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CALIFORNIA INSTITUTE OF TECHNOLOGY
Caltech Instrumentation Note #602
An Integrated Approach to High-Priority
UV/Optical/NIR Instrumentation for TMT
R. Dekany
A. Moore, R. Smith, K. Taylor
4/25/06
Caltech Optical Observatories
California Institute of Technology
Pasadena, CA 91125
Abstract
This document describes a new instrument development strategy that delivers the following
SRD v15 high priority TMT science capabilities with modest scientific compromise, but
significantly reduced cost:



IRIS
IRMOS
WFOS
(Science Priority 1);
(Science Priority 1);
(Science Priority 3);
At its core, the strategy replaces the narrow-angle NFIRAOS facility with a wide-field Adaptive
Optics concept with a view to delivering all 3 capabilities with ~2 years of TMT’s first-light.
Significant cost savings are made by eliminating the adaptive secondary mirror (AM2) from the
TMT development plan. The new strategy rebalances investment in D2 (seeing-limited) science
and facilitates an AO development path that is both robust to risk, minimizes upgrade-based
instrument downtime, and delivers improved Strehl and sky coverage performance in its final
configuration.
Revision Sheet
Release
No.
Rev. 0.1
Rev. 0.6
Rev. 0.7
Rev. 0.8
Rev. 0.85
Rev. 0.9
Rev. 1.0
Date
Revision Description
1/17/06
3/23/06
3/24/06
4/4/06
4/6/06
4/12/06
4/25/06
Initial draft by R. Dekany
Adopted comments by K. Taylor, condensed Section 3
Comments by A. Moore and R. Smith incorporated
Substantial revision by K. Taylor
Minor revision by R. Dekany
Added figure of one multi-feed concept by A. Moore
Clarification to Section 2.4.2 by R. Dekany
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TABLE OF CONTENTS
Page
1
2
General ....................................................................................................................................4
1.1
Acronyms and Definitions......................................................................................................... 4
1.2
Purpose ....................................................................................................................................... 4
1.3
Scope ........................................................................................................................................... 5
1.4
Related Documents .................................................................................................................... 5
TMT Instrumentation Suite ....................................................................................................6
2.1
Guidance from the SAC ............................................................................................................ 7
2.2
Guidance from engineering staff .............................................................................................. 8
2.3
An Offner-centric Solution to TMT Instrumentation ............................................................ 9
2.3.1
2.4
2.4.1
2.4.2
2.4.3
2.5
2.5.1
2.5.2
2.5.3
2.5.4
2.5.5
2.6
3
TiPi Instrument Summary ...................................................................................................................... 9
SRD capabilities in various TiPi modes ................................................................................. 10
IRMOS................................................................................................................................................. 10
IRIS ...................................................................................................................................................... 10
WFOS-petite ........................................................................................................................................ 12
Systems Engineering Issues .................................................................................................... 12
Elimination of AM2 ............................................................................................................................. 12
Switching between instrument capabilities .......................................................................................... 13
Spectrograph redundancy .................................................................................................................... 13
Commissioning schedule and risk ....................................................................................................... 13
Smaller M2 and M3 ............................................................................................................................. 14
Cost Savings ............................................................................................................................. 15
Conclusions ...........................................................................................................................15
3
1 GENERAL
1.1 Acronyms and Definitions
AM2
AO
DL
DM
FoV
IFU
IR
LAO
LGS
LGSF
M1
M2
M3
M6
MEMS
MEMS-DM
MCAO
MOAO
Na
n-DL
NIR
PSF
RMS (also rms)
SAC
SLAO
SLGLAO
Spaxial
SRD
m
nm
”
’
mas
Adaptive M2
Adaptive Optics
Diffraction-Limited
Deformable Mirror
Field of View (the field observed by a single detector array)
Integral Field Unit (optical re-formatting feed to an IFS)
Infrared
Laboratory for Adaptive Optics (at UC Santa Cruz)
Laser Guide Star
Laser Guide Star Facility
TMT primary mirror
TMT secondary mirror
TMT tertiary mirror
The sixth mirror in the telescope train, typically used to describe
the deformable mirror in architectures having a dedicated AO
optical relay
Micro-Electro-Mechanical Systems
MEMS Deformable Mirror
Multi-Conjugate AO
Multi-Object AO
Sodium
Near-Diffraction-Limited
Near InfraRed (typically 1-2.5m wavelength)
Point Spread Function
Root Mean-Squared
Science Advisory Committee
Single-Laser Adaptive Optics (aka SLGLAO)
Single-Laser Ground Layer AO (pronounced "Sly-Go")
The focal plane spatial sampling defined by the IFU
Science-Based Requirements Document
microns
nanometers
arcseconds
arcminutes
milliarcseconds
1.2 Purpose
The purpose of this document is to describe an alternative TMT instrument development strategy
that accelerates science capability delivery while containing cost. It is intended to stimulate
thinking and foster discussion about making acceptable cost/benefit compromises in individual
TMT instrument capabilities for the purpose of delivering an affordable first-light instrument
suite matched to science priority and efficient early TMT operations.
