The document describes the proposed layout of the On Instrument Wavefront Sensor (OIWFS) components that will be mounted inside the GMT Instrument Feed System (GMTIFS) on the Giant Magellan Telescope (GMT). The OIWFS includes channels for the deformable mirror, truth path, tip/tilt sensing, integrated optic segment piston sensing, and focus sensing. All optics will be mounted on a single optical bench inside GMTIFS and maintained at a temperature of 70K. The layout must fit all components in a tight space around the deformable mirror and withstand stresses from cooling from 300K to 70K without degrading image quality or sensor performance.

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GMTAO
THE AUSTRALIAN NATIONAL UNIVERSITY
System Design Note
GMTAO-SDN-02-005
Version Draft
On Instrument Wave Front Sensor Mechanical Design
Research School of Astronomy and Astrophysics
ANU College of Science
The Australian National University
Revision History
Version No. Author &
Date
Approval &
Date
Description
draft R Gardhouse 12-11-11 Draft
Contents
1 Table of Figures........................................................................................................................1
2 Purpose......................................................................................................................................2
3 Applicable Documents.............................................................................................................2
4 Introduction..............................................................................................................................2
5 Mounting Inside GMTIS.........................................................................................................2
5.1 General ...................................................................................................................................2
5.2 Supports..................................................................................................................................3
6 OIWS.........................................................................................................................................4
6.1 General ...................................................................................................................................4
6.2 Deformable Mirror Path........................................................................................................6
6.3 Truth Path ..............................................................................................................................8
6.4 Tip/Tilt Layout......................................................................................................................10
6.5 IOPS Layout.....................................................................................................................1213
6.6 Atmospheric Dispersion Correction/IOPS Dithering.....................................................1314
6.7 Focus Layout....................................................................................................................1415
7 Attachment A:....................................................................................................................1516
1 Table of Figures
Figure 5-1 OIWS Location......................................................................................................................................... 3
Figure 5-2 OIWS Support .......................................................................................................................................... 4
Figure 6-1 OIWS Layout............................................................................................................................................ 5
Figure 6-2 Truth Layout............................................................................................................................................. 9

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Figure 6-3 Deformable Mirror Layout ....................................................................................................................... 7
Figure 6-4 Deformable Mirror Path ........................................................................................................................... 8
Figure 6-5 Tip/Tilt Layout ....................................................................................................................................... 11
Figure 6-6 Tip/Tilt Calibration Layout .................................................................Error! Bookmark not defined.12
Figure 6-7 IOPS Layout ....................................................................................................................................... 1314
Figure 6-8 Dispersion Correction/Dithering ........................................................................................................ 1415
Figure 6-9 Focus Layout ...................................................................................................................................... 1516
2 Purpose
This document describes the proposed layout of the components for the On Instrument Wave Front Sensor
(OIWFS) and how it fits inside GMTIFS
3 Applicable Documents
Document ID Source Title Version Date
GMTAO-SDN-02-004 OIWFS Optical Design Draft 12-09-18
GMTAO-SDN-02-007 IOPS Design 0.2 12-12-21
4 Introduction
The OIWFS optical bench is mounted to the bottom surface of the GMTIFS optical bench and is comprised of four
parts:
- The field optics channel bringing the light form the GMTIS entrance beam through the GMTIS optical
bench to the deformable mirror (DM)
- The Truth channel controlling the on axis reconstruction of the secondary mirrors
- The Tip/Tilt sensor channel controlling the secondary mirrors
- The Integrated Optic Segment Piston Sensor (IOPS) controlling the phasing of the primary mirrors
- The Focus channel controlling the face sheets of the secondary mirrors
5 Mounting Inside GMTIS
5.1 General
The OIWFS optic bench mounts to the lower surface of the GMTIFS optical bench (Figure 5-1). This provides the
cryogenic environment keeping the OIWFS components at 70K.

