An active segmented cophasing imaging testbed with a high accurate optical adjustment and control in nanoscale is set up. A two-dimensional dispersed fringe analysis method is used to calculate the piston error between the adjacent segments. Segments alignment of tip-tilt is fulfilled by overlapping diffraction pattern centroids of the individual segment on the focal plane. The cophasing errors are corrected by sensitive micro-displacement actuators under a computer closed-loop control. A FISBA interferometer is introduced to evaluate that a final total residual surface error of about 0.08λ RMS and the residual piston error of 0.026λ RMS are achieved (λ = 633nm).

Active cophasing and aligning testbed with segmented mirrors

Recently, China is planning to construct a new large optical-infrared telescope (LOT), in which the aperture of the primary mirror is as large as 12m. China's LOT is a general-purpose telescope, which is aimed to work with multiple scientific instruments such as spectrographs. Based on the requirements of LOT telescope, we have compared the performance of Ritchey-Chretien (RC) design and Aplanatic-Gregorian (AG) design from the perspective of scientific performance and construction cost. By taking the primary focal ratio, Nasmyth focal ratio, and telescope's site condition into consideration, we finally recommend a RC f/1.6 design configuration for LOT's Nasmyth telescope system. Unlike the general identical configuration, we choose a non-identical configuration for the telescope system which has a shorter Cassegrain focal ratio compared to the designed Nasmyth focal ratio. The non-identical design can allow for a shorter back focal distance and therefore a shorter telescope fork to guarantee the gravitational stability of the whole telescope structure, as well as relatively lower construction cost. Detailed analysis for the feasibility of our recommended design is provided in this paper.

Recommended conceptual optical system design for China's Large Optical-infrared Telescope (LOT).

One of the major issues in new Extremely Large Telescopes is the phasing of their primary segmented mirror. A cophasing
sensor is mandatory to achieve the ultimate resolution of the telescope. Phase diversity (PD) is a light-hardware cophasing
technique. In this paper, we show that this technique is suited to segmented pupil instruments, such as the E-ELT.

Phasing segmented telescopes with long-exposure phase diversity images

The next generation of optical telescopes will provide high-resolution imaging of celestial objects by using the aperture synthesis technique To preserve the quality of the image, fast corrections of the pistons among subapertures have to be applied, namely, the co-phasing of the array The image-based co-phasing method via an optimization procedure has been newly developed Despite simplicity and strong commonality, when dealing with large piston errors, this correction method is also faced with a problem in which the metric function easily falls into the local convergence, especially in the case of broadband imaging with many subapertures In this study, an improved stochastic parallel gradient descent (SPGD) algorithm based on heuristic search is proposed for co-phasing, termed the metaheuristic SPGD algorithm The heuristic research scheme assists the original SPGD algorithm in getting rid of local extrema By iterations of this algorithm, the synthetic system can be co-phased without any additional instruments and operations The effectiveness of the proposed algorithm is verified by means of simulation Given the efficiency and superiority, it is expected that the method proposed in this study may find wide applications in multi-aperture imaging

Piston error correction of sparse aperture systems using the metaheuristic stochastic parallel gradient descent algorithm

The New Adaptive Optics Module for Interferometry (NAOMI) will be developed for and installed at the 1.8-metre Auxiliary Telescopes (ATs) at ESO Paranal. The goal of the project is to equip all four ATs with a low-order Shack– Hartmann adaptive optics system operating in the visible. By improving the wavefront quality delivered by the ATs for guide stars brighter than R = 13 mag, NAOMI will make the existing interferometer performance less dependent on the seeing conditions. Fed with higher and more stable Strehl, the fringe tracker(s) will achieve the fringe stability necessary to reach the full performance of the second-generation instruments GRAVITY and MATISSE.

NAOMI: a low-order adaptive optics system for the VLT interferometer

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.

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

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