Computerised Electro-Mechanical Control of the Uws Astronomical Telescope and the Integration of a Multi-Tasking Television System
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* COMPUTERISED ELECTRO-MECHANICAL CONTROL OF THE UWS ASTRONOMICAL TELESCOPE AND THE INTEGRATION OF A MULTI-TASKING TELEVISION SYSTEM by FRANK WILLIAM BIRD BSc BTeach A Thesis submitted for the Degree of Master of Science (Hons) at the University of Western Sydney, May 2005. DEDICATION To my lovely wife Treve who patiently entered every (well almost) keystroke in this Thesis as I clumsily dictated the content. Her willingness to help, and the many late nights at the keyboard are very much appreciated. i ACKNOWLEDGEMENTS I would like to thank all those concerned in making this Thesis possible. In particular I would like to thank Dr. Graeme White who invited me to be involved in the UWS Telescope project and gave me great support and assistance during the project. Thanks also go to my co supervisor Dr Paul Jones and to my supervisor in the latter stages of the project Dr Ragbir Bhathal. I thank UWS Nepean for providing a HECS exemption scholarship and other resources to enable me to complete this Thesis. Finally, I thank my wife for enduring the upheavals and changes household routine which an undertaking such as this inevitably inflicts. ii STATEMENT OF AUTHENTICATION I declare that to the best of my knowledge, the work described in this Thesis is original, except as acknowledged in the text, and that the material has not been submitted, either in whole or in part, for a Degree at UWS or any other University. FRANK W BIRD April 2005 iii TABLE OF CONTENTS Page CHAPTER 1. - Introduction 1.1 Purpose of this Thesis 1 1.2 A brief history of Astronomical Telescopes 1.2.1 Early instrumentation 2 1.2.2 Scientific use of Telescopes and the effects of aberrations 4 1.2.3 Use of mirrors 6 1.2.4 Limitations and developments 8 1.2.5 The UWS Telescope 9 1.2.6 Telescope mountings 10 1.2.7 Equatorial mounts 13 1.2.8 Alt-Azimuth mount 14 1.2.9 Adding a layer of complication to a simple problem of telescope mounting 15 1.2.10 Computer control 18 CHAPTER 2. - Mechanical Resonant Frequencies 2.1 Resonant frequencies 20 2.2 Method used 20 CHAPTER 3. - Aligning the Equatorial Head 3.1 Criterion for long exposure 22 3.2 Alignment method 22 3.3 Altitude adjustment 24 3.4 Azimuth adjustment 26 iv CHAPTER 4. - Initial Tracking and Slewing Control System 4.1 Tracking and slewing system 28 CHAPTER 5. - The Video System at the UWS Observatory 5.1 Tasks 31 5.2 Onsite education 31 5.3 Remote viewing 37 5.4 Remote science 38 5.5 Research 39 5.6 Public viewing 40 5.7 A note on Gamma correction 44 5.8 Star-trak 47 5.9 Precautionary note 48 CHAPTER 6. - Engineering Approaches to Telescope Automation 6.1 Go-to 50 6.2 Stepper motor benefits 52 6.3 Stepper motor drawbacks 53 6.4 Brush servo benefits 53 6.5 Brush servo drawbacks 54 6.6 Brushless servo benefits 54 6.7 Brushless servo drawbacks 55 CHAPTER 7. - The Control System at UWS 7.1 Chosen system 56 7.2 Making things easy 58 7.3 Safety 59 v CHAPTER 8. - A Pointing Model 8.1 Options 62 8.2 Observation data 63 8.3 Assessment of pointing model scatter plots 95 CHAPTER 9. – Conclusions and future work 9.1 Current performance 96 9.2 Future work 96 9.2.1 Video System 96 9.2.2 Robotics 97 REFERENCES 98 APPENDIX A - EXTRACTS FROM ACE MANUAL 99 vi LIST OF TABLES page Table 1. Spread sheet layout of HA prime unit and correction 87 Table 2. Spread sheet layout of DEC prime unit and correction 94 vii LIST OF FIGURES AND ILLUSTRATIONS page 1. The first known use of instruments in astronomy 2 2. A wood cut of Hans Lippershey taken from Pierre Borel’s 3 De Vero Telescope Inventore, 1655. 3. Galilao demonstrating his first telescope 4 4. Spherical aberrations 5 5. Longitudinal chromatic aberration 5 6. Lateral chromatic aberration 6 7. George Dollond (1774 – 1852) 7 8. The main components of the Great Paris Refractor 8 9. The f10 UWS observatory 9 10. Altitude azimuth and equatorial mounts 11 11. The Great Dorpat Refractor 12 12. Schneider’s Helioscope 13 13. Relative amplitudes of the telescopes resonant frequencies 21 14. Eliminating non-linear scan error using an electronic pattern generator 23 15. Elevation adjustments and resulting error 25 15.1 Increments in Azimuth and the resulting error 26 16. VFO Circuit diagram 29 16.1 Original system hand paddle 30 17. Video patch panel 32 18. Moon Cam mounted on the side of the main frame 35 19. Rear view of the UHF modulators 36 20. YH438C Q video switcher (ACE smart dome control unit below) 39 21. Video system schematic 43 22. Gamma correction curves 45 22.1 1/3 inch CCD camera with adjustable gamma 46 viii 23. Outer-rotor construction of the Dinaserve motors 56 24. Output torque versus speed of the DM/DR series motors 57 25. U-matic