LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO 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 MeetingBoulder, CO Optical Design & Fabrication Any telescope larger than ~ 8 meters will have a segmented primary mirror. l For mirrors larger than ~ 8 meters, costs increase rapidly for: l blank fabrication l polishing l transportation l coating

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication Three large segmented-mirror telescopes already exist: l Keck I l Keck II l Hobby Eberly Several others are in work or have been proposed: l Gran Telescopio Canarias (GTC) l Large Aperture Multi-Object Spectroscopic Telescope (LAMOST) l Mexican Infrared-Optical Telescope (TIM) l 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 MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues Primary mirror segment size l Segment size will affect the telescope structural design l Cost dictates segments smaller than about 2.5 meters l Blank cost per square meter l Low-cost optical finishing technologies l planetary polishing l replicating l ion figuring l Transportation l Coating facilities

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues Primary mirror segment size l However, smaller segments also have drawbacks: l Increased number of rigid points required on structure l Increased number of actuators -- cost & reliability concerns l Increased control system computational requirements l 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 MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues Spherical or aspherical segments? l Low-cost finishing technologies favor spherical segments l planetary polishing l replication (cost driven by number of masters) l Fixed-mirror telescope designs often use spherical primaries l 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 MeetingBoulder, CO Optical Design & Fabrication Paraboloidal Segments Focal Ratio Asphericity calculated for worst case (outer edge) segments Stress calculated for a 50mm thick segment (varies linearly with thickness) 30-meter telescope Segment Size (meters)

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication Paraboloidal Segments Focal Ratio Asphericity calculated for worst case (outer edge) segments Stress calculated for a 50mm thick segment (varies linearly with thickness) 50-meter telescope Segment Size (meters) For Keck primary: C4 = 101  m C6 = 13  m

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues Petals l All petals in each ring are identical l Harder to polish (not close to round shape) l Edge sensor positions vary from one segment to another Hexagons l Only six copies of each segment type l Closer to circular shape --easier to polish l Edge sensor positions are the same for each segment Choice of segment configuration: petals vs hexagons

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues The optical design will be driven by the requirements of the science instruments l Focal ratio l Field of view (& physical size of focal plane) l Image quality l Curvature of field l Control of distortion l Control of stray light l Photometric stability l 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 MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues Other optical issues l Atmospheric Dispersion Compensation l Segmentation effects in the point spread function l Coalignment effects l Satellite images l Diffraction spikes l Emissivity l Boundaries between segments l Large number of optical surfaces l Contamination control

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues Complementarity of active and adaptive optics l Both active and adaptive optical systems will be needed l Active optics (low bandwidth) l Primary mirror segment position control l Primary mirror segment figure correction l Position control for secondary and tertiary mirrors l Figure control for secondary and tertiary mirrors l Adaptive optics (high bandwidth) l Image stabilization (large tip-tilt mirror) l Atmospheric compensation l Correction of local seeing effects l Fast correction of mirror figure errors These two systems must complement each other.

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues Integration of adaptive optics components l In the Telescope l Traditional AO applications have placed the adaptive components far down the system l In Gemini-ALTAIR the first deformable mirror is M6 l This helps keep the components small l Recent concepts propose adaptive M4, M3, M2 or even M1 l Require locations conjugate to different heights, including zero l In the Instruments l To achieve performance goals, individual instruments may need to incorporate: l wavefront sensors l tip-tilt mirrors l deformable mirrors

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication Key Systems Issues Control of wind buffeting l The telescope may have structural resonances down to ~ 1 Hz l The active segment alignment system must have a relatively low bandwidth to avoid exciting structural resonances l Wind buffeting will cause relatively large structural deformations at frequencies the active optics system may not be able to control l The wind could excite resonances in the structure that have large dynamic amplification factors l Vortex shedding may introduce other oscillations l 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 MeetingBoulder, CO Optical Design & Fabrication CY2000 Studies Develop strawman designs of key science instruments l Diffraction-limited narrow field of view l Diffraction-limited wider field of view l Seeing-limited wide field of view Studies can be performed at NOAO and at Universities

LMS, 3/07/00 GSMT Systems Task Group MeetingBoulder, CO Optical Design & Fabrication CY2000 Studies Measure wind loading on an 8-meter telescope l Two coordinated studies are planned: l Wind pressure on Gemini primary mirror surface l 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 l Principal investigator Dr. Myung Cho of Univ. of Arizona l Response of the Gemini telescope structure to wind loading l Measure the dynamic response of the Gemini telescope structure to the wind pressures measured in the coordinated study l 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 MeetingBoulder, CO Optical Design & Fabrication CY2000 Studies Develop technology for fabrication of aspheric segments on a planetary polisher l First phase, in CY2000, will be to prepare a paper study evaluating feasibility and defining the technical approach l This will be followed in CY2001 by prototype fabrication studies l 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 MeetingBoulder, CO Optical Design & Fabrication CY2000 Studies Investigate parallel work by other projects l NOAO and Gemini staff will investigate work being done for other projects, including OWL, CELT and NGST. l Optical designs l Optical fabrication l Lightweight segment designs l Segment control systems l Actuator designs Where possible, we will coordinate our studies with other projects and we will investigate the possibility of cost- sharing arrangements.