Diffractive Telescope Research Articles

Parallel-filter-based observer control for unknown multiple narrow-band disturbance rejection

An observer-based control method with parallel Q-filter is proposed to reject unknown multiple narrow-band disturbances for line of sight stabilization in telescope systems. In general, wide-band or multiple narrow-band disturbances rejection of observer-based control can be transformed into designing Q-filter, which could be too high-order to achieve fully-accuracy digital implementation, resulting in degrading the closed-loop performance or even causing instability. Therefore, a new parallel type of Q-filters is proposed to flexibly adjust the number of Q-filter in the sensitivity function and effectively reduce the computational complexity. A sufficient condition of the effectiveness for the parallel Q-filter and the closed-loop stability is derived and verified through simulations. Furthermore, disturbance frequencies can be on-line identified to adaptively optimize the Q-filter parameters. The effectiveness of the proposed method is validated on the image-based stabilization subsystem of a segmented lightweight large-scaled diffractive telescope.

High-frequency line-of-sight jitter rejection via an extended Youla-Kucera parameterization controller in segmented diffractive telescope—Theory and experiment

Line-of-sight jitter caused by mechanical vibrations and disturbances in the transmission medium seriously damages the imaging performance in telescope systems. This paper investigates the problem of high-frequency line-of-sight jitter rejection up to Nyquist frequency in bandwidth-limited control systems due to time delays. Although the piezoelectric ceramic-driven tip-tilt mirror has a wide bandwidth, Large time delays caused by the image sensor for high-quality imaging severely restrict the control system’s closed-loop bandwidth. The disturbance cannot be sufficiently rejected, especially outside the bandwidth. Based on the Small Gain Theorem, the gain factor and time-delay factor as an extra flexible budget in the Youla-Kucera (Y-K) parameterization are derived from decreasing the waterbed effect, improving the closed-loop system stability. Thus, this proposed Y-K method accommodates disturbance rejection frequency inside and outside control bandwidth, which can be extended to the Nyquist frequency. Simulation analyses and Experimental results have been effectively verified in the line-of-sight stabilization control of the segmented lightweight large-scaled diffractive telescope (SLLDT) system.

The design of multi-wavelength confocal diffractive optical element based on set operation

A method suitable for the design of large aperture multi-wavelength confocal diffractive optical element (DOE) was proposed. The method is based on set operation for microstructures. As examples, different multi-wavelength confocal DOEs with aperture of 20 mm were designed. The results of the numerical simulations show that the confocal DOEs achieved diffraction-limited focusing performance at all design wavelengths, but the diffraction efficiencies were lower than the ideal DOEs. For experimental verification, one three-wavelength confocal DOE with focal length of 200 mm was fabricated and tested. The optimum full width at half maximum (FWHM) is preferably 1.067 times of the diffraction limit and the highest optical imaging solution reaches 94.6% of the diffraction limit. Both the simulation results and the experimental results verify the validity of this method for designing multi-wavelength confocal DOE. The presented work may find applications in large-aperture multi-wavelength laser communication, wide-spectrum diffractive telescopes, and high-precision multi-wavelength Computer generated holography (CGH).

Large aperture high diffraction efficiency off-axis Fresnel lens fabrication and analysis.

Large aperture high diffraction efficiency off-axis Fresnel lens is one of the most important optical elements in future 10m scale aperture transmissive space telescope systems. Improving diffraction efficiency and effective aperture are long-term goals and bottlenecks for engineering applications. A 4-level off-axis fresnel lens with Ф350 mm effective aperture and 2 µm critical dimension was fabricated through overlay etching technique bylaser direct writing system. Average diffraction efficiency of 75.9% was achieved and certain distribution pattern was observed. Influence of alignment errors on diffraction efficiency distribution was analysed and discussed in detail. This work presented the best results to our knowledge among the same field with similar aperture and critical dimension in open publications, and layed a solid foundation for future large aperture diffractive telescope development.

Design and tolerance analysis of a large-diameter diffractive telescope

The large-diameter diffractive telescope has the advantage of observing space debris. Some of the key issues are the design and tolerance analysis of the telescope with the segmented primary lens. In this paper, we design a space large-diameter sub-aperture stitching diffractive telescope that can achieve a large diameter of 1438 mm, a long focal length of 2000 mm, and a full field of view (FOV) of 0.34°. Considering the manufacturing and stitching errors of the sub-aperture can affect the telescope’s image quality, we provide a tolerance analysis method based on Monte Carlo analysis, which makes the consequence of the tolerance analysis nearly reality. The tolerance ranges are proposed not only for the segmented primary lens but also for the whole telescope system.

