~I ASTRONOMY AND 11 ~ I ASTROPHYSICS LIBRARY LIBRARY Series Editors: I. Appenzeller, Heidelberg, Germany

G. Bömer, Garehing, Germany M. Harwit, Washington, DC, USA R. Kippenhahn, Göttingen, Germany J. Lequeux, Paris, France P. A. Strittmatter, Tueson, AZ, USA V. Trimble, College Park, MD, and Irvine, CA, USA

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For legendsee front matterpage IV

R. N. Wilson

Reflecting Telescope Optics 11 Manufacture, Testing, Alignment, Modern Techniques

With 240 Figures and 39 Tables

i Springer

Raymond N. Wilson

Waaler Str. 29 85296 Rohrbach/llm GERMANY

Cover picture: The two 10 m Keck telescopes at the observatory on Mauna Kea, Hawaii. On the left Keck II, on the right Keck I. At the time of completion of the text of this book (December 1997), these were not only the largest unit telescopes in the world, but also the only very large telescopes using new technology which were aIready in operation. (Photograph courtesy of the W.M. Keck Observatory, through J. Nelson and A. Perala. The high-resolution copy used was kindly provided by Richard J. Wainscoat, who holds the copyright).

Legend to frontispiece: The ESO VLT: Unit Telescope No. 2 (Kueyen), photographed in March 2000 by Hans-Hermann Heyer, ESO. Photo upperleft: A 3-colour composite photo ofthe Crab Nebula (Messier 1) laken with the VLT Unit Telescope No. 2 and the FORS-2 instrument on 10 November, 1999. Distance about 6000 light-years. Exposures 1-5 min with seeing (FWHM) 0.65-0.80 arcsec. Photo upper right: A 3-colour composite photo of the Spiral Galaxy NGC 2997 taken with the VLT Unit Telescope No. 1 (Antu) and the FORS-l instrument on 5 March, 1999. Distance about 55 million light-years. Exposures 3-5 min. With a best star image quality of 0.25 arcsec (FWHM) in the near-infrared band (the other bands gave 0.35 and 0.34 arcsec FWHM), this is probably still the highest-resolution photo ever laken with a large ground-based telescope. Formatting of the photos by Ed Janssen, ESO. All photos and information courtesy of ESO.

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Wilson, Raymond N.: Reftecting telescope optics 1 R.N. Wilson. - Berlin; Heidelberg; New York; Barcelona; Hong Kong; London; Milan; Paris; Singapore; Tokyo: Springer (Astronomy and astrophysics library) 2. Manufacture, testing, alignment, modern techniques: with 39 tables. - 1999 ISBN 3-540-60356-5

Corrected Second Printing 2001

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© Springer-Verlag Berlin Heidelberg 1999 Originally published by Springer-Verlag Berlin Heidelberg New York in 1999. Softcover reprint ofthe hardcover Ist edition 1999 The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.

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To

Gerhard Schwesinger born 1913 in Krappitz, Upper Silesia, who developed the first complete Fourier theory for the support of primary mirrors of telescopes and thereby also stimulated my thinking on active optics, culminating in the ESO NTT

To

The NTT Team at ESO, Garehing, who developed the telescope; to all the consultants and industrial groups who contributed to its manufacture; and to the team at ESO, La Silla, who have ensured correct maintenance and operation

General Preface (Preface to Volume I)

The development of telescope optics is a fascinating story. U ntil this cen­tury, the optical theory of reflecting telescopes was essentially limited to the Cartesian treatment ofaxial imagery. In 1905, Karl Schwarzschild initiated a revolution by applying third order (Seidel) theory to the field imagery of 2-mirror telescopes. Since then, the whole gamut of possible telescope systems has been invented, analysed and, in many cases, tried out in practice.

Over all its history, the optical development of the telescope has also de­pended on technical inventions, above all in mirror materials, glasses, support systems and means of achieving high reflectivity. Over the last 30 years, de­velopments have been particularly spectacular, above all in manufacture and test techniques and generally in enhancing the image quality available.

When I started this work in 1988 there was little literature in book form available on telescope optics. Two of the best were in German: "Die Fern­rohre und Entfernungsmesser" by König-Köhler (1959) and the monograph on "Teleskope" by K. Bahner in "Handbuch der Physik", Vol. XXIX, which appeared in 1967. A major part of this latter work was devoted to a con­densed, but excellent exposition of the theory of telescope optics. Inevitably, more modern technical developments which have since assumed great im­portance could not be included; furthermore, the fact that it was written in German has reduced its impact and dissemination to a limited section of the interested community.

