Secondary Mirror Research Articles

Accurate measurement of deflection of primary and secondary mirrors of large-aperture telescope

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Attitude Alignment Method of Solar Telescope Secondary Mirror Based on Aberration Detection

Attitude Alignment Method of Solar Telescope Secondary Mirror Based on Aberration Detection

Design of Integrated Carbon Fiber Secondary Mirror Supporting Structure

Design of Integrated Carbon Fiber Secondary Mirror Supporting Structure

Attitude correction of secondary mirror in large telescope using coma and imaging point position

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Design and Test of High Precision and Stability Support Structure for Secondary Mirror

Design and Test of High Precision and Stability Support Structure for Secondary Mirror

Calculation of the incidence angle modifier of a Linear Fresnel Collector: The proposed declination and zenith angle model compared to the biaxial factored approach

Since their first appearance as a contribution by Professor Francia at the University of Genova, Italy, the Linear Fresnel Collectors (LFC) demonstrated to be an engineering efficient technology for medium to high temperature solar applications. The strength of the LFC concept is related to the simple mirror motion law, to the compactness of the mirror fields (power to land surface ratios), to the lowest resistance to wind, to the system intrinsic scalability. To perform reliable LCOE analyses, robust performance simulation tools are needed. The Authors developed to this aim a 3D ray-tracing model, able to account for shading, blocking, and end effects as a function of LFC geometry, including primary and secondary mirror curvatures. In this paper, a new approach is implemented to reduce huge yearly ray-tracing datasets and provide very compact analytical equations for fast hourly performance simulations. The present model introduces new Incidence Angle Modifier (IAM) correlations based on the declination and zenith angles. The new model demonstrated to fit subhourly 3D ray-tracing data all year long with an overall error lower than 1.5%, well below the best IAM factored models here compared as a general criticism to the biaxial factored approach related to Fresnel applications.

CanariCam Mid-infrared Drift Scanning: Improved Sensitivity and Spatial Resolution

Ground-based mid-infrared (MIR) astronomical observations require the removal of the fast time variable components of (a) sky/background variation and (b) array background. Typically, this has been achieved through oscillating the telescope’s secondary mirror a few times a second, a process termed “chopping.” However, chopping reduces on-object photon collection time, imposes stringent demands on the secondary mirror, requires nodding of the telescope to remove the radiative offset imprinted by the chopping, and relies on an often-fixed chop-frequency regardless of the sky conditions in the actual observations. In the 30 m telescope era, secondary mirror chopping is impracticable. However, if the sky and background are sufficiently stable, drift scanning holds the promise to remove the necessity of chopping. In this paper, we report our encouraging drift scanning results using the CanariCam MIR instrument on the 10.4 m Gran Telescopio Canarias and the implications to future instruments and experiments.

Design and Implementation of a High Precision Stewart Platform for a Space Camera

In order to design and implement a high-precision Stewart platform to precisely adjust the position and posture of the secondary mirror of a space camera, the following measures were taken: firstly, the inverse mathematical model and ADAMS parametric model of the Stewart platform are established, which are the basis of structural optimization design; secondly, the structural parameters of Stewart platform are obtained through structure optimization design in ADAMS after determining the objective function; thirdly, a 50nm resolution driving strut based on brushless DC motor, ball screw, grating ruler and PI closed-loop control law is designed, which strongly guaranteed the six degrees’ resolution of the Stewart platform that mainly consist of 6 such high-resolution driving struts; finally, the accuracy of Stewart platform is tested via dual frequency laser interferometer and photoelectric autocollimator, and the test results show that the displacement resolution of the Stewart platform is 0.2 μ m, and the angular resolution is 1”, which meets the requirements of the index. The Stewart platform has been successfully applied to the space camera by tuning the secondary mirror precisely in 6 degrees of freedom in the optical alignment experiment, which lays a solid theoretical and practical foundation for on orbit application in future.

Design and Numerical Analysis of an Infrared Cassegrain Telescope Based on Reflective Metasurfaces.

