Mechanical design, calibration, and environmental protection of the REXIS DAM

Abstract

The REgolith X-ray Imaging Spectrometer (REXIS) is a joint effort by MIT and Harvard to build the student-collaboration experiment aboard OSIRIS-Rex, an asteroid sample return mission sponsored by the NASA New Frontiers program. OSIRISREx is scheduled to launch to near-Earth asteroid Bennu in 2016. REXIS is a coded-mask imaging X-ray spectrometer that supports the missions scientific goals by globally and spatially mapping the soft X-ray emission spectrum of Bennu. X-rays corresponding to unique elements are fluoresced from the asteroid by incident solar radiation and enter the instrument through a coded-aperture mask composed of a psuedorandom pinhole pattern. The X-rays that pass through the mask strike an array of four charge-coupled devices (CCDs) that detect the incident photon energy and location on the imaging array. A spatial map of selected elemental abundances on Bennu is constructed by cross-correlating the mask pattern with the collected data. The CCDs are integrated into a Detector Assembly Mount (DAM) that serves three critical functions: Mechanical alignment, calibration and protection of the CCDs. In this thesis we outline the overall design of the REXIS DAM with a focus on its three main functions. Chapter 1 provides background on the OSIRIS-REx mission and the REXIS instrument. Chapter 2 discusses the adaptation of the AXAF CCD Imaging Spectrometer (ACIS) focal plane mechanical design and X-ray Imaging Spectrometer (XIS) flexprint electrical design for the REXIS DAM. Chapter 3 outlines the heritage of the DAM internal calibration sources from XIS and the MIT MicroX project. Driving science and engineering considerations for a calibration scheme are described and then used to inform the mechanical design of a novel calibration set for the REXIS DAM. Chapter 4 illustrates the need for protection from the space environment and analyzes the specific risks to the detectors and DAM in space. Special coatings and a one-time deployable radiation shield are used to protect the assembly and ensure integrity of REXIS science data. Chapter 5 describes the test performed to validate the CCD alignment scheme and the dynamic model of the radiation cover. Chapter 6 summarizes the key results of the present work and outlines plans for future work on the DAM.