In 1987, "Astronomical Optics" by D. J. Schroeder appeared. Harland Epps kindly drew my attention to this excellent book in 1988 and I reflected then whether scope for a furt her work on the subject still existed. I finally concluded that it did: Schroeder's book covers a much wider field, since "astronomicai" optics includes the broad subject of astronomical instruments, whereas my intention was (and remains) only the comprehensive coverage of the optics of the reflecting telescope, in the broadest interpretation of that term. Furthermore, Schroeder's work emerged more from the university orbit and includes much basic optical theory addressed to graduate students who need, and can profit from, the whole physics background.

The aim of the present book is different from Schroeder's. It is addressed primarily to specialists in the field, both in the astronomical community itself and in the industries concerned, although I hope it may also be useful to stu­dents. Consequently, subjects such as practical alignment and test techniques,

VIII General Preface (Preface to Volume I)

aS wen as maintenance aspects, occupy a significant part. Nevertheless, there are inevitably major overlap areas with both Bahner's and Schroeder's books which the informed reader will recognise. This overlap, involving repetitions in a somewhat different context, is unavoidable for a complete presentation.

Bahner's book included sections on achromatic objectives for refracting telescopes, astrographic objectives and oculars. No such material is included in this book. The refractor as such and the optical design of oculars are only of historical interest in large telescope optics and are only mentioned in this context. Of course, refracting elements still play an important role in wide-field telescopes, field correctors and focal reducers, and these are dealt with in Chapters 3 and 4. In general, mirrors supply the optical power while refracting elements have only the sub ordinate but important role of improving the imagery.

I favour the morphological approach with a strong emphasis on the his­torical background of the subject. In this sense, Chapter 5 is to be seen as essential background for understanding the current situation in telescope op­tics. For the background of the general theory of optical aberrations and diffraction, the reader is referred to specialist books in the field of optics. Only the essential consequences of Gaussian optics, third order theory and diffraction theory are given: the emphasis is on a complete treatment of the application to reflecting telescope optics.

At the suggestion of the publisher, the work has been split into two vol­umes. The first volume deals with the historical development (but there is no claim to completeness as a history of telescope optics - that would be a separate work) and the theory of reflecting telescope optics, including that of the refracting corrector elements. The second volume deals with technical aspects and modern developments in general. Although there is considerable cross-referencing between the volumes, the split is a logical one, since each volume has its own entity.

Every attempt has been made to give complete references to the interna­tionalliterature. It is hoped that the work will be useful, apart from its own content, as a "source book" of the subject.

While I was writing the book, three further works on the subject were pub­lished: "Telescope Optics" by Rutten and van Venrooij (1988), "Astrooptik" by Laux (1993) and "Reflective Optics" by Korsch (1991). The first two are primarily destined for amateurs, but have equally great value for profession­als. As with the works of Bahner and Schroeder, there is considerable overlap with my material and I have referred to them liberally in my text. I only became aware of Korsch's work when my own text was finished, but again there is inevitably considerable overlap of treatment. However, not only the content and aim of these five works, an admirable, are very different, but also their styles. In this sense, I feel confirmed in my own enterprise.

Chapter 3 of Vol. I, dealing with the aberration theory of reflecting tele­scopes, is the longest and certainly one of the most important in the whole work. It is in this area that there is the greatest overlap with the above

General Preface (Preface to Volume I) IX

books. However, an illustration of the major, and legitimate, differences in presentation is the data given on the optical quality of systems discussed. Spot-diagrams are the commonest way of representing the quality according to geometrical optics. Rutten-van Venrooij and Laux give virtuaIly complete spot-diagram analyses of the systems they discuss, a very valuable feature. To keep Vol. I within reasonable bounds, I have preferred to limit myself to chosen examples, intended to illustrate with spot-diagrams the key points of the development. Some of these are taken from the literaturej but most of those in Chapter 3 (and a few in Chapter 4) have been optimized by Bernard Delabre of ESO from starting systems I set up from the basic theory, or with minor modifications emerging from the calculations. I am deeply grateful for this major contribution to the work.

