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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Dissertation Heiko Groeneveld


Design, Simulation und Optimierung kodierter Aperturen

Summary.
This dissertation is divided into eight chapters. The first four chapters are of an introductory nature, giving reviews of different topics, whereas chapters five to eight concern my own research in the field of imaging based on the coded aperture or coded mask technique. Chapter 1 reviews the presently known alternatives to this technique and explains the basics of the "coded mask principle".
The mathematics of coded mask imaging, especially concerning aperture codes, are treated in Chapter 2. All codes discussed here in detail have been already used for the aperture design of at least one flown coded mask telescope.
Chapter 3 reviews all flown coded mask teleskopes in the 25 year period from October 1972 to September 1997, starting with the pioniering "Multiple Pinhole Camera" (MPC) and ending with the "Low Energy Gamma Ray Imager" (LEGRI). The focus of this review is set onto the aperture design.
The topic of Chapter 4 - the last introductory chapter - is an overview of methods for judging image quality, especially of images in digital representation as they are gained by the coded mask technique.
Chapter 5 gives a detailed presentation of the software package I developed for the design, simulation and optimization of coded apertures.
Part of this software package are implementations of several imaging algorithms. Since the coded mask technique is an indirect imaging technique, the imaging algorithm that yields the best results generally depends on the application. Chapter 6 introduces a new variant of the so called "IROS" algorithm (Iterative Removal of Sources) which is especially suited for the precise location of the position of point sources. For coded mask telescopes similar to IBIS - the "Imager on Board Integral Satellite" - I found that it is reasonable to assume a point source location accuracy (PSLA) of one tenth of the spatial resolution of the instrument as defined by the size of the aperture holes and the distance between aperture and detector.
Very special research towards the IBIS instrument is the subject of Chapter 7. Focus of the research has been the investigation of the effect of misidentified multiple events of one of the two detector systems of the instrument in regards of imaging. I found that taking double events into account indeed shows some effects (positive as well as negative) on imaging, whereas the influence of (the very few) triple events is negligible.
Finally I present in Chapter 8 an algorithm for the optimization of random aperture codes. One advantage of such codes is their adaptability to a given application, and optimized random arrays - ORAs, as I call them - have been already used a couple of times for the apertures of some flown instruments - as pointed out in chapter 3. The first results with ORAs generated by my algorithm are promising, but strictly speaking the presented algorithm is nothing but a first approach to the subject and further investigation should be performed.
 

Dissertation (18,57 Mb PDF file including figures)


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