To achieve high-resolution image using optical synthesis aperture telescope, it’s necessary to co-phase accurately of all the telescopes so as to reduce the effect of co-phase errors including piston error, tip/tilt error, and mapping error, etc. Though simulation analysis of the optical system, error sources can be identified and thus save time of alignment. This paper introduces the Fizeau-type Y-4 prototype under development, including the layout of the Y-4 prototype, the layout of the reflective mirrors in the delayed light paths and the beam combiner. With the optical transfer function as the evaluation index, the actual equivalent diameter of Y-4 prototype is calculated. Furthermore, the effect of polarization introduced by coating and polarization differences on the contrast of interference fringe is analyzed. At present, the installation and alignment of the prototype in laboratory have been completed, and the interference synthesis of 4 light paths has been realized. One aim of this paper is to share some experiences in optical design and detection for the development of optical synthetic aperture telescopes. Another aim is to expand these new techniques to the larger optical synthesis aperture telescope project in the future.
Deformable mirror (DM) is the most main wavefront corrector in adaptive optics, which can be used to compensate optical aberrations through changing the reflective mirror’s surface frequently. However, a commercial piezoelectric DM can’t have an ideal flat initial surface under zero-voltage condition due to limitation of thin mirror fabrication and support structure of actuators behind of mirror. Optical aberrations generated by this initial distortion will seriously attenuate the performance of DM’s close-loop control, so a flat-surface calibration of mirror needs to be carried out before DM properly correct optical aberrations. In order to properly control the optical figure of the DM we have to obtain an interactive matrix which is the response of optical surface to the DM actuator’s stroke. We measured a serious of surface phase data of OKO 109-channel DM through self-collimation using a ZYGO-GPI interferometer directly, then construct the interactive matrix by zonal and modal methods. After several close-loop iterations, the initial RMS surface error of OKO 109-channel deformable mirror, 1.506λ has been remarkably reduced to 0.145λ.
Liquid rotating about a vertical axis takes a parabolic form, and can be used as the primary mirror in a reflecting telescope. It is a convenient way to get big primary mirror for astronomical telescope. But this kind of Newton type telescope with a parabolic surface has limited field of view, which is not capable for the detection of astronomical and orbital objects. It is necessary to design corrector lens with large field of view to improve the detection efficiency. In this paper, a prime focus corrector with a field of view of 2°, a focal length of 1.5 m, and wavelength coverage of 0.4-0.8 μm is designed for a liquid telescope with a clear aperture of 1 meter. The corrector is optimized by choosing different types of glass for achromatic design. The final corrector consists of 5 lenses and one filter, in which the largest one has a clear aperture of 280 mm and there is only one conic surface is used on a concave lens. The design result shows that 80% encircled energy is limited in one pixel, that is 12 μm for our F9000 3K×3K CCD camera, corresponding to 1.67″ resolution. The distortion is optimized to 0.01% which is very suitable for a Time Delay Integration (TDI) observation. The tolerance analysis for fabrication and assembly is also presented. Space debris and astronomical objects will be observed when the telescope is constructed.
In this paper we report on the laboratory experiment we settled in the Shanghai Astronomical Observatory (SHAO) to investigate the pyramid wave-front sensor (WFS) ability to measure the differential piston on a sparse aperture. The ultimate goal is to verify the ability of the pyramid WFS work in close loop to perform the phasing of the primary mirrors of a sparse Fizeau imaging telescope. In the experiment we installed on the optical bench we performed various test checking the ability to flat the wave-front using a deformable mirror and to measure the signal of the differential piston on a two pupils setup. These steps represent the background from which we start to perform full close loop operation on multiple apertures. These steps were also useful to characterize the achromatic double pyramids (double prisms) manufactured in the SHAO optical workshop.
Fringe test is the method which can detect the relative optical path difference in optical synthetic aperture telescope array.
To get to the interference fringes, the two beams of light in the meeting point must be within the coherence length. Step
scanning method is within its coherence length, selecting a specific step, changing one-way’s optical path of both by
changing position of micro displacement actuator. At the same time, every fringe pattern can be recorded. The process of
fringe patterns is from appearing to clear to disappearing. Firstly, a particular pixel is selected. Then, we keep tract of the
intensity of every picture in the same position. From the intensity change, the best position of relative optical path
difference can be made sure. The best position of relative optical path difference is also the position of the clearest fringe.
The wavelength of the infrared source is 1290nm and the bandwidth is 63.6nm. In this experiment, the coherence length
of infrared source is detected by cube reflection experiment. The coherence length is 30μm by data collection and data
processing, and that result of 30μm is less different from the 26μm of theoretical calculated. In order to further test the
relative optical path of optical synthetic aperture using step scanning method, the infrared source is placed into optical
route of optical synthesis aperture telescope double aperture. The precision position of actuator can be obtained when the
fringe is the clearest. By the experiment, we found that the actuating step affects the degree of precision of equivalent
optical path. The smaller step size, the more accurate position. But the smaller the step length, means that more steps
within the coherence length measurement and the longer time.
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