I owe a great debt of gratitude to many specialist members of the as­tronomical community and associated industrial concerns, particularly Carl Zeiss (Oberkochen) and REOSC (Paris), who have generously supplied in­formation. This debt extends, too, to many ESO coIleagues. Above aIl, I am grateful to the ESO management for supporting the project and for extensive help in establishing the final text. In the detailed work, I wish to thank specif­ically, as weIl as Bernard Delabre mentioned above, Marion Beelen, Samantha Milligan, Baxter Aitken (who has not only played a major role in the text­processing but also kindly read through the entire work), Ed Janssen (who drew and formatted the figures) and Hans-Hermann Heyer for much hard work and enthusiastic support. My gratitude is also due to Richard West for general encouragement and support. Finally, I thank the publisher, Springer­Verlag, for exceIlent cooperation, and, last but by no means least, my wife Anne, for much help with the text and, above aIl, for patience throughout the whole task.

D-85296 Rohrbach January 1996

Ray N. Wilson

Preface to Volume 11

The aim and style of Vol. II follows exactly the intentions expressed in the preface to Vol. I, above. The general approach is, therefore, again historical and morphological, although the subjects of Vol. II are, in general, more re­cent than much of the theory of Vol.1. Most of the developments described are a product of the last 50 years - many of them, indeed, of the last 20 years. Nevertheless, the history of the developments discussed often goes back a long way: in Chap.4 there is a reference to W. Herschel in 1800, in Chap.3 to S. D. Poisson in 1829. It is my hope that the two volumes together include the most complete bibliography of reflecting telescope optics that exists. Chapter 1 of Vol. II, dealing with manufacture and test technology, has 166 numbered references; Chap.3, dealing with modern technical solu­tions for reflecting telescopes in general and the longest in the book, has 260 numbered references. So Vol. II will hopefuBy also serve as a source book.

One important consequence of the fact that the bulk of the material of Vol. II is, in its nature, modern or ultra-modern, is that the exposition given is bound to date fairly rapidly in those areas where development is most intensive. Such a key area is, of course, adaptive optics, the correction of atmospheric seeing dealt with in Chap. 5. Depending on the material involved, the cut-off date for new developments in this book lies between about 1993 and February 1998 at the latest. So developments after 1993 are only partially covered and effectively not at all after 1997.

Since no developments occurring in 1998 could be included in the text, it follows that there is no reference to the remarkable "First Light" results of the ESO VLT UT1, the first telescope with an 8 m monolithic primary to be completed. The image quality (raw) published at "First Light" was remark­able enough, with best star images having a FWHM equal to 0.43 arcsec or even 0.38 arcsec. However, on 6 June 1998 this was bettered by star images of the globular cluster M55 with a FWHM of 0.27 arcsec! Such remarkable results, expected to be achieved only about 3 years later, are a complete vin­dication both of the thin-meniscus active technology of the telescope and of the excellent site of Paranal in Chile. It is clear that the age of very large telescopes based on new technology, initiated with dramatic success by the two Keck 10 m telescopes completed in 1992 and 1996 respectively, will in the next few years be represented by about a dozen or more superb individ­ual instruments using different technologies and aB with apertures between

XII Preface to Volume 11

61/ 2 m and 10 m, some of which are intended for linked operation in the inter­ferometric mode. A marvellous astronomical prospect for the new millenium indeed! It is my hope that I have done justice in this book to all these projects and the remarkable technologies driving them.

It was my intention to inelude specific chapters on solar telescopes, X-ray telescopes using near-grazing incidence, and interferometry with telescopes. Subsequently, I abandoned coverage of these fields for the following reasons. Solar telescopes have very different requirements from those of normal astro­nomical telescopes, and refracting telescopes, which are not treated in this work, still playa significant role. Similarly, X-ray telescopes have very special technical requirements, though the elose link to the Mersenne telescope was indicated in Vol.1. Furthermore, these systems have been admirably dealt with by Korsch in his "Reflective Optics" (Academic Press, 1991). The inter­ferometric requirements for telescopes have been treated briefly in Chap.5; but I coneluded that a treatment in depth of interferometry with telescopes would require a whole book in its own right, even if my own expertise in this field were adequate, which is not the case.

I owe a vast debt of gratitude to a wide cirele of friends and colleagues in the astronomical community and the "big optics" community associated with it. Specific acknowledgments are given at the beginning of Chap. 1 in the areas of manufacture and testing of optical surfaces. The other chapters, above all Chap. 3, are so wide-ranging that it would be impossible to express adequate acknowledgment in this preface to the many sources of generous help and advice. I hope, therefore, it will suffice here if I thank globally all those who have helped with information or figure material. I believe the credits in the text, references and, above all, the figures are the best way to express my deep gratitude.

