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 100m2 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 mis-alignment 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.