4
The audience for this document is the SAC, TMT management, and interested members of the
TMT science user community.
1.3 Scope
This document describes the cost/benefit advantages and scientific compromises that can be
achieved with an Offner-centric approach to delivering the highest priority TMT instrument
capabilities. The analysis is based on first-order budget calculations and is intended to stimulate
discussion rather than to formally propose a definitive development program. The proposed
strategy is not incompatible with the inclusion of other TMT AO capabilities (NIRES, MIRES,
PFI) but their inclusion is beyond the current scope of this document.
1.4 Related Documents







Science-Based Requirements Document (SRD) v15
Caltech IRMOS feasibility study report
Caltech MILES costing study report
NFIRAOS Conceptual Design Report
NFIRAOS CoDR Committee Report
CELT Green Book.
Science Motivated Specifications for the
Switching of Instruments and
Observing Modes of the TMT
5
TMT.PSC.DRD.05.001.REL15
TMT.INS.CON.05.025.REL01
TMT.IAO.CDD.06.008.DRF01
TMT.AOS.CDD.06.010.REL01
TMT.AOS.COR.06.016.REL01
CEL.FAC.CDD.02.001.REL01
TMT.PSC.TEC.04.047.REL01
2 TMT INSTRUMENTATION SUITE
The TMT Science-Based Requirements Document (SRD v15) presents a number of high priority
science capabilities that the telescope must be capable of delivering at first-light. The particular
capabilities that we will be addressing in this document are summarized in Table 1. While
IRMOS retains its priority 1 status, the inherent risks in MEMS-DM and Go-To controls
technology necessary for MOAO were deemed sufficient to relegate it to 2nd generation status.
We will address these issues as part of an overall development strategy herein.
FUNCTION/NAME
InfraRed Imager
and Spectrometer
(IRIS)
Multi-IFU imaging
spectrometer
(IRMOS)
Wide-field Optical
Spectrometer and
imager
(WFOS)
MODE
DL
n-DL
FIELD OF VIEW
2” IFU
10”imaging
2” over 5’
diameter
field
2
SL
75 arcmin
300 arcmin2
(goal)
SPECTRAL
RESOLUTION
WAVELENGTH
RANGE (μM)
4000
0.8 – 2.5
2-50(imaging) 0.6 – 5 (goal)
SRD
PRIORITY
COMMENTS
1
NFIRAOS
2000-10000
0.8-2.5
1
MOAO
[email protected]”
slit
[email protected]”
(goal)
0.31-1.0
0.3-1.3 (goal)
3
(SL)GLAO
Table 1. SRD highest priority science instrument capability summary (from Section 1.5 of SRD v15).
6
2.1 Guidance from the SAC
To the extent to which we have been party to such communication, the SAC, or members
thereof, have made the following observations relevant to the case presented here:
Issue Instrument Short Title
S1
All
Early impact
S2
All
Light/Dark
Time
Balance
S3
WFOS
FoV
S4
WFOS
Throughput
S5
IRIS
Sky
coverage
Detailed Description
Early delivery of high impact
science is a requirement for
TMT (e.g. completion of the
telescope without a significant
completion of strong 1st light
instrument package is to be
avoided)
Similar to S2, but early science
should include at least one
bright-time instrument.
