................................... 5
1.3 IMPORTANT WEBSITES........................................................................................................... 6
1.4 CONTACT INFORMATION........................................................................................................ 7
2. OVERVIEW ................................................................................................................................ 8
2.1 CRIRES IN A NUTSHELL........................................................................................................ 8
2.2 SCIENCE DRIVERS .............................................................................................................. 10
2.2.1 Search for super-earth in habitable zone for low mass stars ....................................... 10
2.2.2 Atmospheric characterization of exoplanets ................................................................ 10
2.2.3 Origin and evolution of stellar magnetic field................................................................ 10
3. THE INSTRUMENT.................................................................................................................. 12
3.1 THE COLD PART:OPTO-MECHANICS.................................................................................... 12
3.1.1 The new Cross-Dispersion unit .................................................................................... 13
3.1.2 The spectrograph unit................................................................................................... 14
3.1.3 The new detectors ........................................................................................................ 14
3.1.4 The cryogenic vessel.................................................................................................... 15
3.1.5 The CRIRES ................................................................................................................. 16
3.2 THE WARM PART................................................................................................................ 17
3.2.1 The Adaptive Optic System MACAO............................................................................ 18
3.2.2 MACAO Hardware Description..................................................................................... 20
3.2.3 The New Calibration Unit.............................................................................................. 23
3.2.4 The Spectro-Polarimetry Unit (SPU) ............................................................................ 28
4. INSTRUMENT PERFORMANCE............................................................................................. 30
4.1 OVERVIEW.......................................................................................................................... 30
4.2 AO PERFORMANCE ............................................................................................................. 30
4.3 DETECTOR CHARACTERISTIC ............................................................................................... 32
4.3.1 Dark and gain ............................................................................................................... 33
4.3.2 Correcting for detectors non-linearity ........................................................................... 35
4.4 CHARACTERISTIC OF THE SPECTROGRAPH ........................................................................... 35
4.4.1 Slit viewer camera field of view .................................................................................... 35
4.4.2 Slit viewer limiting magnitude ....................................................................................... 36
4.4.3 Wavelength settings ..................................................................................................... 37
4.4.4 Wavelength calibration ................................................................................................. 38
4.4.5 Flat Field ....................................................................................................................... 38
4.4.6 Spectrograph Field-of-View, slit width and seeing ....................................................... 39
4.4.7 Spectral resolving power .............................................................................................. 39
4.4.8 Radial velocity precision ............................................................................................... 39
4.5 THROUGHPUT..................................................................................................................... 40
5. PHASE I: OBSERVING PROPOSAL PREPARATION........................................................... 42
5.1 INSTRUMENT MODES OFFERED IN P108 ............................................................................... 42
5.2 PHASE I: GENERAL INFORMATION AND USER CONSTRAINTS.................................................. 42
5.2.1 Turbulence Category .................................................................................................... 43
5.2.2 Atmospheric transmission and Precipitable Water Vapour .......................................... 44
5.2.3 The influence of the Moon ............................................................................................ 47
5.2.4 Sky Transparency......................................................................................................... 47

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5.2.5 Background removal..................................................................................................... 47
5.2.6 Flux calibration and telluric correction .......................................................................... 48
5.3 THE EXPOSURE TIME CALCULATOR ..................................................................................... 50
5.4 INSTRUMENT OVERHEADS.................................................................................................... 52
6. PHASE II: PREPARING THE OBSERVATIONS .................................................................... 54
6.1 SERVICE MODE OBSERVATIONS ........................................................................................... 54
6.2 VISITOR MODE OBSERVATIONS ............................................................................................ 55
6.3 OB PREPARATION............................................................................................................... 55
6.3.1 Target acquisition template .......................................................................................... 55
6.3.2 Science templates ........................................................................................................ 61
6.3.3 Night-time calibrations .................................................................................................. 66
6.3.4 OB Constraint set ......................................................................................................... 69
6.3.5 Ephemeris .................................................................................................................... 70
6.3.6 Finding Charts .............................................................................................................. 70
6.3.7 README file................................................................................................................. 70
7. REFERENCE MATERIAL........................................................................................................ 72
7.1 CALIBRATION PLAN ............................................................................................................. 72
7.2 WAVELENGTH SETTINGS ..................................................................................................... 73
7.3 TEMPLATE SIGNATURE FILES .............................................................................................. 74
7.3.1 Acquisition TSF ............................................................................................................ 74
7.3.2 Science observing TSF ................................................................................................ 77
7.3.3 Calibration TSF............................................................................................................. 80
7.4 DATA FORMAT AND REDUCTION ........................................................................................... 81
7.4.1 Format .......................................................................................................................... 81
7.4.2 Selection of CRIRES FITS header keywords............................................................... 81
7.4.3 Pipeline......................................................................................................................... 81

