Corrals, hubcaps, and crystal balls: some new designs for very-wide-angle visible-light heliospheric imagers

Details Corrals, hubcaps, and crystal balls: some new designs for very-wide-angle visible-light heliospheric imagers Corrals, hubcaps, and crystal balls: some new designs for very-wide-angle visible-light heliospheric imagers

Nov 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998spie.3442...77b&link_type=abstract

Proc. SPIE Vol. 3442, p. 77-86, Missions to the Sun II, Clarence M. Korendyke; Ed.

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Scientific paper

Emerging techniques allow instruments to view very large sky areas, a hemisphere or more, in visible light. In space, such wide-angle coverage enables observation of heliospheric features form close to the Sun to well beyond Earth. Observations from deep-space missions such as Solar Probe, Stereo, and Solar Polar Sail, coupled with observations near Earth, permit 3D reconstruction of solar mass ejections and co-rotating structures, discovery and study of new comets and asteroids, and detailed measurements of brightness variations in the zodiacal cloud. Typical heliospheric features have 1 percent or less of ambient brightness, so visible-light cameras must deliver < 0.1 percent photometry and be well protected from stray background light. When more than a hemisphere of viewing area is free of bright background-light sources, we have shown that corral-like structures with several vane-like walls reduces background light illuminating to wide-angle optical system by up to ten orders of magnitude. The optical system itself typically provides another five orders of surface-brightness reduction. With CCDs as the light-detection device, images of point-like sources must cover typically 100 pixels to average down sub-pixel response gradients and provide the above 0.1 percent photometry. With present-day CCDs this requires images of order 1 degree in angular size. Tolerating such large images in turn enables wide-angle sky coverage using simple reflecting and refracting optical systems such as convex spherical reflectors, toroids and thick lenses. We show that combining these with light- reducing corrals yields practical, light-weight instruments suitable for inclusion on deep-space probes.

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