C G A F This dissertation is submitted for the degree of Doctor of Philosophy by S W B Institute of Astronomy & Magdalene College University of Cambridge December 3, 2010 S Characterisation and mitigation of radiation damage on the Gaia Astrometric Field Scott William Brown In November 2012, the European Space Agency (ESA) is planning to launch Gaia, a mission designed to measure with microarcsecond accuracy the astrometric properties of over a billion stars. Microarcsec- ond astrometry requires extremely accurate positional measurements of individual stellar transits on the focal plane, which can be disrupted by radiation-induced Charge Transfer Inefficiency (CTI). Gaia will suffer radiation damage, impacting on the science performance, which has led to a series of Radiation Campaigns (RCs) being carried out by industry to investigate these issues. The goal of this thesis is to rigorously assess these campaigns and facilitate how to deal with CTI in the data processing. We begin in Chapter 1 by giving an overview of astrometry and photometry, introducing the concept of stellar parallax, and establishing why observing from space is paramount for performing global, abso- lute astrometry. As demonstrated by Hipparcos, the concept is sound. After reviewing the Gaia payload and discussing how astrometric and photometric parameters are determined in practice, we introduce the issue of radiation-induced CTI and how it may be dealt with. The on board mitigating strategies are investigated in detail in Chapter 2. Here we analyse the effects of radiation damage as a function of magnitude with and without a diffuse optical background, charge injection and the use of gates, and also discover a number of calibration issues. Some of these issues are expected to be removed during flight testing, others will have to be dealt with as part of the data processing, e.g. CCD stitches and the charge injection tail. In Chapter 3 we turn to look at the physical properties of a Gaia CCD. Using data from RC2 we probe the density of traps (i.e. damaged sites) in each pixel and, for the first time, measure the Full Well Capacity of the Supplementary Buried Channel, a part of every Gaia pixel that constrains the passage of faint signals away from the bulk of traps throughout the rest of the pixel. The Data Processing and Analysis Consortium (DPAC) is currently adopting a ’forward modelling’ approach to calibrate radiation damage in the data processing. This incorporates a Charge Distortion Model (CDM), which is investigated in Chapter 4. We find that although the CDM performs well there are a number of degeneracies in the model parameters, which may be probed further by better experi- mental data and a more realistic model. Another way of assessing the performance of a CDM is explored in Chapter 5. Using a Monte Carlo approach we test how well the CDM can extract accurate image parameters. It is found that the CDM must be highly robust to achieve a moderate degree of accuracy and that the fitting is limited by assigning finite window sizes to the image shapes. Finally, in Chapter 6 we summarise our findings on the campaign analyses, the on-board mitigating strategies and on how well we are currently able to handle radiation damage in the data processing. Cambridge, December 3, 2010 iii D I hereby declare that my thesis entitled Characterisation and mitigation of radiation damage on the Gaia Astrometric Field is not substantially the same as any that I have submitted for a degree or diploma or other qualification at any other University. I further state that no part of my thesis has already been or is being concurrently submitted for any such degree, diploma or other qualification. This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration except where specifically indicated in the text. I note that the Gaia logo on the title page is credited to ESA. I note that Chapter 1 is intended as a review, and contains a number of figures and tables that have been extracted from or consist of other works, all of which are clearly cited in the appropriate caption. Those parts of this thesis which have been published within the Gaia project are as follows: • Much of the work contained in Chapter 2, section 2.2 formed the basis of the technical note pub- lished as Brown, S. and van Leeuwen, F. (2008) ‘Analysis of the CCN10 data set: CTI effects on a Gaia astrometric CCD.’, Gaia Livelink and Brown, S. and van Leeuwen, F. (2009) ‘Further analysis of Radiation Campaign 1: an extension to SWB-001.’, Gaia Livelink. • Much of the work contained in Chapter 2, section 2.3 formed the basis of the