SPECKLE IMAGING WITH THE MULTI-ANODE MICROCHANNEL ARRAY DETECTOR

Abstract

ABSTRACT This dissertation combines and instrument and a technique that have been studied separately for several years. The instrument is the multi-anode microchannel array (MAMA) detector. This device is a photon-counting imaging system that has been developed primarily for use in ultraviolet astrophysics. Current generation MAMA detectors have the ability to record the arrival time of every detected photon and can distinguish events which arrive only 140 nanoseconds apart in time. The technique is speckle imaging. This is an interferometric method of recovering high-resolution information about astronomical objects from ground-based telescopes, usually carried out in visible or infrared wavelengths. In conventional astronomy, fluctuations in the atmosphere above the telescope blur images so that the resolution is typically 1 to 2 arcseconds, much worse than the theoretical diffraction-limited resolution of large telescopes. Speckle imaging allows for the reconstruction of image features on a much smaller scale by taking many short (typically 0.01 second) exposure images of an object, where over the exposure time the atmospheric fluctuations are effectively frozen. Speckle imaging devices must therefore have the ability to read out images much faster than most other astronomical imaging devices. Since the MAMA detector offers outstanding timing abilities as well as good spatial linearity and is available in a visible light version, it is an excellent choice as the imaging device in a speckle imaging system. A new speckle system based on the MAMA detector has been built at Stanford University and a detailed study of the performance of the system forms a principle part of this work. Under certain observational conditions, the phenonomenon of channel saturation of the MAMA detector can significantly affect the signal-to-noise ratio of the speckle imaging results. In addition, channel saturation leads to a systematic error in the determination of the irradiance ratios (or equivalently, the magnitude differences) of interferometric binary stars. A correction algorithm is described which leads to better estimates of these ratios in simulated data. Four methods of computing the correlation functions necessary for producing high-resolution reconstructed images are compared using MAMA data. Two of these methods explicitly make use of the time-tagging ability of the MAMA detector and have not been previously studied in detail. The highest signal-to-noise ratio in the power spectrum and bispectrum is obtained with a weighted running-window method. Reconstructed images of several objects are presented. The MAMA-based speckle imaging system is shown to determine accurate position angles and separations of sub-arcsecond separation binary stars.