Specifically, I wish here to thank the management of ESO for their con­tinued encouragement and support of the whole project, above all for the major work of the preparation of the figures. This was organised through the kind help of Richard West and executed, as for Vol. I, by Ed Janssen. He has again done a wonderful job in a key area and I express here my grateful thanks to hirn. My thanks are also due to Hännes Heyer for his assistance on the photographic side. On the technical side, many colleagues have given valuable information, particularly Philippe Dierickx, Lothar Noethe, Martin Cullum, Paul Giordano, Bernard Delabre and Francis Franza. The original text processing was done by Marion Beelen, Samantha Milligan and Baxter Aitken, to whom I express my gratitude for an enormous task. Ingrid Weber has given valuable secretarial help in many ways. Finally, the ESO library has been a central factor in the fundamentally important area of literature: my grateful thanks are due to Uta Grothkopf and Angelika Treumann for their efficient and friendly service.

As with Vol. I, the collaboration with Springer-Verlag has been excellent in all respects. I express here my grateful thanks to Prof. W. Beiglböck and his staff for the help and understanding they have constantly given me; also for

Preface to Volume 11 XIII

financial support in preparation of the revised and corrected manuscript. The latter text-processing work has been carried out with admirable efficiency and cooperation by Mr. Adam Leinz, to whom I also express my grateful thanks.

When Vol. II appears, about the end of 1998, the whole project will have covered aperiod of exactly ten years, perhaps 6-7 years full-time equivalent. My wife, Anne, has not only given me much valuable help in checking the style and correctness of the text, but also borne with my obsession with the task with great patience and understanding, for which I owe her a great debt of gratitude.

D-85296 Rohrbach June 1998

Ray N. Wilson

Contents

1. Manufacture and test procedures . . . . .. .. . . .. . . .. . . . . . . . . . 1 1.1 Introduction and acknowledgements ...................... 1 1.2 Grinding, polishing and figuring technology . . . . . . . . . . . . . . . . 1

1.2.1 Background of optical surface working ... . . . . . . . . . . . 1 1.2.2 Lapping techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1. 3 Test technology .............................. . . . . . . . . .. 46 1.3.1 General aspects of test technology development ...... 46 1.3.2 Interferometers................................... 46 1.3.3 Test procedures other than interferometry . . . . . . . . . .. 74 1.3.4 Null (compensation) systems. . . . . . . . . . . . . . . . . . . . . .. 76 1.3.5 Test systems for Cassegrain secondary mirrors ....... 86 1.3.6 Test methods for large flats ........................ 101

2. Sensitivities, alignment of telescopes and test procedures in function ........................... 105 2.1 Sensitivities............................................ 105

2.1.1 Decentering errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 105 2.1.2 Despace errors ................................... 109

2.2 Alignment and adjustment of telescopes ................... 111 2.2.1 General theoretical principles of telescope alignment .. 111 2.2.2 General set-up situation and definition

of the aims of alignment ........................... 122 2.2.3 Alignment at the prime focus of telescopes

with field corrector ............................... 130 2.2.4 Alignment of Schmidt telescopes ................... 130 2.2.5 Field correctors at the Cassegrain focus ............. 131

2.3 Test methods and image analysis of telescopes in function ... 131 2.3.1 Classical qualitative methods ...................... 131 2.3.2 "Pupil plates": geometrical assessments

of defocused star images ........................... 133 2.3.3 Hartmann-based techniques ........... , ............ 144 2.3.4 Curvature sensing: the Roddier test ................. 161 2.3.5 Other methods of testing the optical quality

of telescopes in function ........................... 166

XVI Contents

3. Modern telescope developments: pupil segmentation and techniques to reduce mass ........ 169 3.1 Evolution and revolution in telescope optics ................ 169 3.2 Examples of modern projects using the technologies

of Table 3.3 ............................................ 175 3.2.1 Direct segmentation with a filled aperture ........... 175 3.2.2 Separate telescopes with monolithic primaries

on a single mount (MMT-type indirect segment at ion) . 186 3.2.3 Other large telescope projects

using lightweighted monolithic blanks ............... 192 3.2.4 Projects with thin-meniscus flexible primaries,

controlled actively, or of stiff composites ............. 199 3.3 Blank production for new technology telescopes ............ 216

