Performance of the multiple mirror telescope (mmt) vi. mmt telescope coalignment system

A crucial feature of large telescopes using multiple objectives is the ability to align the optics so that the different star images coincide to within a fraction of the image size. Initially this was intended to be done in the MMT with the "laser active optics" system in which the six telescopes were coaligned by means of artificial laser-star images generated by a seventh guide-alignment telescope. That system was found to be less than satisfactory mostly because of internal seeing and scattered light effects. Instead we have constructed the Telescope Coalignment System (or TCS) in which the telescopes are co-aligned on stellar images. The TCS is an autoguider which works simultaneously on six telescopes and which therefore automatically coaligns the telescopes and tracks them on a field star. When a field star is not present in the small 4 arc minute field of view of the MMT the TCS system coaligns the MMT on a nearby star and then tracks and maintains co-alignment of the six telescopes under computer control using separate flexure corrections for each one.