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A new guide probe, using a Pyramid WaveFront Sensor (PWFS), has been built for the guider port of the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT). The PWFS splits the light from a guide star into a double image of the star at the guider image plane. These two images move with respect to each other with changes in focus. Analysis of the movements in such two-star images allows for simultaneous tracking and focus adjustments while guiding. This paper describes the image processing software developed for the new guidance system. The PWFS is a custom made prism with a 9 degree prism apex angle. Incident light is split as it passes through the prism resulting in two images identical in shape, size and orientation resolving at the guider image plane. A point source, like a star, thus appears as a double image of the star at the image plane. These two images are affected identically with seeing fluctuations allowing one or both images to be used to calculate guidance offsets independently of the focus measurement. The separation of the two images correlates to the focus divergence from the ideal which is used to correct the telescope focus. The prism can be moved off to one side, such that the incident light is not split. In this configuration the guide probe provides guidance offsets without the focus adjustments for maximum sensitivity with dim guide. The software was developed using existing guider images to ensure that the algorithm could handle the image artefacts that are commonly encountered. To reduce processing time and the effects of anomalies the surrounding image is masked out leaving only the star. A centroiding algorithm is then applied to the star to estimate its location in the image. For PWFS double-star images, Zemax was used to model the effect of shifting focus on the light passing through the prism. From this a simulated data set was produced and the software expanded to detect the two stars and measure the separation. The software is capable of measuring star translational movements and double star separation reliability and accurately.
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