The goal of the High Energy Replicated Optics to Explore the Sun (HEROES) mission was to adapt an existing balloon payload, known as High Energy Replicated Optics (HERO), for solar observation. HERO used an on-axis star camera for fine aspect sensing, but this camera was too sensitive to be used when pointed near or toward the Sun. The pitch and yaw aspect system (PYAS) replaced the star camera during solar pointing. The PYAS used a computer vision algorithm to generate aspect solutions based on observations of a carefully constructed scene.

An algorithm was needed to generate pointing solutions in real time with sub-pixel accuracy and minimal computational cost. This algorithm is a machine vision algorithm for generating high-accuracy pitch and yaw pointing solutions relative to the Sun on a high-altitude balloon. It operates on images formed by focusing an image of the Sun onto a plate printed with a pattern of small, cross-shaped fiducial markers. Images are processed to determine relative position of the balloon payload to the Sun.

The algorithm is broken into four problems: circle detection, fiducial detection, fiducial identification, and image registration. Circle detection is handled by an average intersection method, and fiducial detection is handled by a matched filter approach and identification with an ad-hoc method based on the spacing between fiducials. Registration is simplified to a pair of linear fits on point correspondences between fiducial locations in camera and plate coordinate spaces.

This work was done by Alexander Cramer, Albert Shih, and Steven Christe of Goddard Space Flight Center. GSC-17168-1

Photonics Tech Briefs Magazine

This article first appeared in the November, 2015 issue of Photonics Tech Briefs Magazine.

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