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Figure 5-1 OIWFS Location
5.2 Supports
The current 3 post mounting between the two optical benches (Figure 5-2) is only possible if the CTE of the 2
optical benches matches. If more analysis determines that the OIWFS optical bench cannot be matched for CTE,
the mounting supports will be require to not transmit sufficient bending stress to effect the image quality of the
either OIWFS channels or GMTIS sensors. This may a flexure based system such as 3 blades or a wire hexapod.

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Figure 5-2 OIWFS Support
6 OIWFS
6.1 General
All optic channels of the OIWFS are mounted on a single optical bench (Figure 6-1). The optical bench is stiff
enough to maintain alignment between the optical paths in all expected orientations as well minimizing the
additional weight of GMTIFS.
Light entering GMTIS is folded 90° by a mirror to pass through the center of the GMTIFS optical bench. On its
journey, the light is conditioned for the OIWFS by field optics before passing through a cold stop at the bottom of
the OIWFS optical bench.
The optical bench also acts as the primary cold sinkto keep the OIWFS components at 70K. The supports to the
GMTIFS optical bench will be done so that there isn’t undue stress caused by differential cooling between the two
optical benches.
The area around the DM is very tight for space (Figure 6-2). The current DM is mounted to a 100mm x 110mm
aluminum oxide carrier making it more cryogenically friendly. To be as close as possible to the pupil as possible,
the dichroic used to compensate for atmospheric dispersion and IOPS dithering sits as close to the DM as possible.
Directly behind this dichroic is the Tip/Tilt optics with its calibration optics and stage. On the other side of the
dichroic is the dichroic dividing the beam between the IOPS & Focus paths.

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Figure 6-1 OIWFS Layout

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Figure 6-2 Packaging Around DM
6.2 Deformable Mirror Path
The deformable mirror feed light to the Tip/Tilt sensor, IOPS and the Focus sensor (Figure 6-3).Light passes
through the dichroic to a turning mirror feeding a pair of off axis parabolic mirrors feeding the DM.

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Figure 6-3 Deformable Mirror Layout
Light entering GMTIS is folded 90° by a mirror to pass through the center of the GMTIFS optical bench. On its
journey, the light is conditioned for the OIWFS by field optics before passing through a cold stop at the bottom of
the OIWFS optical bench (Figure 6-1). Light continues through to a turning mirror and a pair of off axis parabolic
mirrors (OAP) to form a pupil on the deformable mirror (DM).
This optics will be mounted to provide the required mounting tolerances through the required range of motion. As
well, the optic mounts will be designed to minimize stress induced surface figure cause by cooling from 300K to
70K.
Of note is the large wire harness associated with the DM which will need to exit the cryostat through a vacuum
feed through. Reducing excessive parasitic heat load via the cable will require a detailed design study once the
DM is selected.

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Figure 6-4 Deformable Mirror Path
6.3 Truth Path
The Truth path is the first OIWFS sensor. A dichroic mirror between the cold stop and the DM turning mirror
turns J band light 90° and then through conditioning optics forming a pupil on a lenslet array projecting its image
on a H2RG sensor (Figure 6-5).
As with the previous optics, these optics will be mounted to provide the required mounting tolerances through the
required range of motion. As well, the optic mounts will be designed to minimize stress induced surface figure
cause by cooling from 300K to 70K.

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Figure 6-5 Truth Layout
The sensor for the Truth path is the H2RG (Figure 6-6). It is mounted by flexures to its mount. The flexures are
material matched to the H2RG studs reducing the focus change induce by differential CTE.
The sensor’s accompanying electronics, the Sidecar ASIC, is mounted to a sheet metal mount in behind the sensor
to keep the distance between sensor & electronics as short as possible. The standoffs between board & mount both
thermally isolate the electronics form the cold surface as well as allow the electronics to float as the system is
brought to 70K.
As sensor is thermally regulated by a heater and the Sidecar ASIC is a source of heat, light sheet metal heat shield
prevents stray IR radiation.
This sensor is shared with the Focus path and is mounted on a stage to find best focus between the paths as well as
allowing for compensation of the focus change after the system has reached operating temperature. The mount
bridging the stage and the optical bench will allow tip, tilt and translation movement for alignment.