The 'softest' requirement for
WFOS is FoV. In other words,
WFOS science must achieve
the specified spectral
resolution and wavelength
coverage, but FoV is
negotiable
WFOS, as a D2 science
instrument, must achieve very
high sensitivity ("as high as
any existing spectrometer") to
maintain the aperture
advantage of TMT over 8-10m
telescopes
IRIS sky coverage would
preferably be 'nearly' 100%
Table 2. Guidance from the SAC on Key TMT issues
7
Reference
Various sources, including M.
Bolte, private communications
Implied within Jensen and
Januzzi,
TMT.PSC.TEC.04.047.REL01
C. Steidel, private communication
SRD, Section 2.5.2.8
SRD, Section 2.4
2.2 Guidance from engineering staff
To the extent to which we have been party to such communication, the engineering staff of the
TMT project has commented at various times on the key technical challenges of delivering a
scientifically productive 30-m diameter telescope. The following observations and principles
regarding TMT are applicable to the instrumentation program:
Issue
Domain
Short Title Detailed Description
E1
Telescope
Wind-shake
E2
Telescope
Jitter
rejection
E3
AM2
Cost and
Downtime
E4
M3
Cost and
complexity
E5
NFIRAOS
Sky
coverage
E6
E7
CommissionScheduling
ing
AM2
Complexity
Wind-induced wavefront errors and windshake (pointing jitter) may regularly impact
operations (e.g. require down-wind pointing
of the enclosure aperture).
Residual wind-shake is best rejected optically
using as bright a natural tip/tilt guide star as
available.
AM2 is likely to cost $30M and require
several months of telescope downtime for
installation and commissioning.
M3 is a challenging and long-lead time optic
due to its large diameter and tight optical
tolerances. Segmented M3 options
complicate NFIRAOS (due to fielddependent gaps) and PFI (due to M3's
awkward conjugate height).
NFIRAOS sky coverage is limited by the
diameter of the partially compensated
contiguously anisoplanatic patch diameter.
Realistic commissioning planning will be
important in driving the instrumentation
development schedule
The installation of AM2, the 2nd mirror in the
optical train, has cascading implications to all
TMT instruments (including software
maintenance).
Reference
D. Macmynowski and
J. Nelson, private
communications
J.-P. Veran, private
communication
B. Ellerbroek, private
communication
J. Nelson, B.
Macintosh, private
communications
R. Clare, Interim
NFIRAOS review
documentation
D. Crampton and
G. Sanders, private
communications
Various telescope and
instrument retrofit
experiences
Table 3. Guidance from the engineering staff on key TMT issues coupling with instrumentation
8
2.3 An Offner-centric Solution to TMT Instrumentation
During the development of the feasibility study for Caltech's TMT IRMOS concept (TiPi) we
came to realize the power and versatility intrinsic to a TiPi design based around a classical
Offner relay. As a 5' FoV, AO-feed to TiPi its Offner relay provides the basic architecture to
support all of the capabilities identified in the high priority SRD science instruments called out in
Table 1 while addressing many of the issues and concerns highlighted in Table 2 and Table 3.
2.3.1 TiPi Instrument Summary
TiPi consists of a low-order relay stage ("the Offner relay"), an object selection mechanism
(OSM), and a back-end multi-object spectrograph unit, providing an N=16 target multiplex
through 16 separate spectrograph channels. The Offner relay provides a 1:1 relay of the f/15
TMT Nasmyth beam and differs from the NFIRAOS facility AO system in 4 important respects:




TiPi passes a 5’ field of view (FoV) with excellent image quality;
TiPi has a single, 0 km conjugate, common-mode woofer DM having 40 x 40 actuators
and large stroke (12m of surface, typically), to compensate the science and the guide
star light;
TiPi overcomes fitting and anisoplanatism errors using high-actuator-count, modest
stroke MEMS-DMs in a robust hybrid-MOAO architecture1;
TiPi's LGS wavefront sensors have a large linear dynamic range which, in combination
with relatively modest object-shift aberrations in the sodium laser light, provides for
simplified WFS optical design.
