A description is given of the alt-azimuth mounted 6-meter telescope (BTA) installed in the North Caucasus at an altitude of 2070m above sea level and about 40km to the south of Zelenchukskaya village. Some details of the BTA instrumentation and the local astroclimate are also reported.

The MMT utilizes a number of most unusual features many of which will probably be included in future advanced technology telescopes. We report here on the performance of these unconventional functions as we have determined them until now.

Visiting astronomers have now used the CFH telescope for 18 months in the prime focus and coudé configurations. Many observers have reported very good image quality and high resolution plates have already been obtained. The general concepts and technical details of the telescope and observatory are discussed. The focal plane instrumentation includes more than 25 instruments and detectors, and these are also discussed.

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.

As presently conceived, the large telescope now being planned at The University of Texas will have a Ritchey-Chrétien configuration, f/13.5, with a 7.6m diameter f/2 primary of ultra-thin construction, on an alt-azimuth mount.

In this paper we will describe the current concept for the mounting, the building, mirror handling techniques, and the optical system. We will give preliminary results of the finite element flexure analysis of the mirror on its support system.

Studies have demonstrated a workable concept and have shown that there exists a method of adequately supporting an ultra-thin mirror against gravity and wind loads. We are continuing these studies, including also thermal effects on the mirror figure, in order to determine the best choice of mirror material and construction at the most reasonable price.

The construction of a 6 metre polar-viewing telescope has been proposed (Karachentsev and Gajur, 1979), based upon the use of the paraboloidal primary mirror installed originally in the 6m BTA at Zelenchukskaya and later replaced by a mirror of improved performance. The mirror would be mounted with its axis in a fixed orientation pointing to the celestial pole and observations would be made at the prime focus using instrumentation mounted on a pylon. Object tracking is then performed by rotation of the detector around the point which represents the “image” of the celestial pole. In particular, the polar axis may coincide with the optical axis of the system.

The UK Science and Engineering Research Council (SERC), through the Royal Greenwich Observatory (RGO), is installing on the island of La Palma, a 4.2 m altazimuth telescope, as part of the Observatorio del Roque de los Muchachos, at an altitude of 2300 m in the Canary Islands. The telescope, known as the William Herschel Telescope, is fully designed and manufacture has begun. It is due for completion in the mid-1980's. The telescope, dome and instrument design have all interacted with one another, particularly in the search for economy. Among the proposed instruments is a novel collimator-less spectrograph.

The ordinary coudé train has a single, large, aluminum coated mirror at each reflection point. This introduces considerable light loss because several reflections are required, and the long light path in turbulent air blurs the stellar image by telescope “seeing” effects. The transmission can be increased and the blur decreased by a train of small mirrors with high reflectance coatings. The high reflectance coatings have the added advantages that they can reduce polarization effects and flatten response curves.

Large aperture telescopes usually have an unavoidably long focal length so that stellar image diameters are often much larger than the resolution of the detector. The detection of extended sources can take advantage of a faster focal ratio and both stellar and extended monochromatic sources can be detected with great sensitivity by using interference filters and a focal reducer, without much loss in resolution. Extragalactic results obtained with the 6 meter telescope at focal ratio f/1 are given as examples.

The study and efficiency test of the optics of the Space Telescope Faint Object Camera carried out at the Laboratoire d'Astronomie Spatiale, led the authors to design a similar coronographic mode for large ground based telescopes. Adaptation of this design to the Mount Chiran 1 meter and E.S.O. 3.6 meter Ritchey-Chrétien telescopes has given new levels of performance in the detection of faint features in stellar and planetary observations.

Much effort has been spent at Pic-du-Midi, with increasingly large telescopes, to reduce the instrumental causes of image deterioration and take best advantage of local atmospheric conditions. With the new 2-meter reflector, specifically intended for high resolution imaging, the concept has been, ideally, to separate three air-masses: outside, inside the tube, and inside the dome. As a first step, no parallel plate of required size and quality being available, the tube is open to the outer air, but a connection between tube and dome aims at preventing the dome-air flowing across the light-beam. Results and expectations are presented.

The paper identifies causes of degradation of image quality in astronomical telescopes, describes how the effects of these can be calculated using MTFs, considers the validity of multiplication of MTFs for this purpose and gives the results of measurement of external and dome seeing, system MTF and image quality obtained during the commissioning of UKIRT.

A criterion based upon information content allows one to obtain objective estimates of the efficiencies of astronomical observations for the whole system of light receiving and recording equipment, including the conditions of observation. The proposed method for calculating this information efficiency relies on the analysis of the output observational material and not on the characteristics of optical and electronic components of the instrumentation. The method is quite simple to use for comparison of different astronomical observations and instrumentation (Rylov, 1977, 1979; Karapetian and Oskanian, 1978).

“Grism” and compound prism cross-dispersers have been proposed for the new Cassegrain spectrograph of the 2.5m telescope at La Palma. The RGO image-tube spectrograph has been fitted with a grism to provide us with experience in designing layouts, operating the equipment and handling the data.