Reflective imaging systems such as Cassegrain-type telescopes are widely utilized in astronomical observations. However, curved mirrors in traditional Cassegrain telescopes unavoidably make the imaging system bulky and costly. Recent developments in the field of metasurfaces provide an alternative way to construct optical systems, possessing the potential to make the whole system flat, compact and lightweight. In this work, we propose a design for a miniaturized Cassegrain telescope by replacing the curved primary and secondary mirrors with flat and ultrathin metasurfaces. The meta-atoms, consisting of SiO2 stripes on an Al film, provide high reflectance (>95%) and a complete phase coverage of 0~2π at the operational wavelength of 4 μm. The optical functionality of the metasurface Cassegrain telescope built with these meta-atoms was confirmed and studied with numerical simulations. Moreover, fabrication errors were mimicked by introducing random width errors to each meta-atom; their influence on the optical performance of the metasurface device was studied numerically. The concept of the metasurface Cassegrain telescope operating in the infrared wavelength range can be extended to terahertz (THz), microwave and even radio frequencies for real-world applications, where metasurfaces with a large aperture size are more easily obtained.

Design of a freeform imaging spectrometer based on a solution-diversified automatic design method.

A freeform imaging spectrometer design method is proposed. Only system specifications, including the slit length, numerical aperture, magnification, spectral range, and spectral resolution, are required as design inputs, and imaging spectrometer systems with various optical power distributions and structures are output. Using three-mirror imaging spectrometers as design examples, a series of initial solutions are obtained with a diffraction grating placed at the secondary mirror. In order to understand and explore the utilization of large number of output results, these initial solutions are visualized by mapping from high-dimensional solution space to three-dimensional space using the t-distributed symmetric neighbor embedding (t-SNE) dimensionality reduction algorithm; then a neural network is trained to fit the imaging qualities, showing good generalization performance, and can predict imaging quality for systems with unknown optical power distributions and structures.

Wide-Field Telescope Alignment Using the Model-Based Method Combined with the Stochastic Parallel Gradient Descent Algorithm

To acquire images with higher accuracy of wide-field telescopes, deformable mirrors with more than 100 actuators are used, making the telescope alignment more complex and time-consuming. Furthermore, the position of the obscuration caused by the secondary mirror in the experiment system is changed with the difference of fields of view, making the response matrix of the deformable mirror different in various fields of view. To solve this problem, transfer functions corresponding to different fields of view are calculated according to the wavefront edge check and boundary conditions. In this paper, a model-based method combined with the stochastic parallel gradient descent (SPGD) algorithm is used. The experiment results show that our method can correct the aberrations with a high accuracy in both on-axis and off-axis fields, indicating that the effective actuators are well chosen corresponding to different fields of view.

Design of a Dual-Band Compact Integrated Remote Sensing System for Visible Light and Long-Wave Infrared

This paper presents a design of a dual-band integrated space telescope system for visible light and long-wave infrared. The system can simultaneously image the visible light band of 450–900 nm and the long-wave infrared band of 7700–10,500 nm. The dual-band integrated imaging system can freely switch the observation band to adapt to different scenes and environmental changes. The camera can also further expand its capabilities in the fields of multi-spectral observation and low-light observation by collocation with different detectors. This design is based on a coaxial reflection system, the two bands share the camera’s primary and secondary mirrors, and the separation of the two bands is achieved through a separate field of view design. After simulation, the average Modulation Transfer Function (MTF) value of the visible light band of the system at 50 lp/mm (line pairs per millimeter) reaches 0.45, and the average MTF value of the long-wave infrared band at 50 lp/mm reaches 0.36. In addition, tolerance analysis, ambient temperature analysis and transmittance analysis of the integrated system are carried out in this paper to further improve the integrated system scheme, and the feasibility of the system is further verified.

Analysis and Research on the Performance of Solar Concentration Based on Big Data and Machine Learning

Empirical formulas for PTC optical efficiency calculation are difficult and costly to obtain from rigorous comparative experiments, whereas simpler optical modeling methods inadequately incorporate realistic optical effects. In this article, algorithms are respectively developed to calculate the geometric concentration ratio (Cg) of linear Cassegrainian solar concentrators (CSC) with a secondary flat mirror based on the way of edge rays from solar sources to a flat-plate receiver. On the basis of the large amount of data generated, machine learning and Python language programming methods are used to analyze and process the data, and the functional relationship between the concentration ratio and each parameter is obtained. The learning and training effect is good, and the ideal result is achieved.