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1. Introduction
1.1 Scope
The aim of the CRIRES User Manual is to provide information on the technical
characteristics of the instrument, its performance, observing, calibration and data reduction
procedures.
The document is structured as follows:
•Section 3 provides a technical description of CRIRES and its adaptive optics system.
•Section 4 details the instrument performance.
•Section 5 guides the users through the preparation of the observing proposal (Phase
I) providing a summary of the commonly observing techniques in the infrared, and
their impact on the Phase I constraints and telescope time
•Section 6 provides guidelines for Phase II preparation.
•Section 7 contains reference material. It includes a description of the calibration
plan, the data format, the template reference guide and the defined reference
settings.
1.2 Definitions, Acronyms and Abbreviations
Throughout this document we will use the terms CRIRES+ and oCRIRES to refer to
CRIRES after and before the upgrade, respectively. However, it should be noted that the
instrument name has not changed. This document employs several abbreviations and
acronyms to refer concisely to an item, after it has been introduced. The following list is
aimed to help the reader in recalling the extended meaning of each short expression:
AO
Adaptive Optics
APD
Avalanche photodiode
BOB
Broker of Observation Blocks
CPL
Common Pipeline Library
CRIRES
Cryogenic high-resolution infrared echelle spectrograph
CRIRES+
CRIRES upgrade project
DM
Deformable Mirror
DMO
Data management and operations division
ESO
European Southern Observatory
ETC
Exposure time calculator
FC
Finding Chart
FoV
Field of View
FPET
Fabry-Perot Etalon
FWHM
Full Width at Half Maximum
NGC
New General detector Controller
NGS
Natural guide stars
NIR
Near infrared
NIST
National Institute of Standards and Technology
OB
Observation Block
oCRIRES
Original or old CRIRES
p2
Phase II web-based preparation
p1
Phase I web-based proposal preparation and submission system

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PSF
Point Spread Function
QC
Quality Control
RTC
Real Time Computer (MACAO)
RTD
Real Time Display
SDD
Software Development Division
SM
Service Mode
SNR
Signal-to-Noise Ratio
SR
Strehl Ratio
SV
Slit Viewer
SVGS
Slit View Guide Star
TC
Turbulence Category
TIO
Telescope and instrument operator
TTM
Tip-tilt mount
USD
User Support department
VLT
Very Large Telescope
VM
Visitor Mode
WF
Wave Front
WFS
Wave Front Sensor
1.3 Important websites
All CRIRES related manuals are available on the instrument web page together with the
most updated information:
http://www.eso.org/sci/facilities/paranal/instruments/crires.html
Both Service and Visitor mode Observation Blocks (OBs) should be prepared with the
latest version of the Phase 2 web-based preparation tool (p2), available at:
https://www.eso.org/sci/observing/phase2/p2intro.html
Information for the preparation of the Service mode observations with CRIRES are
available at:
http://www.eso.org/sci/observing/phase2/SMGuidelines.CRIRES.html
Visiting astronomers do not need to submit OBs in advance of their observations. However,
they should prepare them before arriving at the observatory or, at the latest, at the
observatory the nights before their observing run. They will find further instructions on the
Paranal Science Operations web page and the Paranal Observatory home page:
https://www.eso.org/public/teles-instr/paranal-observatory/vlt/
http://www.eso.org/sci/facilities/paranal/sciops.html
In particular, visiting astronomer should read the following webpage:
http://www.eso.org/sci/facilities/paranal/instruments/crires/visitor.html

