Calibrating the aspect systems of the High Energy Solar Spectroscopic Imager HESSI

摘要

HESSI will image Solar flares with spatial resoltuion ranging from 2 and 190 arcsec over hte energy range from 3 keV to approx approx 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 acheive arcsec-quality images with an instrument having only arcmin alignment requirements one needs in additon 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 which focus Sun images on three 2048x(13 mu m)~2 linear CCDs at 1.55 m focal distance. Simultaneous exposures of three chords of the focused solar iamges are made and the pixels spannign 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 fro mthe Sun direction using a commercial lens and a fast CCD. The passage of a star image over hte 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 m_V=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 deisng, construction and calibraiton measurmeents of hte SAS and RAS flight-model instruments.