Двухзеркальная цилиндрическая антенная система с высоким коэффициентом использования поверхности.

A technique for the synthesis of two-mirror cylindrical antenna systems for a given amplitude distribution in the aperture has been developed. At the first stage geometrical-optical synthesis is carried out, at the second stage a refined solution is found using the field of the feed on the secondary mirror found by the numerical method of finite element. As an example, a two-mirror cylindrical antenna system with a horn-lens feed and a uniform field distribution in the main mirror aperture has been synthesized. Using full-wave simulation by the finite element method, the analysis of the radiation characteristics of a synthesized two-mirror antenna system is carried out. The scanning characteristics of the antenna system by turning the main mirror are investigated. The efficiency of a cylindrical two-mirror antenna system with a horn-lens waveguide transition in the E-plane and an H- sectorial horn feed reaches 0.95 i.e. is close to the limiting value. When scanning the main lobe of the pattern in an angular sector of 27 degrees due to the rotation of the main mirror gain exceeds 26 dB, and efficiency exceeds 0.8.

Design and alignment of off‐axis reflection beam expanding and contracting optical system

AbstractThere are special requirements for the laser beam diameter and divergence angle in various optical combination systems, so most of the designs add beam expansion and contraction systems. Off‐axis reflective systems are more widely used in practical applications because of their nonobstructing characteristics and unaffected by chromatic aberration. This paper designs an optical system with both beam expansion and contraction functions by Zemax optical software according to actual needs. The magnification and reduction are 3×. The mirror surface adopts an aspheric design to overcome system aberrations. The off‐axis reflective structure is suitable for lasers of different wavelengths (632.8 nm, 1064 nm, and 1550 nm) without being affected by chromatic aberration. The optical spot diagram shows that the light spot distribution is uniform and meets the magnification and reduction ratio. Besides, combined the dynamic laser interferometer, we install and debug the primary and secondary mirrors of the system and use the laser beam quality analyzer to measure the exit spot of the actual laser beam after passing through the designed optical system. The peak value and root‐mean‐square value of the system transmission wave front are less than 0.1λ. The beam quality test result of the outgoing spot meets the actual application requirements and image quality requirements.

Comparisons between an on-axis three-mirror anastigmat telescope and an off-axis one: polarization aberrations.

Due to the ability to achieve a wide-diffraction-limited field of view, three-mirror anastigmat (TMA) telescopes are widely used in many applications that demand high imaging quality and detection accuracy, in which polarization aberrations play increasingly important roles. In this paper, polarization properties of on-axis elements and those of off-axis elements are analyzed and compared via polarization aberration theory. Then, an on-axis TMA (OTMA) telescope and an off-axis TMA (FTMA) telescope are designed and optimized to be diffraction-limited systems in parallel. Their constructional parameters and first-order parameters are set to be identical. Via polarization ray tracing, polarization aberrations of the 2 telescopes are obtained and compared in terms of diattenuation map, retardance map, Jones pupil, Pauli pupil, and amplitude response matrix. Results demonstrate that polarization aberrations are closely related to the structural features of mirrors and telescopes. The diattenuation and retardance maps of power mirrors in the OTMA telescope are rotationally symmetric, while those in the FTMA telescope are not. A Maltese cross appears in off-diagonal elements of the Jones pupil of the OTMA telescope and disappears in the FTMA telescope. Amplitude response matrices of polarization crosstalk components in the OTMA telescope are with a four-peak structure, while those in the FTMA telescope are with a two-peak structure. After quantitative comparisons, it is found that diattenuation in the OTMA telescope is smaller than that in the FTMA telescope as well as in the cases of retardance and polarization crosstalk. There are also similarities between the 2 TMA telescopes. Polarization aberrations of secondary mirrors (SM) are greater than those of both primary mirrors (PM) and tertiary mirrors (TM). What is more, polarization aberrations of TM are so small that they can be ignored safely. The numerical sum of Pauli coefficients of PM, SM, and TM is nearly identical to the cumulative Pauli coefficients. Hence, the TMA telescopes can be treated as weak polarization elements. This work compares polarization aberration performance of on- and off-axis TMA telescopes and can provide important references for designing and optimizing telescopes in which polarization aberrations matter.