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Reference frames, static calibration frames, information regarding the CRIRES pipeline and
quality control can be found at:
http://www.eso.org/observing/dfo/quality/
1.4 Contact Information
In case of specific questions related to proposal preparation, Service Mode observations,
and the use of the pipeline please contact the ESO User Support Department:
[email protected]
For Visitor Mode observations please contact the Paranal Science Operations Team. For
general information, use:
[email protected]

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2. Overview
2.1 CRIRES in a nutshell
Figure 1: Optical layout of CRIRES
A basic summary of the new and main instrument parameters is given below:
Spectral resolution 50,000 and 100,000
1
Wavelength coverage 0.95 - 5.3 μm | YJHKLM bands
RV precision 3 m/s by using gas cells
Slit length 10 arcseconds
Slit width 0.2 and 0.4 arcseconds
Slit length 10 arcseconds
Polarimetry linear + circular (YJHK bands)
Adaptive optics 60 actuator curvature sensing (MACAO)
Cross-disperser 6 gratings (YJHKLM)
1
See 4.4.7 for more details on ongoing analysis

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Figure 2: Light path sketch of the upgraded CRIRES.
Before the upgrade, the adaptive optics (AO) assisted CRIRES instrument (oCRIRES) was
an IR (0.92 - 5.2 μm) high-resolution spectrograph in operation from 2006 to 2014 at the
Very Large Telescope (VLT) observatory. oCRIRES was a unique instrument, accessing a
parameter space (wavelength range and spectral resolution) up to now largely uncharted.
It consisted of a single-order spectrograph providing long-slit (40 arcsecond) spectroscopy
with a resolving power up to R=100 000. However, the setup was limited to a narrow, single-
shot, spectral range of about 1/70 of the central wavelength, resulting in low observing
efficiency for many scientific programmes requiring a broad spectral coverage.
The CRIRES upgrade project, CRIRES+, has transformed this VLT instrument into a cross-
dispersed spectrograph with the goal to increase the simultaneously covered wavelength
range by a factor of ten. A new and larger detector focal plane array of three Hawaii 2RG
detectors with 5.3μm cut-off wavelength replaced the existing detectors. For advanced
wavelength calibration, custom-made absorption gas cells and an etalon system have been
added. A spectro-polarimetric unit allow the recording of circular and linear polarized
spectra. This upgrade is supported by dedicated data reduction software allowing the
community to take full advantage of the new capabilities.
Figure 2 summarizes the overall concept of the CRIRES upgrade. The main, high resolution
spectrometer unit remains untouched. The new cross-disperser unit substitutes the old re-
imager and pre-dispersing sub-systems.

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Figure 3: Schematic diagram summarizing the differences between oCRIRES and CRIRES
after the upgrade
2.2 Science Drivers
A set of fundamental scientific goals were defined for CRIRES during the Phase A study:
2.2.1 Search for super-earth in habitable zone for low mass stars
A large fraction of all exoplanets has been discovered primarily through radial velocity (RV)
measurements. However, only 5% of the planets detected so far orbit stars with stellar
masses less than about 0.5 Msun. Thus, we lack key knowledge about the process of planet
formation around the most numerous stars in our galaxy –M dwarfs. Low mass stars are
especially interesting because these objects are cold, and the habitable zones are quite
close to the star. The reflex motion of an M star (0.15 Msun) with a 1 MEarth planet in its
habitable zone is about 1 m s-1. Since M dwarfs and brown dwarfs have low effective
temperatures, radiating most of their energy in the IR (1.0 - 2.5 μm), a high-resolution IR
spectrograph is therefore ideal for searching for low mass planets around these objects. A
new gas absorption cell to provide a stable wavelength reference as well as the increase in
wavelength coverage by about a factor of ten should result in an attainable RV precision for
the upgraded CRIRES of 3 m s-1. This would enable the detection of super Earth-mass
planets in the habitable zone of an M-dwarf star in the solar neighbourhood.
2.2.2 Atmospheric characterization of exoplanets
High-resolution spectroscopy of exoplanets provides us with means of studying the physical
(e.g., winds) and chemical composition of exoplanetary atmospheres. CRIRES is well suited
for the observation of close-in, highly irradiated planets that radiate most of their light in the
IR. Furthermore, the IR is a spectral region where lines of molecular gases like CO, NH3,
CH4, etc. are expected to be present in exoplanetary atmospheres.
2.2.3 Origin and evolution of stellar magnetic field
Magnetic fields play a fundamental role in the life of all stars: they govern the emergence of
stars from proto-stellar clouds, control the in-fall of gas onto the surfaces of young stars and

