Since the early sixties, cast aluminum optics has held out a promise of low cost, durability, lightweight, ease of manufacture, the possibility of welding together sections to form large optics and a good match of some astronomical programs such as photometry. Needless to say, this rosy future has been fraught with difficulties. Poor short-term stability, sensitivity to temperature, porosity, bimetallic warping, soft optical surface, poor specularity, nickel adhesion problems, poor image quality, and the inability to generate aspherics, have all contributed to the bad name cast aluminum optics has acquired over the years. With the advent of improved aluminum alloys, long-term dimensional stability data, advances in adaptive optics and the need for very large monolithic mirrors with a relatively low handling risk, aluminum optics deserve another look. This paper is a collection of some of the early attempts to build cast aluminum mirrors using the stable alloy, Tenzaloy. The results of those efforts are discussed together with recommendations for future aluminum metal mirror work.

Future optical observatories in space will require telescopes of very high resolution. To satisfy this demand, technology must be developed for large mirrors capable of diffraction-limited imaging. Conventional monolithic glass substrates (light-weight or not) have serious limitations for future development. In particular, glass is susceptible to fracture during ground- handling, transport and launch. An alternative solution is aluminum. It has lower cost, increased strength, easier and safer methods of fixing, amongst other advantages. It is readily lightweighted and can be produced with good polishing quality with nickel coating. We foresee applications for satellite telescope for astronomy, remote sensing, surveys of asteroids and debris in space. Furthermore, this technology is ideally suitable for lunar mounted interferometric experiments--as mirrors can be easily replicate, saving cost--and for telescopes deployed on planetary surfaces. Some results from the european Eureka LAMA (Large Active Mirrors in Aluminum) are here presented, which show the feasibility of such systems.

A feasibility study of a 25 m class telescope for optical wavelengths is presented. A short summary of the scientific background is given. A possible optical design is presented and discussed. Technical and engineering aspects are detailed. Tentative solutions are proposed for manufacture of mirrors and mirror segments. The suitability of metal mirrors is discussed. Comments are given on procedures for optical testing. Details on the mechanical design of the telescope are given. A first tentative proposal for the enclosure is presented. The control system is briefly discussed. Brief comments are given on the maintenance of the telescope. Finally, a tentative implementation plan is presented, including budget and time schedule.

In order to settle conclusively the best technical solutions to obtain metallic blanks which could be delivered in large size (at least 8-rn for ESO), and to avoid any possible difficulties in the manufacturing process for flexible glass meniscus, a European programme called "L.A.M.A." (for Large Active Mirrors in Aluminium) was set up. The programme is divided into two phases. The first one, which ended December 1991, addressed the following items: (1) tests to select the best aluminium alloy, (2) aluminium welding, homogeneity and stability, (3) aluminium high precision machining, (4) nickel coating, (5) polishing of the nickel layer, (6) active optics. Furthermore, tests have been conducted to demonstrate that the quality of the mirrors is not altered at various temperatures and after a large number of aluminising and cleaning cycles (corresponding to about 50 years' life). The mirror shape (whose specifications are fully compliant with those of the VLT, as the programme is conducted in cooperation with ESO) was computed under several causes of deformations: evidencing gravity as the predominent effect, and very low distortions as the high thermal conductivity limits the thermal transverse gradient to 0.025°C (Table 1). Second phase of the programme, so far under preparation, will be devoted to the completion of a complete active system in its cell: aluminium meniscus supported by passive and active actuators, the latter monitoring the surface form, together with the wavefront sensor, the active control loop system and the software. An extension of the partnership is envisaged, with additional European partners (UK). The final step is to provide the astronomical community with a high-tech, up-to-date primary mirror telescope. Thus, aluminium alloys are attractive candidates for large lightweight mirror blanks. They exhibit good thermal properties, are readily available at low cost. Major concerns, generally linked use of aluminium for precision optics are the material's softness - and thereby poor polishabiity - and the doubts as to its long-term stability, which doubts have now been cleared with the positive results obtained with the ESO's 1.8 m mirror prototype. In fact, the development of industrial processes to coat the surface with about one hundred microns of nickel (through a Chemical prOCeSS made by TECNOL), has successfully solved the former problem. Many experiments and tests undertaken have led to the conclusion that possible, ageing-induced, warping effects, if any, can aptly be minimised as they are readily absorbed through active correction. The expertise gained by the polisher during the first phase of the LAMA programme shows that the same standards of quality can be achieved on nickel coating as on glass, even if the technique needs a different approach. Aluminium shows a worthwhile advantage over glass due to its thermal characteristics (greater thermal diffusibility), ensuring homogeneous temperature within the blanck, and allows the mirror to reach the ambient temperature faster (a few minutes as against several hours, eliminating the risk of thermal gradients). This would greatly improve the efficiency of thermal control (mirror seeing), which is crucial in a dome. The possibility of monitoring distortions like the correction of possible long-wave mirror like those due to thermal expansion of thermal gradients inside the substrate by active optics systems is now one of the major dedsion-making criteria in favor of an aluminium blank. Finally, brittleness of glass blanks is the most important drawback when using such material for large mirrors. The risk of failure during manufacturing, handling and carrying (espedally nowadays where observing sites are far from manufacturing plants) often necessitates procurement of a spare to avoid programme schedule jeopardy. In this respect, aluminium blanks with higher mechanical characteristics, shorter manufacturing sequences, no brittleness and lower cost, offer important gains over glass blanks. In addition, the manufacturing procedure can be easily organised in such a way that it allows a step-by-step process; any non conformance can be detected at the outset and immediately corrected.

The TEMOS concept has been developed by the LOOM in order to evaluate several aspects of the new technologies that will be certainly used in the construction of future giant telescopes.

