Daniel Enard

PROGRESS IN GROUND-BASED OPTICAL TELESCOPES

For several decades, the 5 metre Palomar telescope has set a practical limit to the size of ground-based optical telescopes only exceeded by the Russian 6 metre telescope. Using new technologies to produce either large monolithic mirrors or large mosaic of smaller elements, astronomers in Europe, the United States and Japan are developing a new generation of telescopes. Not only are these new telescopes much bigger in size, offering considerably more collecting power, but they also provide a much better angular resolution. To achieve this they rely heavily on computer control techniques and advanced modelling. Even more progress in angular resolution is expected when modern techniques such as adaptive optics and interferometry, which are being implemented on most of the new telescopes, are fully operational. This article evaluates the impact of new technologies on the development of large optical telescopes, in particular on the realization of their optics. The key mirror technologies are reviewed and a particular project, the ESO VLT is reviewed in some more detail as an illustration. This review was received in January 1996.

VLT primary mirrors: mirror production and measured performance

The primary mirrors of the ESO 8-m class very large telescopes are actively supported, thin Zerodur menisci, 8 - .2-m diameter. The mirror blanks are produced by SCHOTT; the optical figuring, manufacturing and assembling of interfaces and auxiliary equipment are done by REOSC. Three mirror blanks have already been delivered by SCHOTT to REOSC. In November 1995 the project met a critical and very successful milestone, with the completion and testing of the first finished VLT primary mirror at REOSC. Specifications, manufacturing and above all testing methodology are addressed, and the final results are detailed. Optical performance at telescope level is assessed as well.

ESO VLT Project: I. A status report

The ESOs Very Large Telescope will employ an array of four 8-m aperture telescopes which can be operated in either independent or combined modes, as well as in an interferometric mode. The primary mirrors used are of thin, Zerodur glass-ceramic meniscus type, figured for f/1.8; the overall design is optimized for f/15 at the Nasmyth foci. Active optics are employed to compensate for slowly varying deformation and thermal distortion effects, as well as wind buffeting. Wavefront sensing for active figure maintenance is accomplished by means of a Shack-Hartmann CCD wavefront sensor which is integrated with the imaging CCD used for field acquisition and tracking. Two types of enclosure are under consideration for the telescope which attempt to maximize natural ventilating flows.

Two Design Approaches For High Efficiency Low Resolution Spectroscopy

In the course of a systematic analysis of various design philosophies aimed at achieving high optical throughput low resolution spectrometers for telescopes in the range of 2-4 meters, two distinct optical designs were established. The first is optimised for a telescope prime focus having a fast F/ratio, and is based on a concave grating whose aberrations are corrected by 2 or 3 off-axis lenses. The second system is a focal reducer, utilizing lenses and transmission gratings only. The use of a particular glass type having an extremely low dispersion permits excellent correction of various aberrations over a wide spectral range. Both designs provide a two-dimensionally corrected field typical of present day CCD array formats. These designs are highly suitable for long slit or field spectroscopy and offer the possibility of rapid switching to direct imaging mode.

European Southern Observatory (ESO) Coude Echelle Spectrometer

A powerful high resolution spectrometer has been developed, and is installed at the Coude Focus of ESOs 3.6 m telescope. It can also be fed by a 1.4 m auxiliary telescope dedicated to the instrument. With this latter telescope magnitude 10 stars can be observed at a resolving power of 100.000 and with a good instrumental profile. The CES also features a single channel scanner mode which can attain a resolving power of 300.000 with an extremely high spectral purity.

ESO VLT: III. The support system of the primary mirrors

The support system of the 8.2 meter primary mirrors of the ESO Very Large Telescope consists of 150 axial support at the back surface of the mirror and appr. 60 lateral supports at the outer and inner edge of the mirror. This paper describes the general design of the support system and the prototypes of the axial actuators.

Optical figuring and testing of the VLT 8.2-m primary mirrors

The primary mirrors for the ESO very large telescope project consist of four f/1.8 8.2-m diameter actively supported Zerodur menisci, 175 mm thick. Their optical figuring is under way at REOSC (France) plant. Two mirror blanks have been delivered to the optical manufacturer and the two remaining at the blanks manufacturer (SCHOOT Mainz, Germany) are due for delivery by the second hald of 1995. Th polishing of the first Zerodur mirror blank is due for completion in the third quarter of 1995. The REOSC optical fabrication and testing methodology will be presented, and the current results will be compared to the specified performance.

ESO VLT: status of the main 8-meter telescopes

The ESO VLT was first presented in April 1984 and its construction decided in December 1987. The VLT program consists of an array of four telescopes of 8 m diameter capable of working independently or in a combined mode. This paper gives the present status of the construction of the 8 meters Unit Telescope.

Control of image position errors with the VLT

An important contributory factor to the overall quality of the images obtained with the VLT are random tilt errors caused by wind buffeting and the atmosphere. The techniques and strategy adopted to overcome these errors are described. Image position errors caused by wind buffeting are minimized, as far as possible, by optimizing the configuration of the telescope enclosure for the prevailing wind and observing conditions. Residual tilt errors are reduced to acceptable limits by the use of a secondary mirror tilt-tip mechanism in conjunction with the telescope axis control system. In situations where the ratio of D/r0 is small, atmospheric tilt can contribute the dominant source of tilt error. In these cases the same correction procedure using the secondary mirror can even improve the image quality beyond the long integration atmospheric limit. Possible control schemes and results of simulations are presented to demonstrate the effectiveness of these solutions. The limitations of the techniques are discussed as well as their value compared to a full wavefront correction of the VLT adaptive optics system.

8.2 metre primary mirrors of the VLT

The Very Large Telescope (VLT) presently being developed at ESO is described in terms of technological advances which make its use both technically effective and feasible. The VLT capitalizes on advances in materials, polishing techniques, and mirror support systems. The VLT consists of four 8-m alt-az telescopes and a 2-m auxiliary telescope in a single-dish configuration with Zerodur meniscus mirrors passively supported on a lateral system. A discussion of the tradeoffs between glass and metal mirrors is presented, and computerized polishing is described in relation to optical specifications. The mirror is supported with 150 axial and 60 lateral supports with electromechanical actuators to modulate applied force. The active optics concept is employed via the flexibility of the primary mirror, which generates elastomechanical deformations and the position and orientation of the secondary mirror.