Lothar Noethe

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 ...

Chapter 1 – Active optics in modern large optical telescopes

2 Principles of active optics 2 2.1 Error sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2 Classification of active telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2.1 Control loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2.2 Correction strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3 Modal control concept and choice of set of modes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.4 Examples of active telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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.

Use of Minimum-energy Modes for Modal-active Optics Corrections of Thin Meniscus Mirrors

Abstract For thin meniscus mirrors the modes which correct arbitrary wave-front aberrations in the most efficient way are derived. These modes are similar to the free-vibration modes of the mirror. The relationship between the two types of modes is shown. The most important features such as orthogonality and energy of the modes are discussed. This modal approach is applied to a few practical problems in connection with the primary mirrors of the Very Large Telescope of the European Southern Observatory.

The eye of the beholder: designing the OWL

Preliminary requirements and possible technological solutions for the next generation of ground-based optical telescopes were laid down at ESO in 1998. Since then, a phase A study has been commissioned, the objective of which is to produce a conceptual design compatible, to the maximum possible extent, with proven technology, and establish realistic plans for detailed design, site selection, construction and operation for a 100-m class optical, diffraction-limited telescope. There was no doubt about how daunting such a challenge would be, but, somewhat surprisingly, it turns out to be firmly confined to adaptive optics concepts and technologies. The telescope itself appears to be feasible within the allocated budget and without reliance on exotic assumptions. Fabrication of key subsystems is fully within the reach of a properly engineered, industrialized process. A consolidated baseline is taking shape, and alternative system and subsystem solutions are being explored, strengthening the confidence that requirements could be met. Extensive development of wavefront measurement techniques enlarges the palette of solutions available for active wavefront control of a segmented, active telescope. At system level, ESO is developing enabling experiments to validate multi-conjugate adaptive optics (MAD for Multi-conjugate Adaptive optics Demonstrator) and telescope wavefront control (APE, for Active Phasing Experiment).

Removing static aberrations from the active optics system of a wide-field telescope

The wavefront sensor in active and adaptive telescopes is usually not in the optical path toward the scientific detector. It may generate additional wavefront aberrations, which have to be separated from the errors due to the telescope optics. The aberrations that are not rotationally symmetric can be disentangled from the telescope aberrations by a series of measurements taken in the center of the field, with the wavefront sensor at different orientation angles with respect to the focal plane. This method has been applied at the VLT Survey Telescope on the ESO Paranal observatory.

THE ACTIVE PHASING EXPERIMENT PART I: CONCEPT AND OBJECTIVES

In a framework of ELT design study our group is building an Active Phasing Experiment (APE), the main goals of which is to demonstrate the non-adaptive wavefront control scheme and technology for Extremely Large Telescope (ELT). The experiment includes verification and test of different phasing sensors and integration of a phasing wavefront sensor into a global scheme of segmented telescope active control. After a sufficient number of tests in the laboratory APE will be mounted and tested on sky at a Nasmyth focus of a VLT unit telescope. The paper presents APE as a demonstrator of particular aspects of ELT and provides a general understanding concerning the strategy of segmented mirrors active control.

Final alignment of the VLT

In a recent paper McLeod proposed and used measurements of the field dependence of third order astigmatism to collimate a Ritchey-Chretien telescope with the stop at the primary mirror. We adapt this method to the Cassegrain focus of the ESO Very Large Telescope, where the stop is at the secondary mirror and the telescope is only corrected for spherical aberration. In addition, we study the effects of the practical definition that the center of the field is the center of the adapter. We present measurements of the field astigmatism and discuss the accuracy of this collimation method.

Progress of ESO's 100-m OWL optical telescope design

Even as a number of 8- to 10-m class telescopes come into operation worldwide, the scientific challenges these instruments and their space-based counterparts already address imply that future increases in light-gathering power and resolution will have to exceed conventional scaling factors. Indeed, it can be expected that the same progress in telescope diameter and resolution achieved throughout the century must now be realized within, at most, one or two decades. The technologies required to assert the validity of such an extrapolation appear to be within reach. Large telescopes successfully comissioned within the last decade have demonstrated key technologies such as active optics and segmentation. Furthermore, current design methods and fabrication processes imply that the technological challenge of constructing telescopes up to the 100-m range could, in some critical areas, be lower than those underlying, two decades ago, the design and construction of 8 to 10-m class telescopes. At system level, however, such giants are no size-extrapolated fusion of VLT and Keck, but fully integrated adaptive systems. In this paper we elaborate on some of the science drivers behind the OWL concept of a 100-m telescope with integrated adaptive optics capability. We identify major conceptual differences with classical, non-adaptive telescopes, and derive design drivers accordingly. We also discuss critical system and fabrication aspects, and the possible timeline for the concept to be realized.