The primary mirror of the Apache Point 3.5 meter telescope rests on an array of small pneumatic actuators distributed over the back plate of the mirror and within its honey comb cells. A control system constantly adjusts the pressure in the actuators so as to synthesize a perfectly rigid mounting, thus maintaining both the figure of the mirror and its orientation with respect to the mirror cell. In its original configuration, the APO 3.5 meter system exhibited a number of anomalous behaviors that detracted from the optical performance of the telescope. Analysis of the design indicated three basic problems: first that the pneumatic servovalves suffered from mechanical nonlinearities and an adverse dependence of their transfer function on operating pressure; second, that the air supply system could not exhaust air from the actuators rapidly enough near the horizon and zenith; and third, that the control system did not adequately account for the inertia of the mirror under dynamic conditions. Consequently, we have designed and installed a new control system which addresses these deficiencies. The new system employs high-bandwidth, flapper-type proportional valves that eliminate the mechanical problems and permit implementation of a high- performance controller. The new controller utilizes the existing hard points for mirror position sensing, but implements a pressure-feedback inner loop and a more sophisticated dynamic model. The air supply system now incorporates a sub-atmospheric return and operates closed- loop to minimize contamination problems. The new system exhibits substantial improvements over its previous performance, with positioning errors in the sub-micron range. The design has been selected for use on the Sloan Digital Sky Survey telescope.
Yorke J. Brown,
Jon Eric Davis,
"Improvements to the Apache Point 3.5-m primary-mirror support system", Proc. SPIE 3351, Telescope Control Systems III, (26 May 1998); doi: 10.1117/12.308819; https://doi.org/10.1117/12.308819