The cost for the Birr mirror was estimated and quoted at £100 k. The true cost for the complete manufacturing process of the mirror was around £160 k. Most of the excess £60 k was consumed in the production of the base curve to within a few microns of true. Around £30 k could have been saved if the blank had been diamond turned to a few microns accuracy. Another £50 k would have been added to the estimated cost, if a forged or build up welded blank had been used. A glass ceramic blank at the time of enquiry cost £80 k for ULE and £100 k for Zerodur. Using the ceramic would have given an estimated cost for the total manufacture of the mirror from £170 k to £190 k. Dierickx [107] confirmed that the two mirrors for the LAMA programme cost 1.6 million DM around £500 k, £250 k for each mirror. The LAMA programme being a research and development exercise and not a saleable product justified the higher costs. An equivalent glass ceramic blank would have cost around £150 k due to the thicker cross section, with an estimated finished item cost of around £240 k.
The figures show that by producing an aluminium mirror from rolled plate that has been suitably stress relieved has real cost benefits over the expensive forging or welding processes of aluminium and also over glass ceramic.
7. 6. 5 REVIEW
A considerable proportion of the construction time of the Birr mirror was consumed in the accurate generation of the base curvature. If the blank had been accurately diamond turned, as the LAMA mirrors were, this would have considerably reduced the overall manufacturing time scale as reported in chapter 5. It would have traded process time and manpower cost for the cost of accessing a much more expensive facility. No figure however has been reported for the length of time consumed in constructing the LAMA mirrors. The production methods to manufacture the Telas and Linde mirrors are considered by the author to be time consuming and very expensive. Costings conducted by the author on similar optics show that mirrors constructed by electron beam welded forging or build up weld have no real cost saving compared to glass ceramic. However there is approximately a 45% saving when using rolled plate compared to the LAMA mirror processes and a 15% to 30% saving compared to glass ceramic. No comparison was made by the LAMA programme, between the two methods used and rolled plate that has been fully annealed.
The Birr mirror reported in chapter 5 was constructed from rolled plate and did however display some astigmatic characteristics. Tarengi and Wilson [105] report on the NTT (the original concept was for an aluminium primary mirror) and describe an active control system for the mirror cell, which allowed for the figuring tolerances on astigmatism and coma to be relaxed. This allowed the manufacturer Zeiss to concentrate on the high spatial frequency smoothness required for the primary mirror. The author loosely followed this methodology of relaxing the tolerance on astigmatism when deciding to construct the Birr mirror from rolled plate. To control any astigmatism in the rolled plate a warping harness was considered but ultimately not implemented as the mirror met its specification. The reasons for the warping harness and its non implementation are detailed in chapter 5.
As discussed in the literature on the LAMA programme, any large mirror constructed, would have an adaptive support system which would compensate for the lower order deformations. To reduce the cost of aluminium mirrors, rolled plate that has been correctly heat treated and stress relieved should be considered an attractive option.
Chapter 8
A Thin Meniscus
Deformable Mirror
8. 1
INTRODUCTION
The Birr mirror reported in chapter 5, was the proof of concept that large economic monolithic metal mirrors could be successfully manufactured for use in astronomy. However, the future of large telescopes lies with thin deformable mirrors, with computer controlled actuator systems used to correct for form error, or adaptive wave front correction.
To alleviate the signal to noise problems with radio communications, optical communication with space probes is being developed [112]. With the greater use of radio communications, the bandwidth available is rapidly diminishing. Optical links with satellites would give high bandwidth communications enabling higher data transmission rates. The benefits of an optical communication system over normal microwave systems are; high data transmission rates, high gain with the narrow beam and no regulator restrictions on frequencies or bandwidths. Optical transmission of information also offers a high degree of security when compared to radio. However the challenge with optical data links into space is to maintain the optical phase as the beam traverses the earth’s atmosphere. This could be achieved by constantly modifying and adaptively correcting for wave front distortion as the signal traverses through the atmosphere (in a similar manner to astronomical adaptive optics systems). Deep space probes would also greatly benefit from narrow band, high intensity optical links.
The system described in this chapter goes some way to solving the challenges of real time wave front correction for implementation on either an active secondary mirror system or active control of large mirror segments. The demonstrator model reported
here is a seven actuator prototype of the proposed 1 meter diameter Gemini active secondary mirror (100 actuators) [108]. Actuator spacing (100 mm) for the demonstrator is the same as on the proposed mirror. Work on the design was initially investigated by Bigelow [44] and completed by Lee [108]. Lee describes the finite element modelling, the design and testing of the completed demonstration mirror assembly.
The author’s contribution was the research and development of the necessary techniques for manufacturing and testing of a thin meniscus aluminium mirror. This was used to demonstrate the feasibility of constructing an adaptive secondary mirror for use on a major telescope. The challenges in manufacturing the mirror were found to be predominantly in the support structures for polishing and testing. Reported here are the methods employed to polish and test the mirror without inducing distortion. Also reviewed here are the methods used by others in the construction of thin or high aspect ratio mirrors including general support methods for medium to large monolithic and honeycomb structured optics.