Misalignment Error Analysis of Segmented Diffractive Telescope

Misalignment Error Analysis of Segmented Diffractive Telescope

Dynamic modeling for the deployment of the folded truss of space diffraction telescope

Dynamic modeling for the deployment of the folded truss of space diffraction telescope

Design of catadioptric middle infrared diffractive telescope system with large aperture

Design of catadioptric middle infrared diffractive telescope system with large aperture

Development of space-based diffractive telescopes

Membrane diffractive optical elements formed by fabricating microstructures on the substrates have two important characteristics, ultra-light mass (surface mass density < 0.1 kg/m2) and loose surface shape tolerances (surface accuracy requirements are on the order of magnitude of centimeter). Large-aperture telescopes using a membrane diffractive optical element as the primary lens have super large aperture, light weight, and low cost at launch. In this paper, the research and development on space-based diffractive telescopes are classified and summarized. First, the imaging theory and the configuration of diffractive-optics telescopes are discussed. Then, the developments in diffractive telescopes are introduced. Finally, the development prospects for this technology used as a high-resolution space reconnaissance system in the future are summarized, and the critical and relevant work that China should carry out is put forward.

Wide-band high-resolution synthetic imaging of a segmented large-scaled lightweight diffractive telescope

In this Letter, we report a segmented large-scaled lightweight diffractive telescope testbed newly built in our laboratory. The telescope, consisting of one 710-mm-diameter element in the center surrounded by eight 352-mm-diameter elements and a smaller eyepiece of achromatic lenses, can realize wide-band high-resolution imaging of 0.55-0.65 µm. The stitching errors are coarsely corrected by adjusting the motion stage mounted on each element. In particular, an optical synthesis system inserted behind the eyepiece is designed to compensate the residual tip-tilt-piston errors. We present the experimental imaging result of two stitched elements, which is the first successful experimental verification obtained by a practical segmented diffractive telescope to enhance the resolution. Moreover, spatial modulation diversity technology is used to restore the synthetic image so as to improve its quality and contrast.

Simplified unobscured optics design for a diffractive telescope.

An unobscured optical system provides a good solution for a high-performance diffractive telescope. However, the unobscured diffractive telescope, such as an off-axis three-mirror diffractive telescope (OTDT), suffers from the complex calculation of initial parameters, strict tolerance, and fabrication of aspheric mirror diffractive optical elements (DOE). In this paper, a simplified unobscured design method, which is effective in achieving compact and loose tolerance, is proposed. Combining the deflection of an optical path and Schupmann achromatic theory, a novel unobscured diffractive telescope is rapidly converted from a coaxial transmissive system. Not only is the loose tolerance obtained, but the fabrication of an aspheric mirror DOE is avoided. The designed system, with a focal length of 400 mm, $f$f-number of 5, and working waveband of 612.8-652.8 nm, is analyzed. The results show that the imaging quality approached the diffraction limit within a field of view of ${0.04}^\circ \times {0.02}^\circ $0.04∘×0.02∘. Compared with the OTDT, the designed system has great advantages in design step, DOE fabricating, and system alignment. It also provides a reference for unobscured diffractive telescope development.

基于图像复原的衍射望远镜暗弱目标成像

基于图像复原的衍射望远镜暗弱目标成像

Experimental study on imaging and image deconvolution of a diffractive telescope system.

Diffractive telescopes are one of the most promising solutions to build lightweight space telescopes with a diameter over 10 m. However, the performance of the imaging systems is inevitably degraded by the high-order diffractive light from the diffractive system. To address this problem, in this paper we mathematically deduce the imaging model, including multiple-order diffraction, by the scalar diffraction theory. After the imaging characteristics analysis, an adaptive Wiener filtering algorithm based on the principal component analysis method is proposed. The broadband imaging and deconvolution experiments are performed by the 80 mm diffractive optical telescope systems. Results demonstrate that this method increases the average gradient by at least 8.2 times and improves imaging quality and contrast. It could be a useful exploration for high-performance imaging of large-aperture and lightweight telescopes.

Optical design of compact unobscured ground-based diffractive telescope

To realize unobscured, compact structure and maintain loose tolerance, a novel method based on flat-field relay mirror for designing the ground-based diffractive telescope is proposed. Based on this mthod, we perform two different studies devised to design and reduce the manufacturing difficulty of compact unobscured ground-based diffractive telescope. Firstly, the formula of flat-field relay mirror structure based on aberration theory is deduced. Combining the Schupmann achromatic theory, a compact unobscured diffractive telescope with loose tolerance is designed. The designed focal length is 400 mm, F number is 5, the working waveband is 612.8-652.8 nm. After optimization by optical design software ZEMAX, the size of system is 720 mm × 435 mm, the total track distance is only 1/12 of transmissive diffractive telescope. The modulation transfer function (MTF) is greater than 0.73 at 50 lp/mm within 0.30°×0.07° field of view (FOV), and the imaging quality is approached the diffraction limit. Secondly, to further reduce the difficulty, the fabricating parameters of diffractive optical elements (DOE) and the alignment tolerance of system are designed and fully analyzed. The analysis results show that the DOE can be fabricated by existing binary lithography, the designed system meet the requirement of alignment. The designed system provides a good solution for ground-based diffractive telescope.

