Karsten Seiferlin, Sumita Chakraborty, Kurt Gunderson, Josef Fischer, Benjamin Luthi, Daniele Piazza, Martin Rieder, Martin Sigrist, Nicolas Thomas, Thomas Weigel
The BepiColombo Laser Altimeter (BELA), part of the payload of the European Space Agency's BepiColombo mission, is designed to point a telescope with a 200-mm aperture toward a surface that can reach 700 K. Furthermore, direct sunlight can shine into the instrument at angles of 38 deg from the boresight. At Mercury, the solar flux can exceed 14 kW m−2. A baffle for such conditions must both reduce straylight to the best possible extent and minimize the heat load to the spacecraft, i.e., the sum of absorbed visible light and infrared flux. We describe the design and manufacture, including coating, of a reflective baffle. The baffle is made by diamond turning of aluminum and has a clear aperture of 200 mm, about 300-mm length, and a mass of 716 g.
The BepiColombo Laser Altimeter (BELA) is a proposed experiment for the BepiColombo mission to the planet Mercury. BELA is intended to provide payload-to-surface ranging data from a spacecraft in a polar Hermean orbit by measuring the time-of-flight of outgoing laser pulses and their echoes. As proposed, BELA also will provide small-scale surface variation and reflectivity data via characterization of return pulse forms. Primary instrument components include a low frequency pulsed Nd:YAG laser transmitter and a reflective receiver telescope feeding a silicon avalanche photodiode to capture pulse echoes with a direct detection approach. To assist with the evaluation of various design strategies, we have developed a numerical model of the instrument that returns a signal-to-noise-ratio figure of merit, as well as simulated return pulses, according to a diverse set of hardware specifications and viewing geometries as input parameters. An analysis of large sets of simulated pulses assists with the estimation of measurement accuracy. This model has been used to investigate the performance of a variety of instrument configurations, and some tradeoffs leading to the favored design will be described.
The High Energy Focusing Telescope (HEFT) is a balloon-borne, hard x-ray/gamma ray (20-70 keV) astronomical experiment. HEFT's 10 arcminute field of view and 1 arcminute angular resolution place challenging demands on its attitude control system (ACS). A microprocessor-based ACS has been developed to manage target acquisition and sidereal tracking. The ACS consists of a variety of sensors and actuators, with provisions for 2-way ground communication, all controlled by an on-board computer. Ground based pointing performance measurements indicate 1σ jitter of 7" and gyro drift rates of <1" s-1. Jitter is expected to worsen in the flight environment, but star tracker data are expected to reduce drift rates significantly, enabling a predicted 1σ absolute attitude determination of ≥4.7". HEFT is scheduled for flight in Spring 2004.
Complete hard X-ray optics modules are currently being produced for the High Energy Focusing Telescope (HEFT), a balloon born mission that will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented layers are made of thermally-formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. Our novel mounting technique involves constraining these mirror segments to successive layers of precisely machined graphite spacers. We report the production and calibration of the first HEFT optics module.
This paper outlines an in-depth study of the W/Si coated mirrors for the High Energy Focusing Telescope (HEFT). We present data taken at 8, 40 and 60 keV obtained at the Danish Space Research Institute and the European Synchrotron Radiation Facility in Grenoble. The set of samples were chosen to cover the parameter space of sample type, sample size and coating type. The investigation includes a study of the interfacial roughness across the sample surface, as substrates and later as coated, and an analysis of the roughness correlation in the W/Si coatings for N = 10 deposited bilayers. The powerlaw graded flight coating for the HEFT mirrors is studied for uniformity and scatter, as well as its performance at high energies.
The High Energy Focusing Telescope (HEFT) will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented mirrors that form these layers are made of thermally formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. The mirrors are assembled using an over-constraint method that forces the overall shape of the nominally cylindrical substrates to the appropriate conic form. We will present performance data on the HEFT optics and report the current status of the assembly production.
A sounding rocket observation of G191-B2B, a hot white dwarf star, was made on 27 September 1999 over a wavelength range of 220 - 340 angstroms with the Extreme ultraviolet Opacity Rocket (EOR), an EUV spectrograph. EOR acquired over 200 seconds of data above 200 km. Two broadband multilayer- coated diffraction gratings in Wadsworth mounts provide EOR with a peak effective area of 2.5 cm2 near 280 angstroms and spectral resolution of (lambda) /(Delta) (lambda) equals 2500 - 3000. Preliminary examination of the flight spectrum suggests the presence of absorption features which are not apparent in lower resolution spectra.
Sounding rocket observations of G191-B2B are planned for 1999 January over the bandpass 256-304 angstrom with a high resolution spectrograph. The optical system implements two diffraction gratings with broadband multilayer coatings in a Wadsworth mount. Since the spectral resolution of the experiment will be signal limited, preliminary reflectivity test on test gratings have been performed to determine how high the resolution might be.
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