technical note Brown, S. and van Leeuwen, F. (2009) ‘An independent analysis of Astrium’s Radiation Campaign 2 astrometric tests.’, Gaia Livelink. • The work contained in Chapter 3 formed the basis of the technical note published as Prod’homme, T. and Brown, S. (2009) ‘Determination of Gaia Astrometric CCD Physical Parameters using Radiation Campaign 2 Charge Injection Profiles.’, Gaia Livelink, which was completed in col- laboration with T. Prod’homme. I provided the information for section 3.1. Section 3.2 (except part 3.2.2) was put together during a collaborative visit to Leiden Observatory. Afterwards, some of the interpretation and tables were extended by T. Prod’homme. In section 3.3, the plots were produced during the same visit. I performed the curve fittings and associated table of values. The interpretation of the results were done together. • The work contained in Chapter 4 formed the basis of the technical note published as Prod’homme, T., Weiler, M. and Brown, S. (2009) ‘CDM02 Validation: A comparison between CDM02 out- comes and RC2 data.’, Gaia Livelink, and was completed in collaboration with the named authors. My main contribution was in providing the reduced data as described in section 4.2 and helping to set up the simulation as in section 4.3. I include some of their results and interpretation, which I have added to, for completeness. I also add to their conclusions focusing on the quality of the test data and possible improvements that could be made. This thesis contains fewer than 60,000 words. Scott W. Brown Cambridge, December 3, 2010 v A Opportunities to work at the forefront of scientific research on a European space mission do not come along everyday. So for this I have many people to thank. Of course it all began when my supervisor Floor van Leeuwen took me on as a PhD Student and ensured that I became involved with the Gaia project as soon as possible. For this I am extremely grateful. The guidance you’ve given me and the enthusiasm you’ve shown in our work has been invaluable. Thank you Floor. Throughout the Gaia community there are many people with whom I’ve had the opportunity to work with directly, communicate with or present results to at various meetings. In particular, my thanks go to Thibaut Prod’homme for inviting me to work in Leiden for a short time as well as for collaborating on several key Gaia-related issues. This includes Michael Weiler, and the rest of the CTI dream team, with whom I also had a chance to work with. I am grateful to Alex Short, Lennart Lindegren, Anthony Brown, Patricio Ortiz, Nicholas Cross and George Seabroke for their questions and communications on the analysis of the radiation campaigns as well as all of the other members of the Radiation Task Force for their feedback. To those at ESA and at EADS Astrium, I would like to thank you all for your help; in particular, Jos de Bruijne, Laurent Georges and Jean-Francois Pasquier. At the Institute of Astronomy I am very grateful to Francesca de Angeli for helping me out with Java in the early days and Nic Walton for providing feedback on my progress throughout the PhD as well as Sian Owen and Joy McSharry for organising deadlines and training courses. The funding for this research was kindly provided by the UK Science and Technology Facilities Council (STFC), which has allowed me to participate in many Gaia-related schools, workshops and meetings. Closer to home, I cannot begin to thank my Mum and Dad enough for their love and support through- out my time not only as a student but also down the many different paths in life I have decided to venture. I’m sure Dominique would agree with me when I say how much you mean to us both and how you’ve enriched our lives in so many ways - alla famiglia! Speaking of sisters, I cannot help thinking that in some way mine may have lent a helping hand when I decided to ’reach for the stars’. Writing from halfway around the world you certainly planted a seed in my mind that helped lead me here. For this I cannot thank you enough. To my friends both in Cambridge and those scattered around the world it goes without saying how good it feels to share this with you all. Elena, you’ve helped me countless times throughout my PhD, so in a way this has also been a journey for you. Thank you so much for the laughs, the distractions, for your patience, warmth and love. Now, above all, I look forward to our adventure. Scott W. Brown Cambridge, December 3, 2010 vii For my family. C Summary . iii Declaration . v Acknowledgements . vii Contents xi List of Tables xv List of Figures xvi Acronyms xxiv 1 Introduction 1 1.1 Astrometry .