大口径衍射望远系统初始结构研究

大口径衍射望远系统初始结构研究

Design of optical imaging system for RGB three-channel diffraction telescope

Design of optical imaging system for RGB three-channel diffraction telescope

Misalignment measurement of membrane diffractive telescope segments by means of edge sensors with form closure.

The membrane diffractive telescope is an alternative to future 10-meter class space telescopes due to its possible lower cost of manufacturing and launching. To actively control the misalignment, the edge sensor network is utilized to measure the relative in-plane motions as well as out-of-plane motions of neighboring segments. The measurement is vulnerable to noise and malfunction of sensors. We first model the segment kinematics, which is used to simulate the propagation of sensor error to the segment configuration error. Monte Carlo simulation results show that the sensor error is considerably accumulated in the configuration error, which follows approximately normal distribution. On the other hand, the ring-arranged segments imply the form closure with redundancy of edge sensors. It is employed to judge whether there are abnormal sensor readings and then to identify the malfunctioning sensors. A better estimation can further be obtained as correction to the abnormal readings. Simulation results show that without sensor error, the algorithm in most cases can identify two malfunctioning in-plane sensors and two malfunctioning out-of-plane sensors. While considering the sensor error, only one malfunctioning in-plane sensor and one malfunctioning out-of-plane sensor can be identified with a sufficiently big ratio of abnormal reading to the normal sensor error.

Influence of ambient temperature on the modulation transfer function of an infrared membrane diffraction optical system.

In this paper, we present a model that analyzes the influence of the temperature on the modulation transfer function (MTF) of an infrared membrane diffraction optical system. Based on the physical imaging mechanism of diffractive optical systems, the imaging characteristics of the diffraction optical system are characterized by using diffraction efficiency. Then we establish the corresponding calculation model and MTF analysis model based on scalar diffraction theory. In particular, considering the material properties of flexible membrane optical elements and thermal environment effect, the Zernike polynomials are used to establish the thickness distribution model of the membrane diffractive optical components combined with the finite element analysis. After that, we modify the MTF calculation model of diffraction optical system and accordingly propose the model of the infrared membrane diffraction optical system. Finally, we present some experimental results for a 2-m infrared membrane diffraction optical system. The results show that when the temperature is -40°C and wavelength band is 80nm at the center field of view, the diffraction efficiency is reduced to 81%, and MTF at 75 lp/mm is reduced to 0.12. And when the temperature is -40°C at edge field of view, the efficiency at center wavelength is reduced to 71%, and MTF at 75 lp/mm is reduced to 0.02. The established model and method can be used to estimate the imaging performance of a space infrared membrane diffractive telescope and provide theoretical guidance for the processing algorithm and imaging system design.

Stray light characteristics of the diffractive telescope system

Diffractive telescope technology is an innovation solution in construction of large light-weight space telescope. However, the nondesign orders of diffractive optical elements (DOEs) may affect the imaging performance as stray light. To study the stray light characteristics of a diffractive telescope, a prototype was developed and its stray light analysis model was established. The stray light characteristics including ghost, point source transmittance, and veiling glare index (VGI) were analyzed. During the star imaging test of the prototype, the ghost images appeared around the star image as the exposure time of the charge-coupled device improving, consistent with the simulation results. The test result of VGI was 67.11%, slightly higher than the calculated value 57.88%. The study shows that the same order diffraction of the diffractive primary lens and correcting DOE is the main factor that causes ghost images. The stray light sources outside the field of view can illuminate the image plane through nondesign orders diffraction of the primary lens and contributes to more than 90% of the stray light flux on the image plane. In summary, it is expected that these works will provide some guidance for optimizing the imaging performance of diffractive telescopes.

Methods for the performance enhancement and the error characterization of large diameter ground-based diffractive telescopes.

This paper is devoted to the improvement of ground-based telescopes based on diffractive primary lenses, which provide larger aperture and relaxed surface tolerance compared to non-diffractive telescopes. We performed two different studies devised to thoroughly characterize and improve the performance of ground-based diffractive telescopes. On the one hand, we experimentally validated the suitability of the stitching error theory, useful to characterize the error performance of subaperture diffractive telescopes. On the other hand, we proposed a novel ground-based telescope incorporated in a Cassegrain architecture, leading to a telescope with enhanced performance. To test the stitching error theory, a 300 mm diameter, 2000 mm focal length transmissive stitching diffractive telescope, based on a three-belt subaperture primary lens, was designed and implemented. The telescope achieves a 78 cy/mm resolution within 0.15 degree field of view while the working wavelength ranges from 582.8 nm to 682.8 nm without any stitching error. However, the long optical track (35.49 m) introduces air turbulence that reduces the final images contrast in the ground-based test. To enhance this result, a same diameter compacted Cassegrain ground-based diffractive (CGD) telescope with the total track distance of 1.267 m, was implemented within the same wavelength. The ground-based CGD telescope provides higher resolution and better contrast than the transmissive configuration. Star and resolution tests were experimentally performed to compare the CGD and the transmissive configurations, providing the suitability of the proposed ground-based CGD telescope.