The VLT Interferometer - ESO

More documents

Recommendations

Info

7.12. MORE ON BEAMCOMBINATION 7.12.1 Wavefront Mixing 7.12.1.1 Image Plane From the output pupil plane a collimator acting as in a Fraunhofer diffraction set-up forms the image of the source in the final combined image plane. This image consists of a seeing disk with speckles, or a seeing halo with a central speckle, or an Airy disk, depending on what degree of phase correction is achieved on each of the combined wavefronts. The fringes cross the speckles, and their spatial frequency is set by the distance between the individual pupil Images. If the output pupil is homothetic of the input, a large field of view is obtained: the white light fringe moves at the same rate as the image (see Sect. 3.3). The output pupil image can also be reconfigured. The field-of-view is then lost, but this might be advantageous in several cases: • the sizes of the Airy disks of different sized telescopes can be matched allowing a good interferometric efficiency for heterogeneous apertures (see Sect. 3.4). • the ratio of the fringe spatial period to the size of the Airy disk can be adjusted at will, allowing to decrease the number of pixels needed to analyse the image: this is the so-called 'fringe zooming' technique. • redundant input baselines can be separated in a non-redundant output pupil. • the output pupil can be rearranged in a line, allowing the perpendicular direction to be used for e.g. spectral dispersion. • a beam-combiner of smaller size can be used. 7.12.1.2 Pupil Plane From a combined output image plane a collimator forms a image of all the pupils superimposed on each other. This final pupil image is crossed by fringes (possibly distorted by seeing or residual phase errors of the adaptive optics correction) whose spatial period is set by the separation of the individual images in the combined image plane. As a pupil plane set-up has no field-of-view, the reconfiguration of the pupils can be done at will, e.g. matching the pupil sizes to insure maximum interferometric efficiency. It is foreseen to implement pupil plane recombination within the general set-up described in Section 7.7. 135
136 CHAPTER 7. DEFINITION OF THE VLT INTERFEROMETER 7.12.2 Amplitude Mixing This recombination scheme does not involve diffraction. Rather the wavefronts are mixed on a beam-splitter providing two complementary outputs: this set-up can be seen as a beam reconfiguration with null separation. The recombination is done pairwise, i.e. one mixing per baseline. Consequently, for an interferometer with N telescopes, each primary beam has to be split in N - 1. While attractive for a limited number of baselines, this solution seems very difficult to implement for the full VLT configuration: combining 8 beams this way would require 56 detectors and a sprawling optical set-up. The only attractive possibility would be the case of fully or partially coherent individual beams feeding Single Mode optical fibers which could be combined into a gang of X-couplers providing all the 28 possible pair mixings. Several outputs of the couplers could be sent to the same bi-dimensionnal detector. The components needed to build such a beam combiner are not available yet but could be developped in the next few years. A drawback of using SM fibers is however that the field-of-view is at most equal to the diffraction angle of the individual telescopes. 7.13 Array control Systems 7.13.1 Active Stabilization The required stability is only achievable with closed loop active control systems. It will be possible to use a laser beam in the central obstruction of the light path for stabilization of the beams. For the pathlength control of the telescope and combinig optics laser interferometers with closed loop active stabilization of the opto-mechanical system are proposed. The complexity of the total control process makes further studies necessary. 