Raymond N. Wilson

Active optics. I: A system for optimizing the optical quality and reducing the costs of large telescopes

A system of ‘active optics’ control for the optical imagery of astronomical telescopes has been under development in the European Southern Observatory for about ten years. Its first application will be in the 3·5 m New Technology Telescope (NTT) scheduled for operation in 1988. A model test with a thin 1 m mirror (aspect ratio 56) has given remarkably successful results which will be reported in Part II of this paper. Part I gives a complete presentation of the theoretical principles of this technique of active optics and its scope of application. The subject is treated from the viewpoint of the temporal band-pass of error sources, ‘active optics’ being concerned with the low-frequency band-pass. The high-frequency band-pass (‘adaptive optics’) is principally concerned with atmospheric correction and is only briefly referred to for comparison. ‘Active optics’ correction of the low-band-pass system errors should bring major improvements in image quality together with a large cost reduction. While ...

Future of filled aperture telescopes: is a 100-m feasible?

We explore the scientific case and the conceptual feasibility of giant filled aperture telescopes, in the light of science goals needing an order of magnitude increase in aperture size, and investigate the requirements (and challenges) these imply for possible technical options in the case of a 100 m telescope. The 100-m f/6.4 telescope optical concept is of a four mirror design with segmented, spherical primary and secondary mirrors, and 8-m class aspheric tertiary and quaternary mirrors, providing a 3 arc minutes field of view. Building on the experience of the VLT and other large telescope projects, we investigate mirror fabrication issues, a possible mechanical solution, the requirements for the absolutely essential adaptive optics system and for the instrumentation package, and the implications for budget and schedule.

Active Optics II. Results of an Experiment with a Thin 1 m Test Mirror

Abstract Part I of this paper [1] presented the theory, and practical basis, of a complete system for active control of optical quality, with particular application to large telescopes. Reference was made to an experimental verification using a thin model mirror of 1 m diameter. Part II now gives a complete account of these experiments which consisted essentially of two parts: the verification of the theoretical modal calibrations and their orthogonality in practice, and the application of the calibrations to achieve the active modal correction of the mirror. Both aspects of the experiment were successful, with a precision higher than expected. The residual errors left after final correction are, by definition, of a higher order than the five corrected low spatial frequency terms and are small. However, one residual term is dominant (fifth-order astigmatism) but this was omitted in the correction process as being probably too high a mode to appear in practice. A calibration for this term will be performed...

Active Optics: IV. Set-up and Performance of the Optics of the ESO New Technology Telescope (NTT) in the Observatory

This paper is the fourth, and last, in the series on active optics as realized in the ESO New Technology Telescope (NTT). It gives a detailed description of the set-up and optimization of the optical system, based on the three levels (i.e. time frequencies) of active correction. The first results in direct imagery with the telescope optimized only near zenith (‘Astronomical First Light’) yielded immediately the best star images ever recorded in ground-based astronomy with a full-width-half-maximum of 0·33 arcsec on a CCD detector with pixels of 0·123 arcsec diameter.

Active Optics: III. Final Results with the 1 m Test Mirror and NTT 3·58 m Primary in the Workshop

In ‘Active Optics II’, the active correction of a 1 m test mirror was described in detail. A residual aberration term was found which still required correction in order to achieve the aim of the experiment: removal of all systematic defects leaving only statistical or high-frequency noise. The present paper describes the interferometric confirmation of the term and this final correction stage of the 1 m experiment.

New four-mirror optical concept for very large telescopes with spherical primary and secondary mirrors, giving excellent field and obstruction characteristics

The 10 m Keck project has demonstrated that very large segmented primaries can be actively controlled. But there are important technical and above all cost advantages accruing from a spherical primary. This presents, however, a serious problem for optical design solutions with adequate field performance, reasonable central obstruction and good baffling. Two new compact solutions are presented using 4 mirrors with optical power, the primary (f/1.5) and secondary both being spherical. Only the last mirror is strongly deformed (hyperbolic), the third being a weak hyperbola. Studies for the Gemini project and elsewhere have made it clear that technical solutions for improving the reflectivity of large optics will soon be available, making optical design solutions with 4 or more reflections acceptable. The f/no of the final image is about f/6 to f/7.3. The image is only slightly curved. The geometrical basis of the solutions is the use of a second optical axis at right angles, produced by a small folding flat.

Latest developments of active optics of the ESO NTT and the implications for the ESO VLT

The latest developments of active optics of the ESO NTT include the reduction of friction in the lateral supports of the primary mirror and in the positioning system of the secondary mirror. The most important remaining problem is the local air condition. The implications for the ESO VLT and the latest developments in the design of its active optics are discussed.

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.

Evidence supporting the primacy of Joseph Petzval in the discovery of aberration coefficients and their application to lens design.

In 1839 Louis Daguerre published his process for permanently fixing optical images, and created an instant need for a high-aperture, relatively wide-field and well-corrected lens. Within a year, an optical design meeting all of these requirements was provided by Professor Joseph Petzval, a mathematician with no previous background in optics. This optical design was revolutionary in that it was well corrected for aberration over a wide-field at the unprecedented speed of f/3.5. As Petzval never published explicit details of his method for designing lenses, the credit for the invention of an aberration theory applicable to lens design has gone in the first place to Seidel, and later to those who developed high-order coefficients such as Schwarzschild, T Smith and Buchdahl. It is the contention of this paper that this has been an historical injustice, and that sufficient evidence exists, and indeed has existed in part since well before Seidel published his derivation of third-order aberration coefficients, to establish Petzval as the original pioneer of third-and-higher-order aberration theory as a tool for lens design.

History of optical theory of reflecting telescopes and implications for future projects

This contribution, The History of Optical Theory of Reflecting Telescopes and Implications for Future Projects, is a shortened form of the Karl Schwarzschild lecture given in Bochum in September 1993. Some material has been added from an invited paper given in Padua in December 1992. For a full account, with figures and tables, the reader is referred to these two papers.