TiPi utilizes up to 8 laser guide stars (provided by LGSF) and 3 natural low-order wavefront
sensor guide stars to drive the woofer DM. Each of the natural low-order guide stars is
compensated with a MEMS-DM, allowing fainter guide star use and providing nearly full sky
coverage (Issues S5 and E5).
The woofer DM can be driven in SLGLAO mode, GLAO mode, and full tomography mode, to
provide on-axis correction limited only by DM fitting error (see Section 2.4.2 for a discussion of
how this fitting error is eliminated to provide ~120nm rms wavefront error performance.)
1
The Multi-Object AO (MOAO) architecture is based upon the principle of "one DM per science target". This
approach overcomes the usual fitting error limitation of single DM, or two-DM Multi-Conjugate AO (MCAO)
systems, wherein a compromise adaptive correction is made over a wide FoV. In MOAO, each target is given the
exact correction appropriate to it's direction through the atmosphere. Hybrid-MOAO, as suggested for TiPi, uses
both a single low-order DM, which provides approximate correction over a wide-field, and individual MEMS-DM
mirrors that sharpen the exact correction in each target direction. Both MOAO and hybrid-MOAO must correct
some of the atmospheric wavefront error in a 'go-to mode', that is without optical feedback in the wavefront
sensor(s). This imposes tighter constraints on linearity and repeatability of the sensors and MEMS-DM. However,
hybrid-MOAO reduces the amplitude of 'go-to' wavefront correction for TMT from order of 12m of surface stroke
to only 250nm on-axis (and ~3.5m over a 5' field).
9
The Object Selection Mechanism (OSM) is located at the output focal plane of the Offner relay
and consists of ~140 near-contiguous tiles over the focal surface. Each tile can independently
steer the incident beam to any of the 16 spectrographs, 3 NGS sensors or a PSF monitor camera.
This allows the selection of one ~1.5” target within each ~24” tile. An additional of tiles, this
time fixed, provides for contiguously sampled 50mas spaxials, while a final fixed faceted optic
acts as an image slicer to provide diffraction-limited (~5mas) sampling by sending a portion of a
(total) 0.6” FoV to each of the TiPi spectrographs.
The Offner relay and OSM are cooled to ~235K to minimize thermal emissivity down to the
infrared K-band. Operation as cold as 230K is not precluded.
TiPi provides an additional, dedicated PSF monitor camera that enjoys MOAO compensation
and may sample stars anywhere in the full 5' FoV in the same manner at the TiPi spectrographs
and NGS wavefront sensors.
The TiPi spectrographs provide R~5000 spectral resolution and spectral coverage of either the
entire infrared J, H, or K-bands at a single time, using glass image slicers as their field
segmentation technology.
The ~120 tiles not directing targeting light into spectrographs, NGS sensors, or the PSF camera,
can be steered to feed a full FoV acquisition camera. This camera typically produces a low-order
AO-compensated, 5’ FoV, sampled at 75mas/pixel, which can view over 85% of the field. The
15% that has been removed corresponds to the 20 out of ~140 tiles that have fed light to the
spectrographs, NGS-WFSs, and the PSF camera.
2.4 SRD capabilities in various TiPi modes
Next we briefly describe how each of the high priority science instrument capabilities can be
supported by the TiPi Offner relay architecture, with modest and distributed compromises, to
deliver at a feasible cost all of the highest priority science at 1st light highlighted in Table 1.
2.4.1 IRMOS
TiPi is intrinsically an MOAO-based multi-object spectrograph. By design, TiPi supports the
IRMOS SRD requirements, with our concept providing N=16 object multiplex. The IRMOS
capability is developed incrementally, baselining existing MEMS-DM technology, which the
TiPi science team has demonstrated to be scientifically compelling. Future upgrades of the
MEMS-DM technology (from an initial 64 x 64 to a final 128 x 128 actuator format) will further
enhance the ensquared energy performance of TiPi.
2.4.2 IRIS
TiPi's Offner relay can readily provide an on-axis feed for a diffraction-limited IRIS capability
using a MEMS-DM. By starting with existing MEMS-DM technology, the TiPi approach
provides IRIS an upgrade path, via increasing DM actuator format and improved calibration
techniques, to improved Strehl ratio as compared to NFIRAOS. In addition, MOAO sharpening
of natural field stars provides better sky coverage than provided by the MCAO field delivered by
10
NFIRAOS. With MOAO of tip/tilt/focus stars, the diffraction-limited sky coverage for TiPi can
approach ~90%, so critical to much of IRIS science.
