Development of a prototype active optics system for future space telescopes

It is envisaged that future large space telescopes will be lightweight and employ active optics to maintain optical quality throughout the mission lifetime. We have proposed a 4 m, two-mirror space telescope with an active optics system based on reimaging the telescope primary mirror onto a small active mirror (110 mm optical pupil). Using Zemax, we demonstrate the feasibility of using this mirror to correct low-order Zernike aberrations and show that the aberration is well corrected across the 2.5 arcmin field of the telescope, operating at 0.55 μm. We describe the modeling carried out to develop the active mirror design. Using end-to-end modeling, a 25-actuator mirror with polar actuator geometry, and a ratio of mechanical to optical pupil diameter of 2 has been chosen. A single-actuator prototype has been manufactured and used to test stroke, linearity, and hysteresis. Finally, we describe the design of a laboratory breadboard that will image phase screens onto an exact replica of the space active mirror and show the results of measuring the phase screen accuracy.