Support and actuation of six secondaries for the 6.5 m MMT and 8.4 m LBT telescopes

The 6.5 m upgrade of the Multiple Mirror Telescope (MMT) will include a number of new secondary mirrors. For first light, there will be an f/9 Cassegrain secondary manufactured from a 1.0 m diameter Hextek borosilicate honeycomb meniscus blank. This f-ratio is designed to match that of the present MMT to allow the use of existing instrumentation for first light. This will be followed by the wide field f/5 secondary combined with a refractive corrector which includes an atmospheric dispersion corrector (ADC) to give a 1 degrees unvignetted Cassegrain held. The f/5 mirror is made from a 1.7 m diameter lightweighted machined Zerodur blank. Two f/15 0.64 m diameter secondaries are being designed. The first of these is an adaptive secondary consisting of a thin 2 mm thick shell faceplate with 300 voice coil actuators and associated capacitive displacement sensors. A chopping f/15 secondary is planned using a rigid lightweight blank such as silicon carbide. The 8.4 m Large Binocular Telescope (LET) will have two secondaries for each of the two primaries. For first light, 0.87 m diameter f/15 Gregorian adaptive secondaries are planned. These concave mirrors will use the same thin shell faceplate, voice coil actuator and capacitive sensor technology currently being developed for the MMT f/15 adaptive secondary. A pair of 1.25 m diameter f/4 Cassegrain secondaries will be built next. These will be used together with refractive corrector optics to give a 1 degrees held. These mirrors are being polished and tested at the Steward Observatory Mirror Laboratory (SOML) using the recently completed Secondary Fabrication and Test Facility. Stressed lap polishing is used to achieve the fast, highly aspheric surfaces and testing is done with the computer generated hologram (CGH) test plate technique. Each of these secondaries requires a support system, five axis actuation and thermal environmental control. The in-house development of this number of secondaries enables an integrated design approach. As much as possible of the development, design and hardware costs will be shared between secondaries. This paper describes the designs which are being developed for the support, actuation and thermal control for each of these secondaries.