3.3.1 General considerations and physical properties ....... 216 3.3.2 Glass ceramic blank production .................... 223 3.3.3 Fused quartz (silica) blank production .............. 225 3.3.4 Modern blank production

with BSC (borosilicate) glass (Pyrex) ............... 227 3.3.5 Modern blank production in metal .................. 231 3.3.6 Compacted powder, sintered

or vapour-deposited materials for blanks ............ 238 3.3.7 Lightweight composite materials for blanks .......... 240 3.3.8 Liquid mirror telescopes (Hg) ...................... 240

3.4 Mirror support systems in modern telescopes ............... 242 3.4.1 The basic laws ofaxial supports for mirrors .......... 242 3.4.2 Modern work on the theory of mirror supports:

axial support solutions .................... , . . . . . . . . 253 3.4.3 Lateral (radial) supports for mirrors ................ 259 3.4.4 Mirror handling .................................. 272

3.5 Active optics control systems ............................ 274 3.5.1 Introduction and definitions ....................... 274 3.5.2 The principles of the ESO active optics system,

as developed for the 3.5 m NTT .................... 279 3.5.3 Operational results for the ESO 3.5 m NTT

and conclusions from its performance ............... 291 3.5.4 Extension of the active optics system

to the ESO 8 m VLT .............................. 298 3.5.5 Other active optics developments in current projects .. 304 3.5.6 Conclusions on the current state of development

and future potential of active optics in telescopes ..... 313 3.6 Local environmental aspects of telescopes .................. 314

3.6.1 Definition of "local air" and its importance .......... 314 3.6.2 Recent evolution in telescope enclosures

and "local air" error measurements in functioning telescopes .......................... 318

Contents XVII

3.6.3 Other recent investigations by laboratory experiment or theory: mirror seeing . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

3.6.4 Further work on "dome seeing" in general ........... 338 3.6.5 General conclusions on local air seeing and enclosures . 343

3.7 Optical data of the major ground-based telescope projects using new technology ............................... . . . . 345

4. Image quality specification and optical efficiency criteria .. 349 4.1 Classical specification criteria: geometrical angular

or wavefront aberration ................................. 349 4.2 Specifications for modern ground-based telescope projects ... 353 4.3 Optical efficiency criteria ................................ 367

5. Atmospheric optics, adaptive optics, telescope quality for interferometry ........................................ 373 5.1 Atmospheric optics ..................................... 373

5.1.1 Atmospheric refraction and atmospheric dispersion ... 374 5.1.2 Atmospheric turbulence ("seeing") .................. 376

5.2 Adaptive optics ........................................ 396 5.2.1 Definitions and aims: active and adaptive optics ...... 396 5.2.2 The principles of adaptive optics ................... 398 5.2.3 Practical systems for adaptive optics

in astronomical telescopes . . . . . . . . . . . . . . . . . . . . . . . . . 403 5.2.4 Limitations of the isoplanatic patch:

artificial reference sources ......................... 409 5.2.5 Adaptive optics for the ESO VLT

and experimental correction results (COME-ON and COME-ON PLUS systems) ......... 413

5.2.6 Adaptive optics using laser reference sources for military purposes applicable to astronomy ........ 416

5.3 Site selection in height .................................. 418 5.4 High resolution imaging apart from adaptive optics ......... 420

5.4.1 Michelson interferometry .......................... 420 5.4.2 Speckle interferometry ............................ 421

6. Mirror ReHecting Coats: Production and Cleaning ........ 423 6.1 Introduction: evolution to the current situation ............. 423 6.2 Modern perspectives for reflecting coatings ................ 430

6.2.1 Multi-coat enhancement ofreflecting coats ........... 430 6.2.2 Silver reflecting coat with single protecting coat

and other recent developments . . . . . . . . . . . . . . . . . . . . . 434 6.2.3 Cleaning and maintenance of reflecting coats ......... 442 6.2.4 Summary ........................................ 448

XVIII Contents

7. Adapters and beam combination aspects, bafHes . . . . . . . . . . 449 7.1 Adapters .............................................. 449

7.1.1 Background of adapter development ................ 449 7.1.2 The adapter for the ESO 3.5m NTT (Nasmyth focus) 450 7.1.3 The adapters for the VLT 8 m unit telescopes ........ 456 7.1.4 Beam combination aspects ......................... 458 7.1.5 Mountings and field rotation ....................... 459

7.2 BafHes ................................................ 459

8. Maintenance and operation of telescope optics ............ 469 8.1 Ground-based telescopes ................................ 469 8.2 Space telescopes ........................................ 474

Appendix: Mathematical symbols ............................. 475

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

List of figures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

List of tables ................................................. 515

Name index .................................................. 519

Subject index ................................................. 525