7.13.2 General Control Philosophy For reasons of standardization in the VLT the control system for interferometry and its instrumentation will be based on the principle of distributed microcontrollers. The electronics will be incorporated as much as possible locally into electro-mechanical units (Local Control Units). This has the advantage that
Page 3 and 4:
FOREWORD In November 1988 I propose
Page 5:
INTRODUCTORY COMMENTS This document
Page 8 and 9:
3.6.2.1 Spectral Dispersion in Imag
Page 10 and 11:
9.4 Remote Observing . . . . . . .
Page 12 and 13:
7.1 Lay-out of the 8 meter telescop
Page 14 and 15:
Chapter 1 Introduction 1.1 General
Page 16:
1.2. PREMISES AND ASSUMPTIONS 3 and
Page 21 and 22:
8 CHAPTER 1. INTRODUCTION being rou
Page 23:
Chapter 2 Scientific Aspects of Int
Page 27:
14 CHAPTER 2. SCIENTIFIC ASPECTS Ta
Page 31 and 32:
18 CHAPTER 2. SCIENTIFIC ASPECTS Ta
Page 35 and 36:
22 CHAPTER 2. SCIENTIFIC ASPECTS ra
Page 37 and 38:
24 CHAPTER 2. SCIENTIFIC ASPECTS an
Page 39 and 40:
26 CHAPTER 2. SCIENTIFIC ASPECTS [H
Page 41:
28 CHAPTER 2. SCIENTIFIC ASPECTS ti
Page 45 and 46:
32 CHAPTER 2. SCIENTIFIC ASPECTS LM
Page 47 and 48:
34 CHAPTER 2. SCIENTIFIC ASPECTS fo
Page 50 and 51:
2.3. DERIVED DESIRED PROPERTIES FOR
Page 52:
2.3. DERIVED DESIRED PROPERTIES FOR
Page 55:
42 CHAPTER 2. SCIENTIFIC ASPECTS li
Page 65 and 66:
52 CHAPTER 3. OPTO-MECHANICAL ASPEC
Page 70 and 71:
3.2. POLARIZATION EFFECTS IN INTERF
Page 76:
3.3. FIELD-OF-VIEW EFFECTS IN INTER
Page 81:
Page 86 and 87:
3.6. FRINGE ACQUISITION AND TRACKIN
Page 89 and 90:
76 FRINGE DETECTION CHAPTER 3. OPTO
Page 91:
Page 95:
Page 98 and 99: 3.7. "BLIND FRINGE ACQUISITION AND
Page 100 and 101: 4.1. COMPUTER SIMULATIONS OF MULTIS
Page 102: 4.2. REDUNDANT VS. NON-REDUNDANT AR
Page 107 and 108: 94 CHAPTER 4. IMAGE FORMATION ASPEC
Page 109 and 110: 96 CHAPTER 4. IMAGE FORMATION ASPEC
Page 115 and 116: Chapter 5 Adaptive Optics for Inter
Page 117 and 118: 104 CHAPTER 5. ADAPTIVE OPTICS FOR
Page 119 and 120: 106 CHAPTER 5. ADAPTIVE OPTICS FOR
Page 121 and 122: Chapter 6 VLT Site Considerations 6
Page 123: 110 CHAPTER 6. VLT SITE CONSIDERATI
Page 128: 6.3. DISCUSSION OF POSSIBLE CONFIG.
Page 132 and 133: 7.1. SUMMARY OF BASIC REQUIREMENTS
Page 134 and 135: 7.2. THE 8M TELESCOPES 121 Ml M 11
Page 137 and 138: 124 CHAPTER 7. DEFINITION OF THE VL
Page 141: 128 CHAPTER 7. DEFINITION OF THE VL
Page 146: 7.10. IMAGE, PUPIL, AND FRINGE TRAC
Page 155 and 156: Chapter 8 Implementation Plan 8.1 I
Page 158: 8.2. DESCRIPTION OF THE ADVANCED AR
Page 165 and 166: 152 CHAPTER 8. IMPLEMENTATION PLAN
Page 167: 154 CHAPTER 8. IMPLEMENTATION PLAN
Page 170 and 171: B.B. ADDITIONAL SUPPORT NEEDED 157
Page 172 and 173: 9.2. OPERATION OF THE VLTI/ADVANCED
Page 174 and 175: 9.5. OTHER OPERATIONAL ASPECTS OF T
Page 176 and 177: Bibliography [Afanas'jev et al1988]
Page 178 and 179: BIBLIOGRAPHY [Foy 1988] [Forbes 198
Page 180 and 181: BIBLIOGRAPHY [Reinheimer et al 1988
Page 182 and 183: Chapter 11 Appendices List of Appen
Page 184 and 185: 171 6. encourage the funding and co
Page 186 and 187: 3.2 Koechlin's 12T Results I derive
Page 188 and 189: Appendix C Polarization effects in
Page 190: in Table I for incidence angles 9 e
Page 199:
186 3. W. A. Traub, "Polarization E
Page 210:
7. REFERENCES 1. J. M. Beckers, "Fi
Page 214:
telescope diameter d. That conclusi
show all

The VLT Interferometer - ESO

Create successful ePaper yourself

Delete template?

Save as template?