The diffraction-limited IFU FoV provided by TiPi will depend on budgetary constraints and
science priorities. While the TiPi approach is in principle capable of supporting diffractionlimited IRIS science, the optical quality of the current feasibility design spectrograph optics
would at a minimum need to be reconsidered in light of diffraction-limited image quality
requirements never laid upon the initial IRMOS design. Alternative spectrograph designs, such
as multiplication of the IRIS image slicer spectrograph design to support multi-object science,
should also be considered. The cost-optimal solution will depend upon the specific (new)
diffraction-limited field of view requirement and on risk/benefit choices such as whether to
baseline dithering of the spectrum to save detector real estate.
The 10” imaging FoV for IRIS can be provided by TiPi's PSF camera, which enjoys MOAO
correction with graceful Strehl degradation due to natural anisoplanatism across the full 10”
FoV. If simultaneous imaging of the field surrounding the IRIS IFU is required, a special IRIS
macro tile could be fabricated to send the IFU light to the 16 spectrographs while the sending the
immediately surrounding field to the PSF camera (e.g. PSF camera images an 'IFU hole' in the
center of its field).
Achieving high Strehl in a diffraction-limited mode will require go-to control (which includes
both sensor and actuator linearity, and woofer DM metrology) at the 30-50 nm rms wavefront
error level. Because TiPi employs a hybrid-MOAO approach, several 1000's nm rms of
wavefront error are corrected in closed-loop, leaving only about ~230 nm rms on-axis to be
rejected by the go-to controller. Thus, the diffraction-limit can be reached with excellent Strehl
ratio even with 20% residuals from the MOAO control system (this is a large percentage of
leakage in most control systems).
Demonstration of this level of go-to control using real-world MEMS-DM devices is underway at
the UCSC Laboratory for Adaptive Optics (LAO). Led by D. Gavel, the LAO is also planning a
near-term on-sky demonstration on the 1m telescope at Lick Observatory. Recent laboratory
tests conducted by Don's group have shown both MEMS-DM actuator repeatability and
wavefront metrology, two of the enabling technologies for MOAO, at the nanometer level.
Meeting the SRD requirement for wavefront quality (120nm rms on-axis) will also require (the
same) 128 x 128 MEMS-DMs desirous for meeting the SRD requirement for IRMOS. Because
these MEMS-DMs are small and occur very late in the optical train, their upgrade to the latest
available MEMS-DM technology can be readily accomplished.
NGS AO operation would require the interchange of TiPi's current sodium laser D2 wavelength
dichroic with one or more conventional visible/IR dichroic(s), reflecting light short-ward of the
desired science wavelength for IRIS science (typically ~800nm). The full cost of this dichroic
exchanger is included in the cost estimate in Section 2.6.
11
2.4.3 WFOS-petite
As part of our recently completed costing study for the TMT WFOS instrument concept, we
advocated a cost-effective compromise for WFOS science that sacrifices FoV, but introduces
potentially significant sensitivity improvements (up to a factor of 5 or more) using adaptive
optics to provide partial compensation of visible light (issues S3 and S4).
Having a fast tip/tilt (M6) capability for first light of WFOS ensures against unpredicted
dynamical behavior of the telescope in the presence of real enclosure and wind environment
(issue E1). The availability of a 5’ technical FoV for the selection of tip/tilt guide stars could be
useful if wind-induced pointing jitter exceeds current predictions (issue E2).
We hence advocate a single-barrel version of the MILES concept for WFOS, having a square
FoV ~4.5’, fed by the TiPi Offner relay. The Offner relay provides the AO correction,
eliminating the expensive 3-mirror focal reducer (3MfR) that is part of the 4-shooter, full-field
MILES/WFOS design. Details of this proposition can be found in our WFOS costing study
documentation.
