Director's Programmatic Review

Director's Programmatic Review

Systems Issues: Optical Design & Fabrication GSMT Working Group on Optical Design & Fabrication David Anderson Richard Buchroeder Earl Pearson Tom Sebring Larry Stepp -- Chair LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Any telescope larger than ~ 8 meters will have a segmented primary mirror. For mirrors larger than ~ 8 meters, costs increase rapidly for: blank fabrication polishing transportation coating LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO

Optical Design & Fabrication Three large segmented-mirror telescopes already exist: Keck I Keck II Hobby Eberly Several others are in work or have been proposed: Gran Telescopio Canarias (GTC) Large Aperture Multi-Object Spectroscopic Telescope (LAMOST) Mexican Infrared-Optical Telescope (TIM) Southern African Large Telescope (SALT) These projects serve as the starting point for the design of any extremely large telescope LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication

Key Systems Issues Primary mirror segment size Segment size will affect the telescope structural design Cost dictates segments smaller than about 2.5 meters Blank cost per square meter Low-cost optical finishing technologies planetary polishing replicating ion figuring Transportation Coating facilities LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Key Systems Issues Primary mirror segment size

However, smaller segments also have drawbacks: Increased number of rigid points required on structure Increased number of actuators -- cost & reliability concerns Increased control system computational requirements Increased edge sensing error propagation The optimum is likely to be in the range of 1-2 meters. LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Key Systems Issues Spherical or aspherical segments? Low-cost finishing technologies favor spherical segments planetary polishing replication (cost driven by number of masters) Fixed-mirror telescope designs often use spherical primaries For steerable telescopes, the optical design can provide better performance with fewer elements with an aspherical primary The low-cost production and testing of aspherical

segments is a key area for development. LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Paraboloidal Segments 30-meter telescope Segment Size (meters) Focal Ratio 0.5 1.0 1.5 1.0 C4 = 369 m C6 = 22 m astig = 4.3 Mpa coma = 2.9 Mpa C4 = 56 m C6 = 4 m astig = 0.7 Mpa coma = 0.5 Mpa C4 = 18 m

C6 = 1 m astig = 0.2 Mpa coma = 0.2 Mpa 1.5 C4 = 810 m C6 = 72 m astig = 4.2 Mpa coma = 4.3 Mpa C4 = 123 m C6 = 12 m astig = 0.6 Mpa coma = 0.7 Mpa C4 = 38 m C6 = 4 m astig = 0.2 Mpa coma = 0.2 Mpa 2.0 C4 = 1405 m C6 = 170 m astig = 4.1 Mpa coma = 5.6 Mpa C4 = 213 m C6 = 28 m astig = 0.6 Mpa coma = 0.9 Mpa C4 = 66 m C6 = 9 m astig = 0.2 Mpa coma = 0.3 Mpa

Asphericity calculated for worst case (outer edge) segments Stress calculated for a 50mm thick segment (varies linearly with thickness) LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Paraboloidal Segments For Keck primary: 50-meter telescope C4 = 101 m Focal Ratio C6 = 13 m Segment Size (meters) 1.0 C4 = 225 m C6 = 8 m 0.5 astig = 2.6 Mpa coma = 1.0 Mpa C4 = 35 m C6 = 1 m 1.0 astig = 0.4 Mpa

coma = 0.2 Mpa C4 = 11 m C6 = 0.4 m 1.5 astig = 0.1 Mpa coma = 0.1 Mpa 1.5 C4 = 500 m C6 = 26 m astig = 2.6 Mpa coma = 1.6 Mpa C4 = 77 m C6 = 4 m astig = 0.4 Mpa coma = 0.3 Mpa C4 = 24 m C6 = 1 m astig = 0.1 Mpa coma = 0.1 Mpa 2.0 C4 = 826 m C6 = 62 m astig = 2.5 Mpa coma = 2.1 Mpa C4 = 134 m C6 = 10 m astig = 0.4 Mpa coma = 0.3 Mpa C4 = 42 m C6 = 3 m

astig = 0.1 Mpa coma = 0.1 Mpa Asphericity calculated for worst case (outer edge) segments Stress calculated for a 50mm thick segment (varies linearly with thickness) LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Key Systems Issues Choice of segment configuration: petals vs hexagons Petals All petals in each ring are identical Harder to polish (not close to round shape) Edge sensor positions vary from one segment to another LMS, 3/07/00