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Figure 6-6 H2RG Mount
6.4 Tip/Tilt Layout
From the DM, light passes through a dichroic mirror sending K band light to the Tip/Tilt optics and onto a H2RG
sensor (Figure 6-7). There are several CaF optics in this path which in the past have proven to be sensitive to
mechanical damage due to stress induced by their mounts.
The Tip-Tilt sensor requires its calibration optics to be moved into the optic path (Figure 6-8Figure 6-9). These
optics and their mounts are mounted on a cryogenic qualified stage mounted to the Tip/Tilt optic mount allowing
the calibration optics moved into and out of the optic path as required.
The next design cycle will include a canvas of the current thinking regarding CaF lens mounts used in a cryogenic
environment. This thinking will be incorporated into the optic mounts to provide the required mounting tolerances
through the required range of motion.

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Figure 6-7 Tip/Tilt Layout

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Figure 6-89 Tip/Tilt Calibration Stage
6.5 IOPS Layout
The dichroic feeding K band light to the Tip-Tilt path folds H band light to the IOPS and Focus paths. A second
dichroic folds the lower H band light to the IOPS sensor, passing upper H band light to the Focus sensor (Figure
6-9Figure 6-10). Light then passes through conditioning optics to form a pupil on the IOPS sensor’s lenslet array.
The mechanical design of the IOPS sensor is described in GMTAO-SDN-02-007.
As with the previous optic paths, the optic mounts will be further developed to develop CaF lens mounts
applicable to a cryogenic environment, incorporating the specified mounting tolerances through the needed range
of motion.

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Figure 6-910 IOPS Layout
6.6 ADC/IOPS Dithering
A critical component of the IOPS and Focus optic paths is the ADC (Atmospheric Dispersion)/IOPS Dithering
mechanism (Figure 6-10Figure 6-11). This is achieved by moving the Tip-Tilt/IOPS dichroic in tip and tilt: ±100
mas on sky for atmospheric dispersion and ±1mas on sky for dithering. At the dichroic, this translates to
±1.027mrad (212mas) for atmospheric dispersion and ±0.0103mrad (2.12mas) for dithering.
At this position, the ø25mm dichroic needed to be place as close as possible to the pupil image formed on the DM
requiring the mount for the dichroic to be relieved for the incoming beam to the DM. This cut out and the
surrounding optic paths placed strong restriction on the space volume for the assembly.
The current design mounts consists of a pair of concentric rings each mounted on a pair of flexure bearing. The
axiis of the bearing pairs ate perpendicular providing orthoganal tip/tilt motion. The motion is performed by the
arms of each ring being moved by a cyrogenicly compatable piezo actuators.
The arms are actuatoed 100mm from their axis requiring ±102µm for ADC and ±1.02µm for dithering. The piezo
actuators selected are capable of <30nm incremental motion which is well within the 102nm accuracy required for
dithering. Because the actuators are push only, the assembly is preset so that the actuators are required to push the
dichroic into it’s nominal position against the spring force of the flexure bearings.
The upcoming design cycle will refine both components and optic mount for reliable operations at 70K.

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Figure 6-1011 ADC/IOPS Dithering
6.7 Focus Layout
Upper H band light passes through the IOPS/Focus dichroic. As the light passes through condition optic to from a
pupil on a lenslet array, it is folded 40° becoming parallel to the Truth optic path (Figure 6-11Figure 6-12). Relay
optics behind the lenslet array rescaling the image allowing the Focus & Truth path share the same H2RG sensor.
As with the previous channels, the ensuing design stage will refine the optic mounts for reliable operations at 70K.

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Figure 6-1112 Focus Layout
7 Attachment A:
Intentionally Blank