WFOS-petite is the most sensitive single-object implementation of WFOS proposed to date,
supporting tip/tilt-only correction, field-uniform GLAO correction or sensitivity-optimized
SLGLAO correction at first light. The lower total FoV and lower slit length, represents a
scientific compromise which we feel is justified given its D2 status. For wide-field survey work,
observers would be free to choose GLAO correction giving a uniform but limited improvement
in seeing, while many surveys that can take advantage of enhanced sensitivity towards the field
center will be far better compensated by using SLGLAO, an option unavailable to all other
WFOS concepts.
For observations not sufficiently benefiting from AO (e.g. resolved, diffuse targets or UV
observations), we have proposed an elegant 'by-pass' mode for feeding WFOS-petite directly. In
this configuration, the beam would avoid the 3 Offner reflections.
It should be noted that should a wide field, WFOS-grande, capability by required, at least one
early concept for HROS, Steve Vogt's MTHR, provided a very large FoV (> 300 arcmin2) with
intermediate spectral resolution (R~15,000) and very large simultaneous wavelength coverage in
its fiber-fed, multi-object mode.
2.5 Systems Engineering Issues
The adoption of an Offner-centric instrument strategy would have a number of systems level
benefits and repercussions. Some of these arise from the adaptive optics upgrade strategy we
advocate, namely the incremental upgrade of components (MEMS-DMs, wavefront sensors, and
lasers), without large-scale reconfiguration of the telescope or Offner relay, while others are
independent telescope compromises or choices.
2.5.1 Elimination of AM2
One of the most compelling cost and operational benefits of the Offner-centric instrument
strategy is the elimination of the technologically risky AM2 from the TMT program (issue E3).
12
By providing a diffraction-limited near-IR upgrade path based on MEMS-DM technology, the
initial (active) M2 mirror of TMT can last for the entire observatory lifetime.
In addition to the cost savings described in Section 2.6, this provides major science benefits of
never having to:




Take TMT offline for the AM2 upgrade (estimated to require several months of
downtime);
Disturb long-term observing programs with a fundamental telescope reconfiguration (this
is at least as disruptive to programs as a top-end change on 8-10m telescopes);
Perform routine prime-focus source simulator (PFSS) maintenance of AM2, with the
corresponding benefit to simplifying telescope operations;
Take the risk of future funding short-falls which would force the abandonment (or
indefinite deferral) AM2, despite the fact that it was originally judged as “necessary” to
achieving the requirements of the observatory.
2.5.2 Switching between instrument capabilities
TiPi can respond to different atmospheric seeing conditions by switching internally between
IRMOS and IRIS science modes. Furthermore switching from the near-IR capabilities to
WFOS-petite (with or without the Offner feed) can be facilitated through exchanging the
IRMOS/ISIS unit with WFOS-petite. One concept for this, which is by no means optimized, is
shown in Figure 1.
In the configuration shown, the Offner relay is laid out horizontally on the Nasmyth platform,
with a final upward fold mirror feeding a downward-looking IRMOS/IRIS combination IR
spectrograph.
To feed the beam to WFOS-petite, the Offner relay can be bypassed to provide the highest
optical transmission, or used to provide seeing improvement (SLGLAO) over an inner field of
view (typically 2 arcmin) of WFOS-petite. This provides the very highest background-limited
source sensitivity by improving the image quality, particularly at red wavelengths, resulting in
factors of 5 or more savings in integration time (see the TiPi IRMOS feasibility study report).
While this is one plausible approach, other instrument switching configurations to exploit the
Offner relay also exist and would require study in the next phase of the TMT instrumentation
program.
2.5.3 Spectrograph redundancy
The use of the TiPi multi-object spectrograph provides redundancy in cases of subsystem failure
or downtime due to routine maintenance. One can say that it “degrades gracefully” for its
IRMOS and IRIS applications if one or two of the spectrograph channels are taken offline.
2.5.4 Commissioning schedule and risk
Our Offner-centric approach to TMT instrumentation contains a logically straightforward
commissioning scenario, testing the simpler AO capabilities of WFOS-petite and IRMOS, before
moving on to the diffraction-limited IRIS capabilities. This provides a clear risk mitigation
strategy for the project, should the technical challenges of diffraction-limited observations with
13
TMT require additional time to sort out (Issues S1 and S2). It avoids the “Hole-in One” strategy
of NFIROAS which requires that all identified error terms are under control from the very start.
