We have made significant progress in the first stage of the point design process, which is to identify the issues and propose technical solutions. In this section, we present a compliance matrix in which we indicate how far along we are on the path of confirming feasibility and how challenging the remaining steps are: an assessment of relative risk.
We divide the area of performance into the subsystems which individually have to perform at a given level in order for the system to deliver acceptable performance overall. In the matrix that follows, we simplify the terms to compress the information and convey a global picture.
All of the subsystems listed have been defined at some level of detail at this point in the point design process in terms of their requirements. We have proposed and are working through a technical solution to the task asked of each subsystem. (This does not imply that a trade study has been done.) This may involve simply adopting and scaling an existing concept where necessary. The response is "yes" unless otherwise noted.
Next, we define the terms in the matrix.
This is an assessment of the degree of development of the technical concept. To quantify, we classify the degrees as follows:
- Mature
- Prototype - technology is proved
- Prototype - technology is under development
- Theory only - hardware not designed
- Early conceptual stage
Done-Y, N, or P (partial)
Means: This is the means employed for verification up to the present.
- Model - Modeling-by numerical or other technique
- Proto Des. - prototype under design. Modeling may have been used earlier.
- Proto Built - prototype built and tested. Modeling may have been used earlier.
- Market - solution available in market or elsewhere.
- Market test - market solution tested.
- High - great uncertainty, never been done before, may not work.
- Medium - likely to work but requires uncertain investment in development.
- Low - represents only a modest extension of existing designs, no technology development involved.
The following notes amplify on those aspects of the design that have been identified as high risk or medium risk, but whose verification has not really been begun.
4.9.4.1 Note A. Secondary Adaptive Mirror; MCAO Deformable Mirrors
The effort to develop a 65-cm deformable mirror (DM) for the 6.5-m MMT (Multiple Mirror Telescope) by the University of Arizona in collaboration with the University of Arcetri, Italy, is a key technology demonstration effort for many large telescope projects. Three essentially new areas of technology are being developed for the first mirror: (1) fabrication and figuring of a large, very thin deformable facesheet; (2) development of electrostatic sensors to detect the deformation of the facesheet; and (3) development of electromagnetic actuators and associated drive electronics to produce the desired deformations. The project has proceeded fairly slowly, and although the progress is very significant, the system has still not passed the final test of being installed and used in a telescope environment for a significant amount of time. Because of the number of qualitatively significant advances on which this type of mirror depends, we regard the development process as high risk.
This is the most advanced development project that addresses the need for large powered adaptive mirrors, and the GSMT has adopted that concept in the point design for the secondary. If proved, it is also a good candidate for the large DMs in the multi-conjugate adaptive optics (MCAO) system (three 0.5-m mirrors), and for the reflective deformable element in the Prime Focus Corrector. It is our intention to explore other potential designs, particularly in the case of the MCAO DMs, which are nominally flat. It is possible that scaling up current piezo-electric designs would be attractive, although the throw requirements may be excessive.
4.9.4.2 Note B. Secondary Mirror Rigid Body Motion Control
The secondary mirror is a key element of considerable complexity. In addition to the fact that it will be deformable, the entire secondary structure will be required to be tip-tiltable in order to compensate, at least partially, (1) the combination of tip-tilt errors arising because of wind loading on the primary; (2) the displacement of the secondary itself, driven by wind acting on the secondary support structure; and (3) the tip-tilt component of atmospheric wavefront error. These contributions are still under study, but they are roughly of the same order of magnitude in size, depending on wind and atmospheric conditions.
The complicating factors include:
- The tip-tilt correction is "degenerate"; the error can be produced in several different places in the structure and corrected at different places, with varying consequences. The control system has to arbitrate this situation in a way that produces stable behavior and good correction.
- The mass of the secondary system is large.
- The secondary has to share a location with the prime focus corrector, increasing the cost of the implementation.
Mitigating the challenge are the following factors:
- The secondary motions to be compensated are small.
- The secondary motions are taking place at low frequencies.
- An additional stage of tip-tilt correction is available downstream in those AO modes for which the requirements for correction are high.
4.9.4.3 Note C. Prime Focus AO; Ground Layer Compensation Concept
Several authors have advanced the concept for improving the field of view (FOV) of an AO system by selectively correcting ground layer aberrations.1,2 It has been shown that, with knowledge of the contribution to the wavefront error arising close to the aperture of the telescope (including ground layer seeing and telescope primary deformations, for example), it is possible to partially correct the images over a wide field with uniform point spread function (PSF). Effectively, the seeing is reduced to that arising at higher altitude, and the image broadening effects are isoplanatic. However, the amount of wavefront sensing required (how many guide stars, of what order and at what temporal sampling rate) has not been fully evaluated.
Because of the finite thickness of the ground layer, full correction is not possible. With the knowledge of the Cn2 profile for the fiducial site of Cerro Pachón, Rigaut has estimated the degree of compensation possible with a single ground-conjugate DM. (See Section 4.6.A ) This is based on the assumption of full knowledge of the profile rather than on the properties of a real system of WFSs. Rigaut's calculation represents a useful limit on the achievable performance, but does not establish the impact of implementation errors of various types.
The modeling process to include implementation effects is well-defined and is planned for the near future.
4.9.4.4 Note D. Prime Focus AO; WFS
A simple concept for WFS at the corrected prime focus has been sketched in connection with the multi-object multi-fiber optical spectrograph (MOMFOS) instrument. It is based on deployable pickoff probes that will be positioned on the images of bright natural guide stars (NGSs). The number of such guide probes required, the mechanical implementation of the probe-deploying mechanism, and the modeling of the wavefront sensor (WFS) performance are pending.
4.9.4.5 Note E. Narrow-Field High-Order AO; DM
The goal here is high Strehl ratio, and achieving high Strehl requires very-high-order correction, particularly for a 30-m aperture. DMs that are suitable for high dynamic range AO, with tens of thousands of elements, are clearly on the development path of the new industry that is producing micro-electrical-mechanical systems (MEMS). However, that process is out of the direct control of the project and has an uncertain but promising future. We are in touch with the developers and hope to be able to evaluate prototypes in the near future.
4.9.4.6 Notes F. and G. Narrow-Field High-Order AO; WFS and Reconstructor
As with the DM, a very-high-order AO system increases the number of subapertures to be sampled and DM commands to be computed. This produces unprecedented demand on detectors in terms of total pixel rate, and also on the computing power required to process the data and compute the commands. As Section 4.6.2 makes clear, there has already been substantial progress in developing a strategy for both areas, but the process is just beginning and the increment in required performance is large.
4.9.4.7 Note H. Prime Focus/Cassegrain Focus Switch
There is strong science interest in the IR-oriented opportunities implemented at Cassegrain and Nasmyth foci, as well as the optical spectroscopy opportunities implemented at prime focus. Developing a concept for serving both foci and being able to switch between them efficiently -- minimizing telescope down-time -- is a challenge that has not been fully met in a manner consistent with budget and operational concerns.
- Chun, M., "The Useful Field of View of an Adaptive Optics System", PASP, 110, 317-329, (1998).
- Rigaut, F., "Ground-Conjugate Wide Field Adaptive Optics for the ELTs", in Proc. Venice 2001; Beyond Conventional Adaptive Optics, to be published, (2001).
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October 2002
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