The VLT Interferometer - ESO

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Appendix C Polarization effects in astronomical spatial interferometry Jacques M. Beckers European Southern Observatory 0-8046 Garching bei Munchen, FR Germany ABSTRACT One of the operational modes of the European Very Large Telescope (VLT) will be the one in which the four 8.2 meter diameter telescopes will be used together with two or more smaller telescopes with about 2 meter aperture as a spatial amplitude interferometer. One of the main factors affecting the performance of this interferometric array relates to the polarization characteristics of the optics composing the legs of the array. Retardation effects and reorientation of the polarization coordinate frame of reference by reflections are a major cause of fringe contrast decrease (and even disappearance). I have analyzed these effects and developed a ray tracing algorithm to calculate the polarization transfer and the resulting fringe contrast decrease. 1. INTRODUCTION In optical astronomical interferometers, like the European Southern Observatory Very Large Telescope (VLT), there are a number of components in the optical trains of each interferometer leg (telescope and beamcombiner optics) which affect the polarization state of the light. The most common of these are non-normal reflections which act both as partial polarizers and linear retarders to the light which they reflect. Multiple reflections can also cause rotation of the polarization direction (also called here: rotation of Polarization Frame-of Reference, or FOR) in the same way as they can cause image rotation. All three will modify the polarization state of the light and as a result may decrease the contrast and change the position of the interferometer fringes. In the ideal interferometer each incoming polarization state (linear, circular, or in general elliptically polarized light) reaches the final interference location (beamcombiner) in identical polarization states for all legs of the interferometer. These exitting polarization states do not necessarily have to be the same as the incoming polarization states to preserve the fringe contrast, as long as they are all identical. In addition to maintaining high fringe contrast, polarization control in interferometers is of course also of utmost importance if the radiation of the astronomical object under study is polarized. Paper presented at the SPIE Conference Nr. 1166 on Polarization Considerations for Optical Systems II on August 9-11 in San Diego. 175
176 In this paper I will examine the effects on the fringe contrast and position of unequal polarization/retardation effects in the different interferometer legs. To do this I will use the so-called Jones Calculus 1 • 2 for polarized light. Jones Calculus is preferred over the socalled Muller Calculus for polarized light since it preserves the phase information of light which is essential for interferometric applications. Traub 3 also discussed polarization effects in stellar interferometers. The treatment presented here is however more complete. Before going into the analysis it is useful to review the polarization effects of non-normal reflections. 2. POLARIZATION/RETARDATION EFFECTS FOR NON-NORMAL REFLECTIONS Born and Wolff 4 give the magnitude of the polarization /retardation effects by metallic reflections. From it I calculate the values listed TABLE I Polarization/Retardation Properties of Some Coatings. Coating: Wavelength Polarization(%) Retardation (deg) (nm) 0= 45 60 0= 45 60 Aluminium 400 12 17 39 73 450 9 14 29 57 500 6 11 22 44 700 2 4 7 15 1000 1 2 5 11 2000 0 1 2 5 5000 0 0 0 0 Gold 450 15 30 24 49 500 25 34 39 76 700 8 6 78 113 1000 9 10 57 94 2000 1 2 6 12 5000 0 0 0 1 Silver 400 2 1 89 119 450 1 1 89 120 500 1 1 89 119 700 4 3 84 116 1000 8 8 64 101 2000 1 3 12 24 5000 0 0 1 1 Silver over 450 - - 34 64 Chrome 500 - - 30 57 (Traub 3 ) 700 - - 20 40
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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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14 CHAPTER 2. SCIENTIFIC ASPECTS Ta
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18 CHAPTER 2. SCIENTIFIC ASPECTS Ta
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22 CHAPTER 2. SCIENTIFIC ASPECTS ra
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24 CHAPTER 2. SCIENTIFIC ASPECTS an
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26 CHAPTER 2. SCIENTIFIC ASPECTS [H
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2.3. DERIVED DESIRED PROPERTIES FOR
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2.3. DERIVED DESIRED PROPERTIES FOR
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42 CHAPTER 2. SCIENTIFIC ASPECTS li
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52 CHAPTER 3. OPTO-MECHANICAL ASPEC
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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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3.7. "BLIND FRINGE ACQUISITION AND
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4.1. COMPUTER SIMULATIONS OF MULTIS
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4.2. REDUNDANT VS. NON-REDUNDANT AR
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94 CHAPTER 4. IMAGE FORMATION ASPEC
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96 CHAPTER 4. IMAGE FORMATION ASPEC
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Chapter 5 Adaptive Optics for Inter
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104 CHAPTER 5. ADAPTIVE OPTICS FOR
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106 CHAPTER 5. ADAPTIVE OPTICS FOR
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Chapter 6 VLT Site Considerations 6
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110 CHAPTER 6. VLT SITE CONSIDERATI
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6.3. DISCUSSION OF POSSIBLE CONFIG.
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7.1. SUMMARY OF BASIC REQUIREMENTS
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7.2. THE 8M TELESCOPES 121 Ml M 11
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Page 176 and 177: Bibliography [Afanas'jev et al1988]
Page 178 and 179: BIBLIOGRAPHY [Foy 1988] [Forbes 198
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