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Chopping Secondary Mirrors For Infrared Astronomy With The Multiple Mirror Telescope

The technique of rapidly wobbling the secondary mirror of a reflecting telescope to modulate the energy from an astronomical source by moving its image on and off of an aperture in the telescope's focal plane has been used with great success for a number of years on many ground based and airborne infrared telescopes. Such a secondary modulating system involving six synchronized secondaries of 23.5 cm diameter has been designed for use on the Multiple Mirror Telescope. One prototype mechanism using a linear servo drive system has been constructed and tested both in the laboratory and on the MMT itself. For image displacements up to about 20 arc seconds on the sky, the prototype is capable of a rise time of 2 ms and of operation at frequencies up to 80 Hz with a square wave driving signal. Larger displacements up to more than 120 arc seconds are available, but with a rise time of 4 ms. Adjustable hard stops and electronics to drive the secondary mechanism against these stops have been incorporated to provide a back up capability to the linear servo. Seven copies of the prototype are now in production and are expected to be in operation on the MMT in the spring of 1979.