Aberration compensation strategy for the radius of curvature error of the primary mirror in off-axis three-mirror anastigmatic telescopes.

This paper discusses compensation strategies for the aberration fields caused by the error in the radius of curvature (ROC) of the primary mirror (PM) in pupil-offset off-axis three-mirror anastigmatic (TMA) astronomical telescopes. Based on the nodal aberration theory framework, the specific astigmatic and coma aberrations of the off-axis three-mirror system in the presence of the ROC error of the PM are derived. It is demonstrated that some field-dependent aberration components can be induced by ROC error in the off-axis TMA telescopes, apart from the dominating field-constant aberration terms. To reduce the influence of the ROC error on the aberration fields, we propose two aberration compensation strategies: adjusting the position of the PM and introducing axial misalignment of the secondary mirror (SM). Through theoretical analysis and simulations, we conclude that the compensation strategy of changing the axial position of the PM can make the aberration distribution close to the nominal state; the compensation strategy of axially adjusting the SM can make the aberration distribution meet the observation requirements, which is more suitable for space applications. We also discuss compensating the effect of the ROC error using lateral misalignments.

Numerical calculation of the intercept factor for parabolic trough solar collector with secondary mirror

The intercept factor is the most complex parameter involved in determining the optical efficiency of a parabolic trough solar collector. A numerical algorithm method was proposed to calculate the intercept factors of a parabolic trough solar collector using the Gauss error function and a Python program. The results indicate that the intercept factors can be calculated quickly and conveniently using the proposed method. We also introduced a numerical solution to calculate the intercept factor of the parabolic trough solar collector, which can provide accurate interception factors for different types of secondary reflection concentrators. The new analytical method was validated and benchmarked against a Gauss error function and Python program, demonstrating that the deviations were below 0.0015% for most cases studied. The primary goals of this study were to determine an appropriate numerical method to solve the intercept factors and errors for different secondary mirror types and to incorporate the selected intercept factor calculation into a secondary mirror design for a parabolic trough solar collector under various conditions.

Experimental investigation of binodal astigmatism in nodal aberration theory (NAT) with a Cassegrain telescope system.

We present simulations and an experimental investigation of binodal astigmatism in nodal aberration theory (NAT) for a customized, high-precision Cassegrain telescope system. The telescope system utilizes a five-axis, piezo-actuated flexural mechanism to introduce secondary mirror misalignments and generate aberrations intentionally. The induced aberrations are measured interferometrically and quantified for a grid of field points on the telescope system's image plane. For this purpose, a coma-free pivot point of the secondary mirror was simulated for isolating the binodal astigmatism field response. The separation of the nodes is proportional to the introduced misalignments. A simulation of Fringe Zernike coma and binodal astigmatism was generated using a real ray trace model of the optical system and analyzed to compare to the experimental results. A statistical analysis of the measurements was performed to show the experimental results' accuracy and stability. The experimental results were consistent with the simulations, hence experimentally validating NAT for binodal astigmatism for the first time.

Linear aplanatic Fresnel reflector for practical high-performance solar concentration

We propose the linear aplanatic Fresnel reflector (LAFR) as a novel optical design that is suitable for solar thermal power plants. It comprises a segmented primary reflector and a totally stationary secondary mirror and receiver. We show that it is capable of attaining the high intercept factors of conventional parabolic troughs at high concentration, while offering the practical and lower-cost benefits of linear Fresnel reflectors. The fact that dual-mirror Fresnel concentrators can be aplanatic had been overlooked, mainly due to the perception that Fresnel concentrators offer only one degree of design freedom (specifically, the shape of the secondary mirror). The problem is that the two leading orders of geometric aberration must be eliminated to achieve aplanatism, and hence two degrees of design freedom are necessary. After identifying the solar tracking strategy of the segmented primary reflector as the additional degree of design freedom, we derive analytic relations for its placement and curvature. The secondary mirror shape follows directly from the basic aplanatic formalism. We demonstrate that practical high-performance LAFR designs can achieve intercept factors as high as 0.84 at concentration values of ~26.