Hexagons Only six copies of each segment type Closer to circular shape --easier to polish Edge sensor positions are the same for each segment GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Key Systems Issues The optical design will be driven by the requirements of the science instruments

Focal ratio Field of view (& physical size of focal plane) Image quality Curvature of field Control of distortion Control of stray light Photometric stability Location, size and number of instruments Strawman instrument designs are needed as early as possible to guide the optical design work. LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Key Systems Issues Other optical issues Atmospheric Dispersion Compensation Segmentation effects in the point spread function

Coalignment effects Satellite images Diffraction spikes Emissivity Boundaries between segments Large number of optical surfaces Contamination control LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Key Systems Issues Complementarity of active and adaptive optics Both active and adaptive optical systems will be needed Active optics (low bandwidth) Primary mirror segment position control Primary mirror segment figure correction

Position control for secondary and tertiary mirrors Figure control for secondary and tertiary mirrors Adaptive optics (high bandwidth) Image stabilization (large tip-tilt mirror) Atmospheric compensation Correction of local seeing effects Fast correction of mirror figure errors These two systems must complement each other. LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Key Systems Issues Integration of adaptive optics components In the Telescope Traditional AO applications have placed the adaptive components far down the system

In Gemini-ALTAIR the first deformable mirror is M6 This helps keep the components small Recent concepts propose adaptive M4, M3, M2 or even M1 Require locations conjugate to different heights, including zero In the Instruments To achieve performance goals, individual instruments may need to incorporate: wavefront sensors tip-tilt mirrors deformable mirrors LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication Key Systems Issues Control of wind buffeting

The telescope may have structural resonances down to ~ 1 Hz The active segment alignment system must have a relatively low bandwidth to avoid exciting structural resonances Wind buffeting will cause relatively large structural deformations at frequencies the active optics system may not be able to control The wind could excite resonances in the structure that have large dynamic amplification factors Vortex shedding may introduce other oscillations Wind buffeting can be reduced by a fully protective enclosure, but this involves tradeoffs in enclosure cost and local seeing effects Wind buffeting will increase the demands on the adaptive optics system. LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication CY2000 Studies Develop strawman designs of key science instruments Diffraction-limited narrow field of view

Diffraction-limited wider field of view Seeing-limited wide field of view Studies can be performed at NOAO and at Universities LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication CY2000 Studies Measure wind loading on an 8-meter telescope Two coordinated studies are planned: Wind pressure on Gemini primary mirror surface Measure the spatial and temporal variation of wind pressure on the Gemini M1 as a function of wind direction, elevation angle and size of vent openings Principal investigator Dr. Myung Cho of Univ. of Arizona Response of the Gemini telescope structure to wind loading Measure the dynamic response of the Gemini telescope structure to the wind pressures measured in the coordinated study

Principal investigator Dr. David Smith of Univ. of Mass. These studies will provide key information for design of the telescope structure and adaptive optics system. LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication CY2000 Studies Develop technology for fabrication of aspheric segments on a planetary polisher First phase, in CY2000, will be to prepare a paper study evaluating feasibility and defining the technical approach This will be followed in CY2001 by prototype fabrication studies Possible contractors include Brashear, Carl Zeiss, Eastman Kodak, Raytheon, REOSC, Tinsley and Zygo This is a key investigation that could lead to a five-fold reduction in the cost of polishing aspherical segments. LMS, 3/07/00

GSMT Systems Task Group Meeting Boulder, CO Optical Design & Fabrication CY2000 Studies Investigate parallel work by other projects NOAO and Gemini staff will investigate work being done for other projects, including OWL, CELT and NGST. Optical designs Optical fabrication Lightweight segment designs Segment control systems Actuator designs Where possible, we will coordinate our studies with other projects and we will investigate the possibility of costsharing arrangements. LMS, 3/07/00 GSMT Systems Task Group Meeting Boulder, CO

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