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aid the formation of planetary systems. Measurements of magnetic fields have mostly been
confined to A- and B-type stars, so our knowledge of magnetic fields in Sun-like stars, and
the low end of the main sequence, is still poor. The upgraded CRIRES will make it possible
to measure with greater accuracy magnetic fields in M-dwarfs and brown dwarfs for several
reasons:
1) The Zeeman splitting of a spectral line is proportional to λ2, so there is a huge leverage
in going to the IR; 2) For cool objects most of the flux is in the IR so there is also a gain due
to the increased signal-to-noise ratio. 3) In order to disentangle Zeeman broadening from
other broadening effects one must compare the broadening of Zeeman sensitive lines to
magnetically insensitive lines. The large wavelength coverage of CRIRES will include many
more lines of different magnetic sensitivities needed for an accurate determination of the
field strength. 4) The capability of CRIRES to take circular and linear polarized spectra will
support these measurements.
The spectro-polarimetric mode is not offered in P108.

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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 two 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.

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3.1.1 The new Cross-Dispersion unit
The fore-optics of the upgraded CRIRES is shown in Figure 4. It consists of an off-axis
parabola, which creates a collimated beam with a diameter of 50 mm, being followed by two
flat mirrors with distances and angles adjusted to match the new fore-optics with the already
existing three-mirror anastigmatic (TMA) relay optics and the echelle grating which
remained from the original CRIRES instrument.
Figure 4: Top view of the new CRIRES fore-optics assembly
Figure 4 shows that the beam from the f/15 focus at the new entrance slit (A) is collimated
by a parabolic mirror (B) and arrives at the cross-disperser wheel (E) via two flat mirrors
and a long pass filter wheel (C) to block the 2nd and higher orders of the cross-disperser
gratings. The jitter mirror (D) has two piezo actuators that allow the echellogram to be
translated at sub-pixel accuracy on the detectors. The order-sorting filter can be accordingly
selected from one of three filters (YJ, KH, LM or an open position) to the chosen cross
disperser grating. The cross-disperser wheel contains six reflection gratings, one for each
of the wavelength bands Y, J, K, H, L and M. The Metrology system ensures accurate
repeatability of the cross-disperser wheel.
A
C
B
D
E
F

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Figure 5: Grating wheel design with locking mechanism and build prototype
Following the cross-disperser grating, an achromatic camera (F) working at a fixed focal
length brings the collimated beam to an f/8 focus at the field stop. In order to avoid time
consuming thermal cycling during the AIT phase the camera is mounted on a small and
simple focusing stage. This focusing functionality is only intended for integration and
maintenance and not for regular operations.
3.1.2 The spectrograph unit
The echelle grating subsystem is unchanged relative to the oCRIRES. It consists of a 40 x
20 cm, 31.6 lines/mm, 63,5deg blaze echelle grating plus a TMA (three-mirror anastigmatic)
which acts first as a collimator and then as a camera to image the spectrum on the new
three Hawaii 2RG detectors effectively forming an 6144x2048 pixels array. More details on
the optical and opto-mechanical designs can be found in Lizon et al. (2014) and Oliva et al.
(2014), respectively.
3.1.3 The new detectors
Another major part of the upgrade project was to increase the coverage of the focal plane
by introducing a set of new detectors. To accommodate the echelle spectral format, a larger
field was required to cover the ten orders per band with a slit length of 10 arcseconds. Figure
6presents a comparison between the oCRIRES focal plane array area and the actual array
of CRIRES detectors after the upgrade. The actual detector array, composed by three
Hawaii 2RG detectors (the CRIRES H2RG detectors are shown in Figure 7 on the right
together with the detector mount on the left), span over 6144 x 2048 pixels (111mm x 37mm)
at a pixel size of 18μm. For comparison, the old Aladdin mosaic spanned only 4096 x 512
pixels (111mm x 14mm) with a pixel size of 27μm, as described in Dorn et al. (2006).

