The CCD stellar sensor of the MASCO telescope pointing system Original Research Article

We describe the development of a CCD stellar sensor capable of determining the pointing coordinates with an error smaller than 0.5′. This instrument is part of the attitude control system of the balloon-borne γ-ray MASCO (acronym in Portuguese for coded mask — MÁScara COdificada) telescope. It acquires an image of the sky, recognizes the most brilliant stars in the image, creates with them a “constellation”, and searches for this pattern in an on board star catalogue of the observed region. The searching algorithm is based on the analysis of the geometrical configuration of the “constellation” and considers the distances and angles among the stars and their relative brightness. This instrument consists in an inexpensive CCD camera coupled to a telephoto lens and a frame grabber which digitizes the images at 512×400 pixels resolution in a field of view of about 6°×5°. The implementation of interpolation algorithms allowed an accuracy in the determination of the pointing coordinates of 110 of a pixel, i.e., a few arcseconds. The tests carried out with this sensor showed the following results: 100% of correct matching of the point patterns in more than 1500 sky images, error dispersion in the determination of the pointing coordinates smaller than 30″ and dispersion smaller than 4″ in the determination of the star imageʹs positions.