Technical Challenges for the Next Generation of Large Telescopes

1. Technical Challenges for the NextGeneration of Large Telescopes David R. Smith September 25, 2000 Edinburgh, Scotland New Initiatives Office

2. Goals Enable the science: Investigate known ‘tall pole’ engineering challenges Determine the scale of problems Develop potential solutions Uncover unexpected ‘tall poles’ before major design effort

3. Approach Develop strawman ‘Point Design’ to Focus technical studies Test implications of design approaches Reveal critical interfaces Provide ‘real’ constraints for potential solutions Determine which specifications are easy/hard to meet Simultaneously conduct studies on known challenges Wind buffeting of M1 and M2 Effects of enclosures Control system interactions MCAO (Rigaut; this meeting)

4. Current Status Point Design Optical and Structural layout generated Interfaces developing Optics/structure (central hole, segment size) MCAO/structure (DM stroke, space requirements) Instruments/structure (size, location) Instruments/optics (location, scale) Controls/structure (heirarchy and interaction) Wind buffeting studies First round tests performed Initial results available Further data reduction underway

5. Point Design: Optical Layout Cassegrain layout 30m f/1 primary 2m secondary Final focal ratio of f/15 Seeing-limited FoV of 15' at Cassegrain focus Considering prime focus camera MCAO FoV of ~2' What it provides Basic interface dimensions for structure Geometry for discussions of optical fabrication issues Reference point for MCAO or instrument ideas

6. Point Design: Structure and Controls Cable-braced tripod Primary reflector truss Elevation axis

7. Raft and Segment Layout

8. Point Design: Controls Active optics (quasi-static) Hierarchical control Rafts controlled on structure Segments controlled on rafts Edge sensing (high spatial frequency) Wavefront sensing (low spatial frequency) Adaptive optics Removes residual structural errors

9. Controls Issues Correction and stabilization of the primary High bandwidth, large displacement, large moving mass Potential control-structure interaction Single (or small count) point correction spillover Highly distributed correction expensive Optical correction via AO Stabilization of the secondary Disturbance suggests high bandwidth Potential control-structure interaction Model-based controllers

10. Wind Studies Results from NRO 45m (SPIE 2000) Scale and structural frequencies well matched Deflection scale Experiment at Gemini North/South 8-m Target of opportunity Well characterized telescope (good benchmark) Wide range of enclosure configurations Simultaneous structural and wind measurements

11. Gemini South Test Modal Testing Controlled, calibrated input Allows validation of FEA Operating Data Testing Total (uncontrolled) response 75 channels of accelerometers 24 channels of wind pressures 15 channels of wind speed Factors varied Azimuth angle (w.r.t. wind): 0, 45, 90, 135, 180 Elevation angle: 30, 45, 60, 75 Upwind vent gate: open, half, closed Downwind vent gate: open, half, closed

12. Experimental Approach Full factorial test impractical (5*4*3*3 = 180 tests) Wind not constant Time (equipment availability and cost) Other activities at site Statistical coverage in 34 tests using orthogonal arrays Two ‘L8’ (Four factors, two levels each) Combined into an optimal L16 (full-factorial) Some interaction information Two ‘L9’ (Four factors, three levels each) No interaction information Additional 20 tests (impact, checkout, elevation sweep)

13. Results: Wind Pressure Vent Gates Open Vent Gates Closed

14. Results: Structural

15. Next Steps Develop Point Design and interfaces System Engineering Structure performance estimates Controls Identify and support studies Generate SOW’s, schedules, costs Seek funding to support community studies Optical fabrication, wind studies, structural trades, etc.