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Figure 6: The original CRIRES detector mosaic focal plane array area compared to the new
detectors with an increase of a factor of 2.7 in the cross-dispersion direction.
The new detector mosaic does not only provide a larger area but also lower noise, higher
quantum efficiency, better cosmetic quality and much lower dark current. Also, the gaps
between the detectors in the mosaic are smaller. The detectors operate at 35K with
cryogenic preamplifiers located next to the focal plane.
Figure 7: The 3 CRIRES H2RG detectors are shown (right) together with the detector mount
(left)
All detector systems, including the slit viewer camera, is upgraded to the current ESO
standard New General detector Controller (NGC). This detector upgrade does not only
significantly increase the coverage of the focal plane, but the increased spatial homogeneity
of the pixel response combined with lower readout noise, dark current and higher QE will
result in improved data quality. All detectors have been tested at the ESO detector labs and
the full detector system is in operation in the upgraded CRIRES instrument.
3.1.4 The cryogenic vessel
CRIRES is located at the Nasmyth B focus of VLT-UT3. The instrument is mounted in a 3
m-diameter, 1 m high vessel. Including its support structure, the total weight of the
oCRIRES Aladin detector
mosaic 4096 x 512 pixels,
27 microns pixel size
Current Hawaii2RG detector
mosaic 6144 x 2048 pixels,
18 microns pixel size

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instrument is 6.2 t, spread between 2 t for the warm part or AO system and 4.2 t for the cold
part. The optics inside the cryo-vessel is cooled to 65 K. The detectors are stabilized at 35
K within ~5 mK.
3.1.5 The CRIRES
In order to stable exposures with high repeatability, the concept of a metrology system was
developed to allow for a 0.1pixel reproducibility. The strategy is to centre a defined set of
emission lines of the Kr and Ne pen ray lamps on the science detector by finetuning the
positions of the cross-disperser grating and Echelle grating and refining further via the use
of a piezo driven tip-tilt mirror that has actuators aligned with the main- and cross-dispersion
axes. This is an iterative process which may take a few minutes, the exact duration depends
upon the unpredictable behaviour of the cross-disperser grating and Echelle grating
functions. The metrology then ensures that these emission lines are indeed located at their
fiducial positions on the science FPA before any science exposure (or any calibration
exposure when used during daytime) follows. Those science/calibration exposures
obtained after a successful application of metrology will have the following metrology
keywords written to their headers (values below are examples):
HIERARCH ESO OCS MTRLGY DX = 0.002 / [pixels] Final mean x residual relative to
fiducial
HIERARCH ESO OCS MTRLGY DY = 0.039 / [pixels] Final mean y residual relative to
fiducial
HIERARCH ESO OCS MTRLGY ECHCORR = -54 / [Enc] Difference between the nominal echelle
encoder value and the resulting value after metrology
HIERARCH ESO OCS MTRLGY ECHNMOV = 1 / Number of echelle grating moves
HIERARCH ESO OCS MTRLGY NITER = 5 / Total number of iterations performed
HIERARCH ESO OCS MTRLGY PIEZO1 = 4.340 / [V] Cross-dispersion piezo voltage at the
conclusion of metrology
HIERARCH ESO OCS MTRLGY PIEZO2 = 3.990 / [V] Main-dispersion piezo voltage at th e
conclusion of metrology
HIERARCH ESO OCS MTRLGY ST = T / Success or failure of metrology
HIERARCH ESO OCS MTRLGY TIME = 1612768172.0 / [s] Timestamp for the successful
conclusion of metrology
HIERARCH ESO OCS MTRLGY TOTDX = -1.430 / [pixels] Average total applied correction
in the main dispersion direction
HIERARCH ESO OCS MTRLGY TOTDY = 0.194 / [pixels] Average total applied correction
in the cross-dispersion direction
HIERARCH ESO OCS MTRLGY XDGWCORR = 0 / [Enc] Difference between the nominal XDGW
encoder value and the resulting value after metrology
HIERARCH ESO OCS MTRLGY XDGWNMOV = 0 / Number of XDGW moves
The metrology can be activated or deactivated in the acquisition and observing templates.
When it is enabled during the acquisition the metrology runs in parallel to the telescope
preset, therefore no overheads are associated to the metrology (see Table 9).

