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18 July 2000 Calibrating the aspect systems of the high-energy solar spectroscopic imager (HESSI)
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HESSI will image Solar flares with spatial resolution ranging from 2 and 190 arcsec over the energy range from 3 keV to approximately equals 100 keV and as low as 35 arcsec for energies up to 20 MeV, respectively. The system is based on Fourier- transform imaging in connection with high-resolution Ge- detectors. In order to achieve arcsec-quality images with an instrument having only arcmin alignment requirements one needs in addition two precise aspect systems: (1) The Solar Aspect System (SAS) will provide Sun aspect data with high precision (< 0.2 arcsec relative and 1 arcsec absolute) and at high frequency (100 Hz). It consists of three identical lens/filter assemblies with focus Sun images on three 2048 X (13 micrometers )2 linear CCDS at 1.55 m focal distance. Simultaneous exposures of three chords of the focused solar images are made and the pixels spanning each solar limb are recorded. (2) The Roll Angle System (RAS) will provide precise (arcmin) information on the roll angle of the rotating spacecraft. The RAS is a star scanner which points out radially and observes stars at 75 degrees from the Sun direction using a commercial lens and a fast CCD. The passage of a star image over the CCD will induce a signal in one or several pixels and the timing of this signal defines the roll angle, once the star has been identified by comparing its pixel position and amplitude with a star map. With a limiting magnitude of mv equals 3 we expect to observe at least 1 star per revolution (during direct Sun view) over 1 year; on the average we will detect about 10 stars/revolution. We report on the design, construction and calibration measurements of the SAS and RAS flight-model instruments.
© (2000) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Martin Fivian, Jacek Bialkowski, W. Hajdas, Reinhold Henneck, A. Mchedlishvili, P. Ming, Knud Thomsen, Alex Zehnder, Gordon J. Hurford, David W. Curtis, David H. Pankow, and Brian R. Dennis "Calibrating the aspect systems of the high-energy solar spectroscopic imager (HESSI)", Proc. SPIE 4012, X-Ray Optics, Instruments, and Missions III, (18 July 2000);


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