APE: the Active Phasing Experiment to test new control system and phasing technology for a European Extremely Large Optical Telescope

Abstract

The future European Extremely Large Telescope will be composed of one or two giant segmented mirrors (up to 100 m of diameter) and of several large monolithic mirrors (up to 8 m in diameter). To limit the aberrations due to misalignments and defective surface quality it is necessary to have a proper active optics system. This active optics system must include a phasing system to limit the degradation of the PSF due to misphasing of the segmented mirrors. We will present the lastest design and development of the Active Phasing Experiment that will be tested in laboratory and on-sky connected to a VLT at Paranal in Chile. It includes an active segmented mirror, a static piston plate to simulate a secondary segmented mirror and of four phasing wavefront sensors to measure the piston, tip and tilt of the segments and the aberrations of the VLT. The four phasing sensors are the Diffraction Image Phase Sensing Instrument developed by Instituto de Astrofisica de Canarias, the Pyramid Phasing Sensor developed by Arcetri Astrophysical Observatory, the Shack-Hartmann Phasing Sensor developed by the European Southern Observatory and the Zernike Unit for Segment phasing developed by Laboratoire d'Astrophysique de Marseille. A reference measurement of the segmented mirror is made by an internal metrology developed by Fogale Nanotech. The control system of Active Phasing Experiment will perform the phasing of the segments, the guiding of the VLT and the active optics of the VLT. These activities are included in the Framework Programme 6 of the European Union.

色散瑞利干涉仪在“共相位检测与控制平台”上的实验结果

“…As the required sensing and real-time control accuracy of cophasing and alignment of segmented mirrors is typically a small fraction of a wavelength [3] , which equals several tens of nanometers in optical spectrum range, it is one of the most difficult and also key technologies in the new generation telescopes [4] . Therefore, segmentedmirror testbeds are established to validate wavefront sensing (including piston, tip-tilt, and high order abberations) and control algorithms, for example, the famous wavefront sensing and control testbed (WCT) [5] and active phasing experiment (APE) [6] . There are also some segmented mirror testbeds in China, such as the "experiment system of the segmented-mirror active optics" in Nanjing Astronomical Instruments Research Center [7] which is used to validate real-time control algorithms, the "segmented mirror system with three sub-mirrors primary" in Harbin Institute of Technology [8,9] which helps the study of synthetic aperture optics theory and image quality evaluation, and the "active cophasing and alignment testbed (ACAT)" in Beijing Institute of Technology established in 2008 aiming to validate piston, tip-tilt sensing, and real-time control algorithms.…”