Mirror Support System
for Large Honeycomb Mirrors
J. M. Hill, Steward Observatory
Large Binocular Telescope Project
Technical Memo
originally UA-94-02
revised to UA-95-02
originally September 27, 1994
revised December 20, 1995
http://medusa.as.arizona.edu/lbtwww/tech/ua9502.htm
Abstract
This is a 1995 revision of Technical Memo UA-94-02 which itself was a
1994 revision of Technical Memo UA-90-04
This memo describes our concept for the support of the 8.4 meter
honeycomb mirrors in the Large Binocular Telescope. This support
scheme is also going to be used in the 6.5 m MMT Conversion, the
6.5 m Magellan telescope, and the AFWL 3.5 m telescope. The 8 meter
honeycomb mirror is supported on 186 axial supports and
100 lateral supports. All supports are attached to the
backplate of the mirror. The axial supports apply a set of corrective
forces to correct the overturning moment when the telescope is
pointing at the horizon. Pneumatic actuators provide push and pull
forces over a range of 2800 N. Load cells on each actuator assure a
force accurate to 0.1% . Load cells on the mirror hardpoints will be
used to adjust the actuator forces when wind blows on the primary
mirror. This memo also summarizes the other functions of the mirror
cell such as the ventilation and thermal control system.
This memo has two purposes: to provide a broad overview of the mirror
support strategy and to document the engineering requirements for the
detailed design of the support systems. Note that this is the input to the
support system design, not a detailed description of the final design.
In some cases, design drawings have been included to clarify the
discussion.
Revisions since September 1994
- Added block diagram of actuator servo from Bob Nagel (drawing 7392).
- Added references to BCV reports #158, #159 and #160 by Parodi.
- Added reference to Gray et al. and Olbert et al. papers at SPIE Kona 1994.
- Removed the words ``2 G'' from Table 4 regarding static support drop height.
- Added missing specification on ``Cross-Lateral Decoupling Factor of
Lateral Supports'' based on a suggestion by W. Davison.
- Clarified stray force requirements on hardpoint mechanisms based on recent discussions.
- Refined air supply volumes for the actuators to accomodate dual servo
valves on each cylinder and the measured air consumption.
- Added support air supply pressure sensors for the distribution plenums to the
list of computer readable data.
- Replaced Table 8/9 with the final optimized 8.4m forces for case 94.6a from BCV report
#157a.
- Updated the number of support actuators for the LBT 8.4m mirrors using case 94.6a.
- Added a figure showing the four mirror support control groups on the 8.4m using the
distribution groups assigned by Davison.
- Added a photograph of an assembled MMT Conversion 6.5m 2-axis actuator.
Table of contents
1. Strategy for Mirror Support
2. Requirements for Mirror Support
3. System of Actuators
4. Hardpoints for Mirror Alignment
5. Ventilation and Thermal Measurements
6. Overall Mirror Cell Requirements
7. Mirror Cell Software
8. Actuator Control System
9. Mirror Support Safety
10. Maintenance
11. Support for Secondary Mirrors
12. Conclusions
13. Acknowledgements
14. References
15. Appendix A: Calculated Finite Element Forces
16. Appendix B: Comparison with Meniscus Studies
