The Production of Metal Mirrors for use in Astronomy A Thesis Submitted to the University of London for the Degree of Doctor of Philosophy by David Brooks UCL Optical Science Laboratory Department of Physics and Astronomy University College London 2001 ProQuest Number: U643140 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest U643140 Published by ProQuest LLC(2015). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 The Production of Metal Mirrors for use in Astronomy Abstract This thesis demonstrates the possibility of manufacturing larger mirrors from nickel coated aluminium with a considerable cost and risk benefits compared to zero expansion glass ceramic or borosilicate. Constructing large mirrors from aluminium could cut the cost of production by one third. A new generation of very large telescopes is being designed, on the order of 100 meters diameter. The proposed designs are of mosaic type mirrors similar to the Keck Telescope primary. The enormous mass of glass required inhibits the construction, simply by its cost and production time. Very little research has been done on the processes involved in the production of large metal mirrors. However the thermal efficiency and potential improved mirror seeing benefits are documented. Space telescopes and optical telecommunications could also benefit with the application of metal mirrors. Presented here are the processes and results that culminated in the rebirth of the Birr Telescope. The main section concerns the material selection and processes in the construction of a 1.83 meter diameter 1.4 tonne aluminium primary mirror. The aluminium mirror technology developed was also applied to the construction of an aspheric thin meniscus deformable mirror. Methods employed in its production are described. Documented are the advanced computer controlled polishing methods employed in producing a one third scale model of the hyperbolic secondary mirror for the Gemini Telescopes. These were developed using an active polishing lap. List of Tables 3.1 Physical properties of potential metal mirror blank materials ......... 55 4.1 Grades of silicon carbide .................................................................. 72 4.2 Composition of silicon carbide ........................................................ 73 4.3 Moh hardness comparison ................................................................ 74 4.4 Polishing compounds for the production of an optical surface on metal . 74 4.5 Aluminium oxide ................................................................................. 75 4.6 Commercially available polishing cloths suitable for polishing metal... 78 5.1 Types of temper as defined by British and European Standard BS EN 515. 97 5.2 Types of aluminium alloy .................................................................... 98 5.3 Mechanical and physical properties of selected alloys ....................... 100 5 .4 Chemical analysis of the composition of the 5083 in “O” condition .... 102 5.5 Grades and quantity of SiC used during grinding ............................... 112 5.6 Typical set of spherometer readings ................................................... 113 5 .7 Hardness of grinding lap materials tested ........................................... 121 5.8 Lapping materials ............................................................................... 123 5.9 Mask dimensions ............................................................................. 135 5.10 Distances are away from the mirror ................................................... 135 5.11 Typical data set measured at focus ..................................................... 138 9.1 Slip gauge settings for the ten-probe profilometer ............................. 207 List of Figures 2.1 German type polishing machine at OSL .............................................. 35 2.2 General layout of a German type polishing machine ............................ 36 2.3 The general layout of a Draper type polishing machine ......................... 37 2.4 Mismatch of form when de-centring the lap .......................................... 39 2.5 The ERP-600 aspheric polishing machine ............................................... 48 3.1 Final interferogram of the test mirror .................................................... 66 3.2 Test samples ........................................................................................... 68 4.1a Polishing cloth and compound, surface finish comparison .................... 81 4. lb Detailed view of textures achieved .......................................................... 82 4.2 Surface texture produced by 1 micron AI 3 O2 and MultiTex ................... 83 4.3 Surface texture produced on aluminium ................................................. 83 4.4 Surface polished with 0.3 micron alumina and MultiTex ....................... 84 4.5 Micro scratches ....................................................................................... 85 4.6 Micrographs of alumina powder (300X) and particle size distribution . 85 4.7 MultiTex polishing cloth ........................................................................... 86 5.1 Side view of the telescope ....................................................................... 88 5.2 Diagram of proposed warping harness ..................................................... 96 5.3 Turning the 1.4 m dia lap ......................................................................... 108 5.4 Diagram of the initial contact area of the lap ........................................... 112 5.5 Example centre to edge mirror profile ..................................................... 114 5.6 1.4 m lap with cloth facets ....................................................................... 116 5.7 Damage polishing cloth ........................................................................... 117 5.8 Test path dimensions ............................................................................... 118 5.9 Lead lap facet with inclusion ................................................................... 121 5.10 Perspex faced lap ..................................................................................... 124 5.11 Optical test path rooms ............................................................................. 125 5.12 Enhanced video image of 10.6 micron fringes on polished aluminium . 126 5.13 Histogram of nickel c o a t ......................................................................... 130 5.14 Nickel plated witness piece, bend test ..................................................... 130 5.15 Surface pitting ......................................................................................... 131 5.16 Magnified image of pits ......................................................................... 131 5.17 Null test arrangement ............................................................................. 136 5.18 Aspheric departure ................................................................................. 137 5.19 Typical graph of the slope differences (centre to edge) ........................... 138 5.20 The final zonal measurement (centre to edge) ......................................... 140 5 .21 Image spread at focal plane ..................................................................... 140 5.22 Testing the mirror on the support ............................................................. 142 5.23 Disk of least confusion at the focal plane ................................................. 142 5.24 F.E.A. of mirror segment ......................................................................... 145 5.25 The primary mirror after turning ............................................................. 148 5.26 The primary being nickel coated at Nitec ............................................... 148 5.27 The primary mounted on the polishing machine .................................... 149 5.28 The primary mounted in the test support frame....................................... 149 6.1 The Birr Telescope ................................................................................... 150 6.2 Installing the primary mirror ................................................................... 152 6.3 Diagram of the mirror box ....................................................................... 153 6.4 Mirror box at the base of the telescope ................................................... 154 6.5 The telescope tube .................................................................................... 155 6.6 The universal joint and jacking screws ................................................... 156 6.7 The Newtonian mirror and support ......................................................... 157 6.8 The eyepiece interchange .......................................................................... 159 6.9 The Moon taken through the Birr Telescope .............................................. 161 6.10 Staining of the mirror surface .................................................................