Citation
Abstract
Optical reception antennas (telescopes) must be capable of receiving communications even when the deep-space laser source is located within a small angle of the Sun (small solar elongation). Direct sunlight must not be allowed to shine on the primary reflector of an optical reception antenna, because too much light would be scattered into the signal detectors. A conventional sunshade that does not obstruct the antenna aperture would have to be about five times longer than its diameter in order to receive optical communications at a solar elongation of 12 degrees without interference. Such a long sunshade could not be accommodated within the dome of any existing large-aperture astronomical facility, and providing a new dome large enough would be prohibitively expensive. It is also desirable to reduce the amount of energy a space-based large-aperture optical reception facility would expend orienting a structure with such a sizable moment of inertia. Since a large-aperture optical reception antenna will probably have a hexagonally segmented primary reflector, a sunshade consisting of hexagonal tubes can be mounted in alignment with the segmentation without producing any additional geometric obstruction. The tubes can be extended downward toward the primary reflector, until they reach the envelope of the focused beam to the secondary reflector. If the optical reception antenna is ground-based, the other ends of the tubes may be trimmed so that both the sunshade and the antenna will fit within a sphere whose diameter is only six-fifths the diameter of the primary reflector. If the segmentation involves four rings of hexagons with the central segment absent from the primary, then this sunshade is useful when solar elongations are as small as 12 degrees, Additional vanes can be inserted in the hexagonal tubes to permit operation at 6 or 3 degrees. The structure of the sunshade is very strong and can be used to support the secondary reflector instead of an independent support. An analysis of the duration and recurrence of solar-conjunction communications outages (caused when a deep-space probe near an outer planet appears to be closer to the Sun than a given minimum solar elongation), and the design equations for the integral sunshade are appended.
Details
- Volume
- 42-95
- Published
- November 15, 1988
- Pages
- 180–195
- File Size
- 757.4 KB