Systems Issues: Optical Design & Fabrication

1. Systems Issues: Optical Design & Fabrication GSMT Working Group on Optical Design & Fabrication David Anderson Richard Buchroeder Earl Pearson Tom Sebring Larry Stepp -- Chair

2. 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

3. 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

4. Optical Design & FabricationKey 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

5. Optical Design & FabricationKey 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.

6. Optical Design & FabricationKey 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.

7. Optical Design & FabricationParaboloidal Segments 30-meter telescope Segment Size (meters) Focal Ratio Asphericity calculated for worst case (outer edge) segments Stress calculated for a 50mm thick segment (varies linearly with thickness)

8. Optical Design & FabricationParaboloidal Segments For Keck primary: C4 = 101 m C6 = 13 m 50-meter telescope Segment Size (meters) Focal Ratio Asphericity calculated for worst case (outer edge) segments Stress calculated for a 50mm thick segment (varies linearly with thickness)

9. 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 Hexagons Only six copies of each segment type Closer to circular shape --easier to polish Edge sensor positions are the same for each segment Optical Design & FabricationKey Systems Issues Choice of segment configuration: petals vs hexagons

10. Optical Design & FabricationKey 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.

11. Optical Design & FabricationKey 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

12. Optical Design & FabricationKey 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.

13. Optical Design & FabricationKey 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

14. Optical Design & FabricationKey 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.

15. Optical Design & FabricationCY2000 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

16. Optical Design & FabricationCY2000 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.

17. Optical Design & FabricationCY2000 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.

18. Optical Design & FabricationCY2000 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 cost-sharing arrangements.