Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Allen Telescope Array wikipedia , lookup
Spitzer Space Telescope wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
CfA 1.2 m Millimeter-Wave Telescope wikipedia , lookup
Optical telescope wikipedia , lookup
James Webb Space Telescope wikipedia , lookup
- COO CALTECH OPTICAL OBSERVATORIES 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 2 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 1-2.5m 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 (12m 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 12m of surface stroke to only 250nm on-axis (and ~3.5m 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: 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). 15 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. 16