Roll Angle System (RAS) for the High-Energy Solar Spectroscopic Imager HESSI

Details Roll Angle System (RAS) for the High-Energy Solar Spectroscopic Imager HESSI Roll Angle System (RAS) for the High-Energy Solar Spectroscopic Imager HESSI

Oct 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999spie.3765..518h&link_type=abstract

Proc. SPIE Vol. 3765, p. 518-523, EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy X, Oswald H. Siegmund; Kathryn A. Flan

Astronomy and Astrophysics

Astronomy

1

Scientific paper

The purpose of the HESSI RAS is to provide information on the roll angle of the rotation spacecraft. Precise knowledge of the roll angle is a necessary ingredient for image reconstruction. The RAS is a continuously operating star scanner that 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 charges one or several pixels above threshold 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. Roll angles at intermediate times are inferred by assuming uniform rotation. With a limiting star magnitude of mv equals 3 we expect to observe at least 1 star per revolution over 1 year; on the average we will detect about 10 stars/revolution.

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