It is well known that telescope mirrors can produce seeing effects when they are at a different temperature than the adjacent air. The mirror acts as a reservoir for heat that can be convected to/from the air. In this paper we will discuss some tactics for minimizing mirror seeing in nighttime and daytime (solar) telescopes. Aluminum mirrors with internal fluid cooling appear to be the optimum choice for mirror substrates as a means for reducing mirror seeing. However, the mirrors must be actively corrected for the distortions caused by thermal gradients. The planned provisions are described.

New aspherization methods have been developed and applied in the field of astronomical optics. These methods, based on the elastic properties of substrates, have been used separately as well during the polishing process as in situ as the telescope. The goal of this proposal is to combined for the first time, on a primary mirror of telescope, these two possibilities in order to achieve the best optical figure from active optics capabilities.

Within the framework of the NTT 3.5-m and VLT 8-m telescopes' projects, ESO has been trying to assess the potential interest of Aluminum as a substrate for large astronomical mirrors. After a series of test on 500-mm diameter Aluminum mirrors two 1.8-m diameter Aluminum blanks were ordered and produced according to totally different manufacturing processes. Both blanks were figured within specifications, and successfully underwent a series of thermal cycles aimed at verifying their long-term stability towards relaxation of internal stresses. While in the 8-m range the baseline solution selected by ESO, partly for schedule reasons, was the glass-ceramics substrate, the results obtained so far with Aluminum have been undoubtedly positive. Aluminum is considered as a potentially cost and performance- effective substrate for active telescopes, at least in the four- meter range. Further tests are however still required to validate the technology.

The accuracy requirements and design philosophy of the JCMT reflector structure are briefly reviewed, leading to a steel space-frame with separate reflector panels. The choice of material for the panels is discussed, with particular emphasis on the properties of aluminum honeycomb composites. The development of the manufacturing process and the details of the process are described. Finally, statistics on the results of the manufacturing process are given.

A 0.9 m diameter f/1.8 paraboloidal mirror of moderate quality (s2 armit) has been produced by spinning an epoxy-coated spherical substrate. This mirror has been used to build a prototype flux collector for stellar seismology. The objective is to produce 2 m mirrors for this application.

A 0.9 m diameter f/1.8 paraboloidal mirror of moderate quality (approximately 2 arcmin) has been produced by spinning an epoxy- coated spherical substrate. This mirror has been used to build a prototype flux collector for stellar seismology. The objective is to produce 2 m mirrors for this application.

Metals have important advantages as mirror substrates. Existing metal mirrors exploit the bulk properties of the material for mechanical and thermal purposes, and some mounting details and useful optical features which are very difficult with glass can be more readily produced in metal. In the near future, metal substrates promise to extend the possibilities for large active optics (with control of the optical figure), on account for their mechanical properties and methods of attachment. Looking further ahead, enhanced active optics will make it possible to produce large non-axisymmetric or variable optical figures. I suggest that such optics will open new dimensions for the telescopes of the future. One potential example is a two-aperture telescope with coherent imaging. Its elements are off-axis Cassegrain telescopes. The two beams combine (if required) in an optically ideal manner and with no obstructions or further reflections. Other suggestions for further study include the reflecting Schmidt camera, beam-steering systems and systems with a non- circular field of view.

Traditionally glass represents the common choice for mirror substrates and the current development of new techniques such as glass ceramics, lightweighted blanks or honeycomb structures even increase the fields of application of glass mirrors. However, the research on alternative substrates for mirrors covers a large variety of materials: metals, silicon, silicon carbide, metal matrix, etc... This presentation concentrates on the machining, polishing and testing of aluminum mirrors to demonstrate that the optical performance of these substrates can be compared to the more classical glass mirrors.

Lightweight and high performance mirrors are currently required for telescope systems. Today, a lower cost option is available with aluminum-alloy blanks, especially for large size (more than about 2-m), with a combination of active supports to compensate any distortions. This paper deals only with the properties of the substrate, manufactured by FORTECH.

The current interest in ever larger solid metal mirrors has highlighted several fabrication problems, amongst them the method of joining the largest available forgings. Thick section fusion welding is still predominantly carried out by multi-pass arc welding processes, but substantial development of electron beam welding (EBW) technology, particularly over the last decade, now warrants re-appraisal of this situation. The electron beam welding process offers not only a single pass welding capability for practically any thickness, but also increased joining rates, reduced distortion and in most cases elimination of consumables. Increases in beam power level have necessitated careful design of both the electron source and electron optics; in addition, to avoid problems of weld defects, special power supply developments have been required. Metal vapor and weld spatter are prevented from entering the electron gun by the introduction of a magnetic trap device and a low stored energy switch mode power source is employed to provide continuous operation, even for the most volatile workpiece materials. Reference is made to the latest in- vacuum external and in-chamber 100 kW gun column developments and, amongst the range of industrial applications, to the particular problems associated with welding large aluminum alloy components.

The laser induced damage thresholds (LIDT) of metal surfaces and mirrors can be directly related to the absorption properties of the surface and to the laser temporal profile. This paper presents both the theory and the experimental data for the reflection/absorption properties for a range of metals together with the relevant laser induced damage thresholds. The effect of the surface finish, roughness and chemical state, on the absorption and on the damage thresholds is discussed and quantified.

At TELAS we have been working for several years to develop the technology for mirrors of telescopes using stiff and light blanks in other materials than glass. Aluminum alloys offer good properties and this paper deals with how we can quantified the aging effect, by accelerated cycles in order to simulate about 50 years of the lifetime of a real mirror.

Aluminum mirrors polishing can not be effected if the surface is not properly coated. Electroless nickel coatings are still the best answer to this demand but they have several backdraws that become prohibitive for producing very large mirrors. These critical points have been investigated by the author and solved by means of chemical and physical techniques.

Abstract not available.