2.5.5 Smaller M2 and M3
As a further cost savings, not integral to the TiPi approach, TMT could independently consider
permanently foregoing the 20’ FoV provided by the current M2 and M3 mirrors, utilizing only a
5’ diameter FoV for WFOS and IRMOS science. The reduction in unvignetted FoV from 20’ to
5’ would only reduce the M2 diameter by about 20cm, all else being equal, but M3 would be
reduced by more than a meter in diameter, making a monolithic mirror readily manufacturable
without driving the construction schedule critical path (issue E4). In order to further reduce the
size of M2, the option of a slower Nasmyth (to ~f/20) should be revisited since not only are there
formally no instruments (other than the fiber-fed MTHR concept) that benefit from the 20’ field,
there are also no instruments that clearly require the faster f/15 Nasmyth focus.
Figure 1. Initial instrument switching concept to feed IRMOS/IRIS and a WFOS-petite. Left Top: Isometric
view of Offner relay with IRMOS/IRIS feed, WFOS SLGLAO feed, and WFOS Offner-bypass feed. Left
14
Bottom: Side view, showing IRMOS/IRIS down-looking and WFOS-petite up-looking (located beneath the
Nasmyth platform. Right Top: Optical path for IRMOS/IRIS feed. Right Middle: Optical path for WFOSpetite SLGLAO feed . Right Bottom: Optical path for WFOS-petite with throughput-optimized Offner
bypass.
2.6 Cost Savings
Perhaps the primary motivation to consider an Offner-centric instrumentation strategy is that it
accelerates the science capabilities of TMT while reducing the integrated lifecycle cost of the
instruments.
Suggested First-Light Instrument Plan for WFOS + IRIS/IRMOS
Offnercentric
Cost
Estimate
(US$M) (US$M)
Current
Cost
Estimate
INSTRUMENT
Science
Compromise
Off-setting
Benefit
Infra-Red Imager and Spectrometer (IRIS)
25
~8
Smaller FoV (0.6")
5' Imager;
PSF camera
Wide-field Optical Spectrometer and imager (WFOS)
45
18
Smaller FoV
(5')
SLGLAO; GLAO
Multi-IFU imaging spectrometer (IRMOS)
36
36
1st light not SRD
compliant
NFIRAOS
26
0
Not planned
AM2
28
0
Not planned
Instrument/AO suite subtotal (no contingency)
160
62
Low initial risk;
Upgrade path
Functionality
provided by TiPi
Reduced risk; no
upgrade downtime
Table 4. Instrument Cost (WFOS; IRIS; IRMOS; NFIRAOS+) comparison for feasibility study instrument
configuration vs. an Offner-centric instrument configuration. We allocate here $8M in order to revise the
original TiPi concept to support diffraction-limited image quality requirements for IRIS science.
3 CONCLUSIONS
Caltech's Offner-centric concept provides an alternative approach for delivering a major portion
of the TMT science capabilities at lower cost and risk. Specifically, an Offner-centric approach:



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Provides Offner-fed capabilities for IRMOS, IRIS and WFOS;
Accelerates the science return for the three highest priority instrument capabilities;
Eliminates the need for AM2, and thereby also eliminates the need for TMT downtime
for AM2 installation and commissioning;
Results in an estimated US$100M in cost savings in the instrument/AO budget.
Although some performance penalties are necessarily paid, we believe these are primarily:


Reduction in WFOS FoV to 4.5’ x 4.5', offset by partial AO compensation at first-light;
Defers to a (far-)future consideration the fate of the 30" MCAO-fed contiguous-FoV
WIRC imager (having SRD priority #7), offset by TiPi's 10" FoV anisoplanatic IR PSF
camera and 5' full-field visible imager (sampled at 0.75"/pixel).
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Finally, we recommend that the TMT Project Office undertake a formal systems-level study of
an Offner-centric approach to instrumentation development, in comparison to the existing
instrument development model, in parallel with other promising approaches offering similar,
substantial cost savings and AO risk mitigation strategies.
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