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In Y-, J-, H- and K-bands the metrology converges reliably resulting in residual errors in
relative alignment of 0.1pix in main dispersion and 0.5pix in cross-dispersion. If the
metrology is not used the relative alignment is an order of magnitude larger. In P108 the
metrology should not be used for L- and M-band observations, its behaviour has not yet
been characterised in these bands (whilst it is known to be more challenging due to the
scarcity of suitable emission lines and the significant continuum from the pen-ray lamps).
An important note regarding main-dispersion stability following metrology: During
commissioning it was observed that the drift in main-dispersion echellogramme alignment
was somewhat higher (~0.2pix over 30mins) following metrology alignment than it was
without metrology (0.05pix over 30mins, consistent with PAE measurements and
specifications). This effect is still being investigated and several mitigation strategies are
under consideration, but users should keep in mind that during P108 spectral resolution and
alignment of data obtained within 30mins of the application of metrology may be degraded
due to this effect.
3.2 The Warm Part
The Warm Part of CRIRES consists of different subsystems (see Figure 8 for an overview):
the AO Unit, the Calibration Unit which also includes a Fabry-Perot Etalon System and a
carriage stage with the new Polarimetry Unit and new sources for wavelength calibration
described in detail in Section 3.2.3.
Figure 8: The upgraded CRIRES warm part assembly with etalon system, calibration slide,
AO system and de-rotator mechanism
MACAO
FP Etalon
System
Carriage Stage
Calibration Unit
Spectro-Polarimetry
Unit

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3.2.1 The Adaptive Optic System MACAO
The adaptive optics system of CRIRES is described in Paufiqué et al. (2004, SPIE 5490,
216). The multi-application curvature adaptive optics system (MACAO) for CRIRES corrects
a turbulent wavefront and provides diffraction limited images at the focal plane. The overall
sensitivity is thereby improved by about a factor two for point-sources. To highlight the
advantage of combining MACAO and CRIRES a PSF is shown in Figure 9 in AO open loop
(uncorrected) and closed loop, where the PSF is reconstructed from wavefront
measurements. The non-circular PSF in open loop is due to the very short integration time
used.
Figure 9: PSF without (left) and with (right) MACAO correction. Images have been taken
in lab using a turbulence generator.
The following section provides an introduction to the field of adaptive optics and
atmospheric turbulence, and essentially is taken from the NACO user manual. For further
reading, see for example: “Adaptive optics in astronomy”, Rodier 1999, Cambridge
University Press, or “Introduction to adaptive optics”, Tyson 2000, Bellingham/SPIE.
3.2.1.1 Atmospheric Turbulence
The VLT theoretical diffraction limit is 1.22×l/D = 0.07 arcsec at a wavelength of 2.2 μm.
However, the resolution is severely limited by atmospheric turbulence to l/r0~1 arcsec,
where r0is the Fried parameter. r0is directly linked to the strength of the turbulence and
depends on the wavelength as l6/5. For average observing conditions, r0is typically 60cm at
2.2μm.
Temperature inhomogeneities in the atmosphere induce temporal and spatial fluctuations
in the air refractive index and therefore cause fluctuations in the optical path. This leads to
random phase delays that corrugate the wavefront (WF). The path differences are, to a
good approximation, achromatic. Only the phase of the WF is chromatic. The coherence
time of WF distortions is related to the average wind speed V in the atmosphere and is
typically of the order of r0/V = 60 ms at 2.2μm for V = 10 m/s.

