The new technologies: Active optics
 

The previous generation of large telescopes involved the production of large monolithic primary mirrors to collect the flux from celestial sources and to produce images for detection. In order for the image quality to be maintained as the telescope is oriented to different parts of the sky, it was important to have mirrors with good mechanical stability. To achieve this, the mirror design required a thicknessto- diameter ratio of the order 1:9. As telescopes growin size, the requirement of producing thick blanks from which the mirror is figured causes difficulties in their production. In addition, primary mirrors with large mass require very heavy engineering to allow the telescope to be readily manoeuvrable.
As the telescope alters its orientation in moving from one target object to another, the mirror tends to flex under its own weight. Such flexure allows the optical surface to deform with the consequent deterioration of image quality.
However, by using ‘active’ supports on the underside of the mirror in the form of a distribution of pistons, it is possible to readjust the shape of the optical surface of the mirror and maintain the required figure. By continuously monitoring the image quality of some reference object in the field, the adjustments may be applied continuously by computer control. Because of the inertia of the large optical system, the response to the feedback is relatively slow (∼ tens of seconds) but easily sufficient to allow ‘continuous’ adjustment as the telescope tracks a celestial object. Thus, it is now possible to use lighter mirrors with smaller thickness-to-diameter ratios. Such mirrors are obviously more likely to suffer flexure but can also be corrected more easily. Many modern monolithic mirrors are now manufactured with material removed from their underside to form a honeycomb pattern with the correcting pistons in contact with the rib structure.

An additional development involving active optics is the idea of producing a large collection aperture by constructing the main mirror from a mosaic of smaller ones. Such a system is referred to as a multiple mirror telescope (MMT). Small mirrors are so much cheaper and easier to make that the cost of the resulting MMT is still considerably less than it would be to construct a single mirror equivalent in size. The idea of using mirror mosaics is not new but the concept has only recently come to fruition with the advent of inter-active computer control systems to maintain the mosaic alignments while the telescope is being guided.
Perhaps the most famous MMTs are those of the Keck ObservatoryW 20.1 on Mauna Kea, Hawaii, at a height of 4160 m. Two identical 10 m reflectors have been built, each with 36 hexagonal segments. The telescopes are only 85 m apart and can be used as an interferometer. The Observatory, owned and operated by the California Association for Research in Astronomy, provides an unprecedented resolving power at visual and near-infrared wavelengths. It may be noted too that the James Clerk Maxwell telescopeW 20.2 designed for use in the mm spectral region and also sited on Mauna Kea is an MMT system. It comprises a primary dish of 15 m diameter made up of 276 aluminium panels, each of which is adjustable to keep the surface as near to perfection as possible. Another interesting approach is that of the combination of images provided by individual telescopes which are linked to combine their foci either by superposition or in some other way.
Technically, such a system should be referred to as a multi-aperture telescope (MAT) but the term of MMT is more frequently applied. The best known example of a multi-telescope combination is that sited at the Whipple ObservatoryW20.3 in Arizona at an altitude of 2382 m under the auspices of the Steward Observatory and the Smithsonian Astrophysical Observatory. Fully operational in 1980, it consisted of six 1·8 m mirrors arranged in a hexagonal array on an alt-azimuth mounting. This MAT produced a collecting area equivalent to one 4·5 m single mirror telescope. Star images of less than 0·5 arc sec were achieved. The individual images from each of the mirrors were sent to a six-sided hyperbolic secondary mirror system which produced a single image. Computer-controlled repositioning of the secondary mirrors provided a correcting system to enhance the final image. One of the engineering advantages of this approach is that the length of the system depends on the diameter and focal ratio of each of the component telescopes. For a single telescope with a diameter equal to that of the effective combination but working with the same focal ratio, the length of the system would be that much greater. More recently, however, the six primary mirrors in the mount have been replaced by a single 6·5 m mirror.
Currently, the largest multi-telescope system is operated by the European Southern Observatory (ESO) in Chile at Cerro Paranal. This system comprises four individual and independent 8·2 m telescopes that can be linked to provide interferometry and aperture synthesis (see later) across baselines according to their separation.
All the plans for future large optical telescopes are based on expansion of current MMT technology. The grandest project yet, the OWL (overwhelmingly large) telescope proposed by ESO, is for a diameter of 100 m, using 1600 segments. Each piece will need to be polished to an accurate shape and positioned with nanometre precision and kept in place by a system of sensors and activators, this being the essence of active optics.
The new telescopes with active optics satisfy one of the main purposes of using collectors with large diameters, i.e. the collection of larger amounts of flux to boost the strengths of recorded signals from celestial sources and improve the measurement signal-to-noise ratio. However, their potential of achieving improved angular resolution according to the telescope diameter, D, is spoiled because of seeing. This problem is being addressed through the principle and technology of adaptive optics.

Figure 1. A schematic layout of an adaptive optics system.