# Presentation: 005477

## Mirror Support: Edge, 70%, or both?

- M.Lampton
- Space Sciences Lab
- University of California Berkeley
- Oct 2003

## Overall optical test plan

- Each component is tested alone, 1-G environment, by manufacturer
- Mirrors, structure will have vendor test protocols
- Entire telescope interferometric tests can be run vertical or horizontal
- Interferometric tests yield pupil WFE distribution
- If interferometer resolution is good enough, +1G and –1G interferograms can be combined to give a 0-G interferogram   <<ACCURACY IS THE ISSUE
- no flotation system is then needed, horiz beam OK
- Entire telescope autocollimation imaging tests yield PSF information
- Secondary, flat, tertiary mirrors have small 1-G deformation when mounted on flight support systems; flotation support is unnecessary
- Primary mirror deforms 2 to 20 microns (huge!) on flight support bipods
- Therefore primary mirror needs a GSE flotation system to verify the 0-G performance for autocollimation test
- Would also improve sensitivity of interferometric tests.
- We want both interferometry and imaging.

## What does industry recommend?

- Contraves-Brashear: vert, 70% w/float
- Yields both interferometry and PSF
- Eastman Kodak: vert, 70% w/float
- Yields both interferometry and PSF
- Goodrich: horizontal, 70% no float
- Interferometry only
- Tinsley: horizontal, 100% no float
- Interferometry only
- But they have not done a two-meter mirror
- SAGEM: horizontal, 100% no float
- Interferometry only
- But on all their big mirrors, they use vertical beam testing with multipoint support

## What does common sense say?

- 70% support better for minimizing launch stress
- Puts top plate into tension: top is stronger
- 70% support better for fringe visibility
- Horizontal beam testing better for test flat
- Flat stands on edge; figure is constant
- Horizontal beam testing better operationally
- no hoisting; less danger of dropped wrenches
- Vertical beam test mandatory for PSF image test

## Conclusions

- Edge support at R=100% is a nonstarter
- Poorer launch stress margins
- Poorer interferograms, either H or V
- Recommend we adopt R=70% support now
- Recommend we simplify our blank drawings: “universal” as regards H and V but adopt six attach zones at 70% radius, delete edge support zones
- Most tests can be run H or V interferometrically
- All tests will be witnessed by Sholl BITE
- One test will have to be flotation: perhaps at level 5?
- This one test shows that our zero-gravity system meets reqts
- Sholl BITE shows that system has not changed from one-gravity figure

## Mirror flotation systems

- Employed at Perkin-Elmer for HST
- M.H.Krim “Metrology mount development and verification for a large spaceborne mirror” Proc.SPIE 332#57, 1982
- Approx 100 jeweled counterweights, individually calibrated
- Employed on all large ground-based monolithic mirror telescopes
- Subaru (8.2m, Mauna Kea) uses 120 pneumatic actuators
- Gemini (two 8.1m; Mauna Kea & Cerro Pachon) use 148 pneumatic actuators
- LBT (two 8.4m, Mt Graham) use ~100 pneumatic actuators
- Function is to distribute the load according to the local weight
- Maintains a condition of near-zero local strain, hence stress
- Allows mirror surface figure to remain independent of orientation

## SNAP primary mirror flotation conceptH.Heetderks

- Strongback

- Primary Mirror

- Control box

- 10 psig air

- Pressure controller

- Cylinder

- Load cell

- }

- Repeated 100 places

## Frictionless piston + cylinder

- * Outer cup
- * Inner cup
- * Rubber flex joint
- ---------------
- No sliding friction
- No leaks or maintenance
- Low cost
- Easy tolerances

## Slide 9

## Slide 10

## Slide 11

## Compressed air is not a problem

## Air Pressure control is not a problemBellofram T1500 Controllerhttp://www.lesman.com/downloads/s13sp_accessories.pdf

## Sensing force is not a problemEntran load cells http://www.entran.com/elps.htm

## Conclusion:  flotation test is valid

- Zero-gravity is approximately simulated when the local weights and local supports are equal
- Neglects gravity quilting
- FEM weight model& fab metrology errors are < 1%
- Cross checks: total mass; center of gravity; local force/deflection compared to FEM predictions
- Force errors are <1% thanks to good accuracy of load cells
- Anticipate imbalance of weight+force < 2% of each load
- Typical 40 newton load, so load imbalance < 0.8 newton
- Net large scale mirror compliance is 4nm/newton
- Typical surface errors ~4nm due to imperfections in the flotation test
- Acceptably within 1-gravity release surface error budget