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3.2.1.2 Adaptive Optics
A technique to overcome the degrading effects of atmospheric turbulence is real-time
compensation of the deformation of the WF by adaptive optics (AO, Figure 10).
Figure 10: Principle of Adaptive Optics. Note that in practice, and contrary to the this
schematic design, CRIRES has not dedicated tip-tilt mirror, but perform low- and high-order
correction with a single deformable mirror mounted on a tip-tilt stage (see Figure 12).
The wavefront sensor (WFS) measures WF distortions which are processed by a real-time
computer (RTC). The RTC controls a deformable mirror (DM) to compensate the WF
distortions. The DM is a continuous thin plate mirror mounted on a set of piezoelectric
actuators that push and pull on the back of the mirror. Because of the significant reduction
in the WF distortions by continuous AO correction, it is possible to record near diffraction-
limited images with exposure times that are significantly longer than the turbulence
coherence time. The residual error from the WF compensation (WF error) directly
determines the quality of the formed image. One of the main parameters characterizing this
image quality is the Strehl ratio (SR), which corresponds to the amount of light contained in
the diffraction-limited core relative to the total flux.
An AO system is a servo-loop system working in closed loop. The DM flattens the incoming
WF and the WFS measures the residual WF error. A commonly used WFS is the Shack-
Hartmann WFS, used for example in NACO. However, CRIRES, as well as other ESO
MACAO systems, relies on a curvature WFS: it is designed to measure the WF curvature
as opposed to the WF slope. This is achieved by comparing the irradiance distributions of
two planes placed behind and in front of the focal plane. In practice, a variable curvature
mirror (membrane) is placed in the telescope focus. By vibrating, inside and outside focus
blurred pupil images can be imaged on a detector array: in the case of CRIRES, a lenslet
array feeds avalanche photo-diodes (APDs). The modulation frequency of the membrane
corresponds to the temporal sampling frequency of the WFS. The difference between the
inside and outside pupil image measures the local WF curvature.

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The performance of an AO system is related to the number of lenslets in the lenslet array,
the number of actuators behind the DM, and the rate at which WF errors can be measured,
processed and corrected (the server-loop bandwidth).
The performance of an AO system is also linked to the observing conditions. The most
important parameters are the seeing, the coherence time, the brightness of the reference
source used for WFS and the distance between the reference source and the object of
interest. In case of good conditions (i.e., seeing < 0.8” and coherence time > 3ms) and a
bright (i.e., R< 7), nearby (i.e., within ~5 “) reference source, the correction is good, and the
resulting point spread function (PSF) is very close to the diffraction limit. A good correction
in the K-band typically corresponds to a SR larger than 30%. At shorter wavelengths
(particularly in the J-band) or in the case of poor conditions or a faint, distant reference
source, the correction is only partial - the SR may only be a few percent
3.2.2 MACAO Hardware Description
The MACAO system for CRIRES is based on a 60-actuator deformable mirror, inserted in
a so-called relay optics. These optics and the wavefront sensor optics are mounted on a
bread-board located between the Nasmyth focus and the spectrometer. It is about 1.5m
wide and a top view of the warm optics overlaid by the optical path is shown in Figure 11,
the assembly of the deformable mirror is displayed in Figure 12 .
Figure 11: Top view of the warm optics of the MACAO-CRIRES system. From f/15 Nasmyth
focus and after the optical derotator, one notices the deformable mirror and the tip-tilt mount
assembly. Light enters from the dichroic to the cold and warm part of the instrument. On the
right the wavefront sensor and some analysis tools are visible.