Next generation UVOIR space telescopes with primary mirrors larger than four meters are forecasted to answer important questions in astronomy. However, these instruments are exposed to harsh environmental conditions in launch and operation and their optical quality will consequently decrease. An active optical correction chain can measure and compensate for aberrations within the telescope using wave front sensing and deformable mirrors.
Within the project STOIC, Fraunhofer IOF and NUI Galway developed a concept for such a correction chain. The baseline scenario was the ‘HYPATIA Space Telescope’, a Ritchey-Chrétien reflector with a 4-meter-primary mirror. With the help of analytical and numerical simulations, we created an optomechanical design and manufactured a breadboard to demonstrate the functionality of the entire system – the wave front sensor, deformable mirror and control algorithm. Typical aberrations of the space telescope’s mirrors were replicated using a phase plate for correction using the active mirror. Fraunhofer IOF concentrated on the design concept and the subsequent mechanical design and manufacturing of a spherical shaped active metal mirror.
The theoretical analyses and the mechanical design lead to the fabrication of the active metal mirror in a sophisticated manufacturing process. Through ultraprecision diamond turning and magnetorheological finishing (MRF), we acheived a low surface error of less than 100 nm RMS within the optical pupil (100 mm diameter). Implementation of the active optics mechanisms through activation of the 25 'set-and-forget' actuator mechanics reduced this to a final surface error of 15 nm RMS. Integration of the mirror into a test breadboard from our partner NUIG enabled the correction of specified Zernike modes of anticipated aberrations. Most of these aberrations could be corrected to within a maximum residual deviation of 20 nm RMS.
The funding support of the European Space Agency ESA is gratefully acknowledged. This work is funded by ESA under contract number AO/1-7955/14/NL/KML.
Fraunhofer Institute for Applied Optics and Precision Engineering IOF