Fix Version/s: None
Sprint:TSSW Sprint - Oct 24 - Nov 07
Team:Telescope and Site
Review the Harris documents related to mirror test.
- is triggering
DM-36773 Draft the Idea of M2 Mirror Safety Test
Review the document: LSST M2 Mirror Final Acceptance Testing Review - Final v1.pdf
Document F14900-0115-001, Final Optical Test Report
All optical testing of the LSST M2 was completed with the mirror held in its in-use orientation (face down with the vertex axis parallel to gravity). It was mounted in its cell with all actuators connected and active. There is no quilting to be considered outside of what is measured and reported in the final surface figure.
If I am not wrong, it looks like Harris only applied +-2 N in the optional test based on the test report. In addition, only one elevation angle orientation (90 degree) was tested. I think this is just to adapt the simplest case in LTS-107 220.127.116.11.
When analyzing the total force (FE+F0+FF), only the gravity correction was considered in the report.
Max force over all elevations: 296.8 N (80 ̊)
Min. Force over all elevations: -275.6 N (0 ̊ [Horizon])
System meets 305 N max force magnitude requirement Factory correction net impact on peak force magnitude: 38.7N
It looks like Harris did not perform the surface damage test based on CR-0127. But I could not find it.
The values in page 79 Nominal Forces do not look real.
Note: I think in this report, Harris did use the real mirror for some optical tests. They used the single operation orientation and check the force error is +- 2 N. However, it looks like they only applied the optical bending mode 1 as the simplest case in LTS-107.
Review the document: M2 FAT - Pending Issues and Verification - Issue Tracking.pdf
Review the documents:
Review the document: CR-0127 LTS-107_approved.pdf
I think the following paragraphs are important:
The original requirement limited only the magnitude of force used for large scale factory error correction. This revised requirement sets force limits on the actuators in terms of the sum of weight support, gravity induced figure error correction, and large-scale factory error correction. The sum of these forces at any elevation angel will not exceed the overall force budget of 305 N. It is expected that the sum of large-scale factory error correction and weight support forces will be reported through test, and the addition of gravity induced figure error correction forces will be applied via analysis.
The nominal weight-support force, independent of the static large-scale factory correction force, will be reported to LSST. These values may be useful for system-level force calibrations. The expected support forces are 254.1 N (Ring 1, radial position 1.002 m, 18 supports), 230.5 N (Ring 2, radial position 1.284 m, 24 supports), and 190.4 N (Ring 3, radial position 1.601 m, 30 supports).
Based on Doug, people believed it is safe to put the actual mirror on M1M3 support system because it passes all the requirements. This makes sense. However, for the M2, it might be hard to verify all the requirements at this moment since we are replacing all the existed software, which takes some time.
Based on Harris's document, the way they judged the mirror will be safe or not is the actuator force. That means they make sure the maximum actuator force will not be more than a specific threshold, and believe the mirror should be safe.
If this is reasonable, we could evaluate the actuator forces under all possible conditions (0-90 elevation angles, possible temperature fluctuation, reasonable applied bending mode forces, etc.). The test process can be developed on the test stand first. When we test this on the summit, we may want to rotator the cart to see the force change.
Another thing needs to check is the LUT parameters and others. Need to compare the configuration files for mirror and surrogate provided by Harris to have the idea of difference. When Bo and I did the test at 2019, we used the configuration files for the surrogate mirror. I think we may need to use the configuration files of mirror on the surrogate mirror to see the force changes.
I have the concern for the natural frequency of mirror. From the LTS-107, we can see several frequencies are listed. I do not know how to measure this on the real hardware. May need to check with the hardware engineers.
For the possible altitude of active forces, the existed AOS closed-loop simulation may or may not be a good source of judgement. In that simulation, the simulated force errors comes from the M1M3 instead of M2. There are two possible ways to make the simulation is for M2:
1. Update the AOS simulation to support to generate the mirror error from M2.
2. Change the penalty matrix to enforce to use the M2 to correct the error from simulated M1M3.
The above AOS tests can be parallel with the test of M2. For the M2, we could apply the bounce test to check the affection of each actuator. Maybe we could increase the applied force. In my memory, we only tried 10-30 N for each actuator before. I know there is the threshold for the actuator force in the cell control system. Maybe I should check the code logic there to see the code implementation. I am not sure it is based on the measured force, or just parts of corrections. When I reviewed Harris's documents, it is weird to me to see that they did not use the measured force in the judgement of safety. I do not know why at this moment.
Thanks for reviewing all these documents to get ready for the M2 testing - this will help considerably, even if Harris' documentation is severely lacking.
It disturbs me a bit that the nominal forces listed don't seem reasonable.
Sounds like you are starting to formulate a good plan for testing. Let me know if there is anything I can do to help move this forward.
Review the document: (100) F14900-0115-001, rev 1, LSST M2 Final Acceptance Optical Test Report.docx
The only thing I felt is related to mirror safety is:
The total force magnitude at any actuator shall not exceed 305 N (68.6 lbf) for any telescope elevation angle in the range of 0 to 90 degrees (horizon pointing to zenith pointing).