The VLT Interferometer - ESO

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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
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FOREWORD In November 1988 I propose
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INTRODUCTORY COMMENTS This document
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3.6.2.1 Spectral Dispersion in Imag
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9.4 Remote Observing . . . . . . .
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7.1 Lay-out of the 8 meter telescop
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Chapter 1 Introduction 1.1 General
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1.2. PREMISES AND ASSUMPTIONS 3 and
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8 CHAPTER 1. INTRODUCTION being rou
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Chapter 2 Scientific Aspects of Int
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2.3. DERIVED DESIRED PROPERTIES FOR
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3.2. POLARIZATION EFFECTS IN INTERF
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3.3. FIELD-OF-VIEW EFFECTS IN INTER
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3.6. FRINGE ACQUISITION AND TRACKIN
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76 FRINGE DETECTION CHAPTER 3. OPTO
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186 3. W. A. Traub, "Polarization E
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7. REFERENCES 1. J. M. Beckers, "Fi
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telescope diameter d. That conclusi
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