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.
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 www.techbriefs.com/tsp under the Electronics/Computers category. NPO-42552
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Photodetectors on Coronagraph Mask for Pointing Control
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Overview
The document titled "Photodetectors on Coronagraph Mask for Pointing Control" discusses advancements in the design and functionality of coronagraphs, which are essential for detecting exoplanets by suppressing starlight. The primary focus is on integrating a detector array directly onto the coronagraph mask to enhance pointing accuracy and control.
Coronagraphs work by blocking out the light from a star, allowing for the observation of planets orbiting around it. However, achieving precise alignment and centering of the coronagraph mask is critical, as even slight misalignments can hinder the detection of faint planetary signals. The document outlines a method to capture and process the small amount of starlight (approximately 1%) that is reflected by the mask, which is crucial for feedback in the pointing control mechanism.
The proposed design incorporates a linear array of photodetectors situated at the center of the mask. This configuration allows for effective utilization of the starlight available at the focus, facilitating direct control over the mask's position. The document details how the signals from the detector elements can be processed through simple arithmetic operations to provide feedback for adjusting the mask's alignment.
The detector array can consist of various configurations, ranging from 4 to 64 elements, depending on the extent of the occulting pattern and the functionalities required. The document suggests that a practical approach would be to use two sets of 4 or 8 detector elements arranged in a square configuration. This setup enables differential detection in both x and y dimensions, improving the system's ability to sense and correct for any displacements.
Additionally, the document references the use of specific mask designs, such as a linear 1-sinc² pattern, which is effective for occulting functions. The integration of these photodetectors not only enhances the coronagraph's performance but also simplifies the complexity of signal extraction, making it easier to implement in practical applications.
Overall, this document highlights a significant innovation in the field of astrophysics, aiming to improve the capabilities of telescopes in the search for exoplanets by ensuring precise control over the coronagraph mask's positioning.
