A Deployable Telescope Secondary Mirror Positioning System

The PH-1 system was developed as a remotely controllable actuator mechanism for accurately positioning secondary mirrors within a deployable cassegrain optical telescope assembly. Such a means of adjustment was needed because the accuracy to which the secondary mirror support structure could be deployed exceeds the allowable optical error budget. In the particular configuration of interest, the primary was composed of three discrete deployable petals in a sparse configuration. Performance and cost had to be balanced to achieve the most effective solution for demonstrating the fundamental technologies embodied in the actuator mechanism.

An unusual “actuator below the secondary mirror” configuration was mandated by the desire to lower the effective c.g. of the positioning system within the deployable secondary mirror support tube in order to lower the expected resonant frequency response of the entire telescope tube. This configuration however constrains the allowable hexapod geometry due to the need to avoid the light of the three primaries being focused on the secondary as they pass up through cutouts in the deployed secondary mirror support tube onto the secondary mirror itself. The realized hexapod configuration and a representative light path is shown in here.

The PH-1 system was chosen for incorporation into an optical telescope brassboard test article for demonstration of advanced technologies at the Air Force Research Laboratory, Kirtland Air Force Base, NM. The ultimate objective is to demonstrate technologies to enable future designs of Deployable Optical Telescope (DOT) systems for space based applications.

Summary

In more generic terms, the purpose of the PH-1 is to enable accurate positioning of its attached payload in three dimensional space over a relatively large range of motion. A key advantage of this system, over other potential approaches, is that once the desired position has been attained, all power to the actuators and the power amplifier itself can be removed resulting in a true zero power hold condition. Other advantages include its small/compact size, low mass, volume, cost, and no need for launch locks to withstand launch loads.