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We propose an optimized design for the redeployment of the Multiple Mirror Telescope (MMT) primary optics as a 25 m Adaptive Steerable Imaging Array (ASIA) providing direct focal plane image formation. The design incorporates concepts of active telescope alignment and adaptive optics proven in the existing MMT. Basing predictions on contemporary radio telescope mount technology, 45 m to 100 m optical baselines may be practical.

One of the questions which has endured for as long as man has looked at the heavens is ”Are we alone?”. Are there beings on other planets who are also looking into the heavens? The first requirement for alien life similar to our own is that there be planets orbiting other stars. The early stages of planet formation as dictated by theory appear to be robust and several large planets have recently been detected by indirect means[1][2]. Direct imaging is currently being used to search for less ambitious, yet still scientifically interesting faint objects called Brown Dwarfs. The most convincing evidence of a Brown Dwarf, a cross between a star and a planet, was discovered by direct imaging with the aid of a 7.7 arcsecond separation from a companion star[3].

A prototype optical wavelength ultrahigh resolution echelle cross-dispersed spectrograph has been tested at the Starfire Optical Range (SOR) 1.5 m telescope (Woolf et al. 1995; Ge et al. 1996). To our knowledge this is the first high resolution spectrograph to take advantage of the diffraction limited images produced by an adaptive optics system. Because of the sharpened images produced by the adaptive optics at visible wavelength, about r0/D ~ 1/15 in the seeing-dominant domain, the narrow slits necessary for high resolution can be used without a large loss of light. This is a great advantage when compared with conventional high resolution spectrographs (e.g. Lambert et al. 1990; Diego et al. 1995). In addition, the smaller image widths inherent with the adaptive optics system allow the orders to be spaced closer together on the chip, allowing more orders to be observed simultaneously.

High resolution spectroscopy of stars has been rarely used because at visible wavelengths, the atmosphere produces a star image typically 10-20 times the diffraction limit of the telescope. Either the grating must be used at a far poorer resolution than its diffraction limit, or much light must be lost at the slit.

Thermal emission from "planetary" discs is known in other stars such as β Pictoris. The emission, discovered by the IRAS satellite, is comparable in strength to that from the star itself at 60 µm wavelength, indicating a large quantity of material at radius ~ 100 AU and temperature ~ 50 K (ref 1). Such discs are uncommonly bright, far more dense and extensive than the known zodiacal dust in our solar system. Here the emission is concentrated mostly within 1 AU, where the dust temperature is ~ 300K and it is optically very thin, causing an increment of only 10-5 in the 10 µm luminosity of the Sun as seen from afar.

The new 6.5 m single mirror Multiple Mirror Telescope (MMT) will be equipped with adaptive optics capabilities. In particular, high resolution infrared astronomy will use a new adaptive secondary on a regular basis. Before we design the adaptive secondary in detail, we fabricated a smaller prototype mirror using similar actuators and actuator geometry. The prototype's purpose is to debug the components and allow comparisons with theoretical predictions. We are studying system interactions and looking for unexpected performance characteristics. Manufacturing techniques and reliability are also being tested, to reduce risks when building the adaptive secondary.

The case for making large mirrors for astronomy with borosilicate glass honeycomb structure has been made in an earlier paper (Angel and Hill, 1981). Honeycomb structure gives rigidity in a lightweight mirror as well as low thermal inertia. Mirrors which are in thermal equilibrium with the air hold the promise of better images, since convection at the mirror is eliminated. Thermal expansion of borosilicate glass is problematical in thick mirrors which are likely to be out of thermal equilibrium and hence distorted. This will not be a problem in honeycomb mirrors with thermal time constants of only a few minutes. We believe a single casting technique holds the greatest promise of making borosilicate honeycomb mirror blanks up to 7 meters in diameter. Our work to develop the technique now involves casting test blanks 60cm in diameter but which have the full thickness (33cm), cell size (15cm) and faceplate thickness (2.5cm) needed for a 1.8m mirror.

The full potential of the next generation of larger telescopes will be realized only if they have well instrumented large fields of view. Scientific problems for which very large ground-based optical telescopes will be of most value often will need surveys to very deep limits with imaging and slitless spectroscopy, followed by spectroscopy of faint objects taken many at once over the field. Improved instruments and detectors for this purpose are being developed. Remotely positioned fibers allow the coupling of light from many objects in the field to the spectrograph slit. CCD arrays, operated in the TDI or drift scan mode, will make large area detectors of high efficiency that may supercede photographic plates. An ideal telescope optical design should be based on a fast parabolic primary, have a field of at least 1° with achromatic images < 0.25 arcseconds and have provision for dispersive elements to be used for slitless spectroscopy and compensation of atmospheric dispersion over the full field. A good solution for a general purpose telescope that can satisfy these needs is given by a three element refractive corrector at a fast Cassegrain focus. A specialized telescope dedicated to sky surveys, with better image quality and higher throughput than presently available, might be built as a scaled up Schmidt with very large photographic plates. Better performance in most areas should be obtained with a large CCD mosaic detector operated in the drift scan mode at a telescope with a 2-mirror reflecting corrector.

Instruments to study simultaneously the spectra of many objects in the field of view of a telescope can be made with the aid of fused silica fibers. The spectrograph at the 2.3m telescope of the University of Arizona has been modified for such operation, and is used routinely to investigate the dynamics of clusters of galaxies (Hill et al. 1981). The system presently in use locates up to 40 fibers in the telescope's Cassegrain focal plane, with the aid of plates previously drilled with holes in the configuration of the objects to be studied. Each field must be set up by hand by inserting fibers into a hole plate. An obvious improvement to this method would be to mount each fiber on a mechanical actuator, so new field configurations can be set up by remote control. It is our intention to make such a system with 32 fibers for the 2.3m telescope, over the next two years.