Preliminary mechanical design of an adaptive secondary unit for the MMT-Conversion telescope

The potential advantages of using the secondary mirror of a telescope to perform adaptive correction of the wave front distorted by propagation through the atmosphere are known for long time. Among them are the drastic reduction of the number of optical surfaces, that increases the transmission and reduces the emissivity, and the possibility of providing adaptive corrections to different focal stations. The reduced emissivity makes adaptive secondary mirrors particularly attractive for work at infrared wavelengths where thermal background is of concern, provided that the design of the unit doesn’t affect emissivity in other ways (e. g., with high emissivity surfaces surrounding the secondary). Another desirable feature of an adaptive secondary (AS) is the capability of providing, in addition to wave front correction with many degrees of freedom, also a large (> 0.1 mm) range of motion to perform “chopping”, that is useful for sky subtraction even with modern two dimensional IR detectors. A conceptual approach to the design of an AS with the above features (+) was followed by a variety of studies and experiments that are still in progress (Bruns et al., Biasi et al., Biliotti et al., in this Conference). This paper reports the results of a preliminary study of the various problems related to the general layout, to the support of the optics and to the thermal aspects, in the specific case of the secondary mirror unit for the F/15 Cassegrain focus of the MMT Conversion telescope.