Structural Design of a Unique Passive Telescope

Most telescopes designed today assume that, as the telescope is pointed at different locations on the celestial sphere, there will be some interactive human or computationally-activated mechanical adjustment for defocus or misalignment during routine use. As part of a collaboration between the University of New Mexico and the University of Texas at Austin, design of the second iteration of the CCD/Transit Instrument (CTI-II) is underway. With a one degree square scientific focal plane mosaic of charged coupled devices (CCDs) operated in the time-delay and integrate readout mode, the stationary, 1.8-m telescope will accomplish a multi-bandpass photometric and astrometric imaging survey of more than 300 square degrees of the sky. The structural design goal for CTI-II is to make the telescope as passive as possible, so that little or no physical human or computationally-activated mechanical adjustment is required during routine use. Because the telescope will never be repositioned during normal operations, structural deformations due to the earth's gravity will not vary during operation. We are designing the telescope so that its optical behavior will be as nearly as possible thermally invariant, and we are also attempting to minimize vibrations due to wind and ambient seismicity. The structural design philosophy, methodology, and provisions for thermally and optically tuning the telescope (hopefully an infrequent operation) are discussed.