GMAGAO-X R-02 Deformable mirror technology

[dmkelly42]

Risk creation date: 6/15/2022

Last Modified: 10/4/2022

Risk type: Technical

Risk Approach: Watch

Risk Owner: Doug Kelly

Risk Statement:

If we cannot get the 7 parallel tweeter DMs to work together for wavefront correction, then we would have to buy a large, monolithic DM that does not currently exist.

Risk Effects:

The largest monolithic DM that you can buy today is a 64x64 DM by ALPAO. They are working to develop a 128x128 DM. We would want 190x190 to have the same AO performance as the 7x3k. If this risk is realized, we will have to use lower count DMs, and the coronagraph contrast will suffer.

Risk Assessment:

Risk Ratings	Likelihood	Consequence
Initial	2-low	4-high
Current	2-low	4-high
Target	0-Nil	3-medium

Initial Risk Notes: The wavefront sensor needs to correct >1000 aberration modes, and it needs to operate at >2 kHz. Our preferred option is to buy 7 3k BMC MEMS (one for each GMT mirror). These have a 400 micron pitch, 3.5 micron stroke, a temporal response of >10 kHz, linear response, 0 hysteresis, 0 creep. We have experience with a 2k device, and it is proven to have fast (~kHz) low stroke (~400nm) phasing.
BMC is confident that they can produce 3k MEMS at high yield, but this is unproven. MEMS have low robustness to actuator failure so spares would be required. They can be swapped in in a reasonable time, but procurement might be slow so we would have to consider buying multiple spares.
The second option is an ALPAO 128x128 (or larger) monolith. This would simplify the optical design, and it would provide high stroke so would obviate the need for a complex woofer. It is supposedly highly robust to actuator failure and repairable. It suffers from creep. It has lower temporal response, and current results of R&D development indicate that it might not meet our 2 kHz requirement. A very large ~8 inch DM membrane would be required, and no device has been delivered with this size or actuator count.
The parallel DM is being demonstrated in the lab as part of the HCAT project.

Cost Impacts:

Estimated cost if risk is realized:
TBD

Explanation for cost risk:
We would have to purchase a large ALPAO DM, which is not in our current budget.

Risk mitigation list:

• Maintain flexibility to be able to change between BMC MEMS and ALPAO optical designs

• Lab demonstration of parallel DM approach, using flat mirrors (R-02 mitigation) #18

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[dmkelly42]

There is some concern that BMC could go out of business before we procure our DMs. Ironically, if we get funded and order our suite of DMs, that would likely provide enough funding to keep them in business.