It has been proposed to install a symmetrical array of photodetectors about the center of the mask of a coronagraph of the type used to search for planets orbiting remote stars. The purpose of this installation is to utilize the light from a star under observation as a guide in pointing the telescope. Simple arithmetic processing of the outputs of the photodetectors would provide indications of the lateral position of the center of the mask relative to the center of the image of the star. These indications could serve as pointing-control feedback signals for adjusting the telescope aim to center the image of the star on the mask.

A Square or Rectangular Array of four photodetectors would provide indications of the x and y displacements of a star image from the origin, which would lie at the center of a coronagraph mask.

The widths of central mask areas available for placement of photodetectors differ among coronagraph designs, typically ranging upward from about 100 μm. Arrays of photodetectors can readily be placed within areas in this size range. The number of detectors in an array could be as small as 4 or as large as 64. The upper limit on the number of detectors would be determined according to the extent of the occulting pattern and the number of functionalities, in addition to pointing control, to be served by the array.

In the simplest case, differential position measurements along two orthogonal axes (x and y) could be effected by use of four photodetectors in a square or rectangular array similar to familiar quadrant detectors. Denoting the reading from each photodetector by the letter designation of the photodetector as shown in the figure, the x displacement between the star image and the center of the mask would be proportional to

[(A + D) – (B + C)]/(A + B + C + D),

while the y displacement would be proportional to

[(A + B) – (C + D)]/(A + B + C + D).

This work was done by Kunjithapatham Balasubramanian of Caltech for NASA's Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at under the Electronics/Computers category. NPO-42552

This Brief includes a Technical Support Package (TSP).
Photodetectors on Coronagraph Mask for Pointing Control

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This article first appeared in the July, 2007 issue of NASA Tech Briefs Magazine.

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