idler wheel coupled to encoder below 59 26. Control desk with CCTV monitor 61 27. Scatter plot of HA prime unit versus correction 88 28. Scatter plot of DEC prime unit versus correction 94 ix ABSTRACT Obtaining a very high level of precision and sophistication in automated computer control is now available inexpensively from a variety of hardware and software sources. Applying this automated technology to an astronomical telescope broadens the scope of applications of the instrument, particularly in areas such as photo electrics, CCD imaging and remote control. The ultimate design goal of the UWS telescope was that of full roboticism, giving access of the facility to off campus clients both in Australia and overseas. The first phase toward full robotic control is automation of the required optical and mechanical parameters, providing precision targeting and object tracking. This Thesis describes the mechanical aspects of the UWS Telescope and the procedures and equipment involved in its automation, including the drive system, electro mechanical design and associated computer hardware and software. Sample performance test data shows that using a high percentage of inexpensive proprietary robotics components, a very sophisticated and accurate measuring device can be produced. x CHAPTER 1 - INTRODUCTION 1.1 PURPOSE OF THESIS This thesis describes and discusses the installation, testing and commissioning of the control system and TV camera systems of the 24 inch, f/10 Ritchy-Chretien telescope of the UWS Observatory at the University of Western Sydney. The intention at the University of Western Sydney (UWS) was to firstly put the telescope under computer control, secondly to have the telescope fitted with various cameras which could be used for teaching and remote viewing (see video system chapter 6) and thirdly to have the telescope controlled robotically over the internet by a distant observer. The telescope at UWS was designed to carry out two principal roles for the centre of astronomy. The first one was to provide a research tool for its undergraduate and early level post graduate students. The second role was to act as a teaching observatory for the University, and to allow the university to interact with the public and with the school sector of Western Sydney. A principal design parameter of the facility was to put TV images into various places around the building and hopefully eventually over the internet. As such, it is appropriate to start this thesis with a brief historical review of the instruments used in astronomy, their mounting systems, their control systems, and how this knowledge was applied to the instrumentation systems at the UWS Observatory. 1 1.2 A BRIEF HISTORY OF ASTRONOMICAL TELESCOPES. 1.2.1 Early instrumentation. Historically, astronomy became a science possibly as early as 3000 years BC. The earliest scientific instruments that are recorded, and that are in collections, consist of plumb bob lines which were used for sighting from one observer to the other. These were used for the practical application of astronomy, namely for navigation, and for surveying for the determination of land etc. after flood. The earliest of these dates back to Egyptian times . fig 1. The first known use of instruments in astronomy. (Reproduced from.King 1955). The first of the instruments that attempted to measure the elevation of an object in the sky came about in the Greek times and one particular instrument called Ptolemy’s Rule, which used the combination of two axes of rotation, the vertical axis and the horizontal axis. This instrument allowed the measurement of both the azimuth and the elevation of the object . The telescope itself as we know it was invented in 1608 by a Dutch optician named Hans Lippershey. 2 . fig 2. A woodcut of Hans Lippershey. ( Pierre Borel’s De Vero Telescopii Inventore,1655 .Reproduced from King 1955) Lippershey patented the concept, although it is reputed that it was actually invented sometime before that, in Holland, as being a mechanism for measuring the strength of an army. In other words it was patented as a military device. 3 fig 3. Galileo demonstrating his first telescope (American Museum of Natural History. Reproduced from King 1955) 1.2.2 Scientific use of telescopes and the effects of aberrations The first scientific use of the telescope was by Galileo in 1610 and his observations are well recorded as they are the basis of the dispute between the Catholic Church, the Pope and Galileo which resulted in Galileo being immortalised. The telescope used by Galileo was a very simple device consisting of a tube with 2 lenses attached, one being a convex lens which converged the light from a distant object to an image and the second, being a negative lens which then translated that image back to the retina of the eye.