A New Mirror Alignment System for the VERITAS Telescopes

Imaging atmospheric Cherenkov telescopes (IACTs) used for ground-based gamma-ray astronomy at TeV energies use reflectors with areas on the order of 100m$^2$ as their primary optic. These tessellated reflectors comprise hundreds of mirror facets mounted on a space frame to achieve this large area at a reasonable cost. To achieve a reflecting surface of sufficient quality one must precisely orient each facet using a procedure known as alignment. We describe here an alignment system which uses a digital (CCD) camera placed at the focus of the optical system, facing the reflector. The camera acquires a series of images of the reflector while the telescope scans a grid of points centred on the direction of a bright star. Correctly aligned facets are brightest when the telescope is pointed directly at the star, while mis-aligned facets are brightest when the angle between the star and the telescope pointing direction is twice the misalignment angle of the facet. Data from this scan can be used to calculate the adjustments required to align each facet. We have constructed such a system and have tested it on three of the VERITAS IACTs. Using this system the optical point spread functions of the telescopes have been narrowed by more than 30%. We present here a description of the system and results from initial use.