Secondary Mirror Research Articles

Development of an off-axis TMA telescope for coastal water remote sensing

A proto-model of an off-axis reflective fore-optics in a new hyperspectral sensor onboard a small unmanned aerial vehicle (UAV) is developed for low-altitude coastal remote sensing. In spite of the substantial advantages of off-axis optics over refractive or on-axis reflective optics, it is difficult to fabricate and align the mirrors in off-axis systems because of their asymmetric configurations. The fore-optics is designed by Korea Institute of Ocean Science & Technology (KIOST) as a three-mirror system including aspheric primary and secondary mirrors, with an entrance pupil diameter of 70 mm and F-ratio of 5.0. The off-axis mirror is fabricated by a single point diamond turning machine (SPDTM) with a form accuracy of 89.3 nm (RMS) and surface roughness of 7.2 nm (Ra). Coarse alignment of this three-mirror off-axis optical system is accomplished by using a designated base plate and a coordinate measuring machine. The optical performance is tested using the point spread function of the returned collimated beam of a Fizeau interferometer.

The mini-array of ASTRI SST-2M telescopes, precursors for the Cherenkov Telescope Array

In the framework of the Cherenkov Telescope Array (CTA) Observatory, the Italian National Institute of Astrophysics (INAF) has recently inaugurated in Sicily (Italy), at the Serra La Nave astronomical site on the slopes of Mount Etna, a large field of view (FoV, ~ 9.6°) dual-mirror prototype (ASTRI SST-2M) of the CTA small size class of telescopes (SST). The CTA plans to install about 70 SST in the southern site to allow the study of the gamma rays from a few TeV up to hundreds of TeV. The ASTRI SST-2M telescope prototype has been developed following an end-to-end approach, since it includes the entire system of structure, mirrors optics (primary and secondary mirrors), camera, and control/acquisition software. A remarkable performance improvement could come from the operation of the ASTRI mini-array, led by INAF in synergy with the Universidade de Sao Paulo (Brazil) and the North-West University (South Africa). The ASTRI mini-array will be composed of nine ASTRI SST-2M units and it is proposed as a precursor and initial seed of the CTA to be installed at the final CTA southern site. Apart from the assessment of a number of technological aspects related to the CTA, the ASTRI mini-array will, if compared for instance to H.E.S.S., extend the point source sensitivity up to ~ 100 TeV, also improving it above 5-10 TeV. Moreover, the unprecedented width of the FoV, with its homogeneous acceptance and angular resolution, will significantly contribute to the achievement of original results during the early CTA science phase.

An autocollimator alignment system for a Schwarzschild-Couder Cherenkov telescope

We present a digital autocollimator which will be used in the alignment system of a prototype medium-sized telescope, which is part of the U.S. contribution to the Cherenkov Telescope Array (CTA). The Schwarzschild-Couder optics in the prototype telescope (which is currently under construction) requires the precise alignment of three components: the primary and secondary segmented mirrors, and the gamma-ray camera. The approximately 9 meter separation between the mirrors necessitates remote optical measurement. Our autocollimator will measure the angle of a segment in one mirror relative to the center of the other mirror with a precision better than 5 arcsec over a range of ±0.126°. We present a detailed description of the instrument and describe its performance in the laboratory.

Optical Analysis of a Two Stage XX Concentrator for Parametric Trough Primary and Tubular Absorber With Application in Solar Thermal Energy Trough Power Plants

A new symmetric nonimaging parametric trough collector (PmTC) for an evacuated circular receiver is proposed having an absorber diameter of 70 mm similar to the LS3/Eurotrough absorber one, and a circular secondary. The optical design method is explained and a sensitivity analysis is conducted with respect to the diameter of the secondary and to the acceptance angle. In the design process, transmission, absorption, and reflection losses are neglected. A secondary mirror radius of 145 mm and an acceptance angle of ±14 mrad were chosen as realistic values. For these values a concentrator geometry having 8.7 m gross aperture and 100% intercept factor for rays impinging on the primary within the referred acceptance angle is obtained producing a net concentration ratio relative to the thermodynamic ideal limit of 0.52 compared to 0.31 for traditional parabolic trough collectors. The new concentrator is composed of a primary discontinuous reflector with two symmetrical portions with parametric geometry, and a central parabolic portion located between the other two. The ensemble secondary receiver can be dressed up with the secondary concentrator manufactured by partially mirroring a diameter adapted glass tube—either internally or externally—or alternatively by means of a commercial evacuated receiver and an independent 145 mm radius arc of circumference external secondary reflector. Monte Carlo ray-tracing results show that only 15% of the rays undergo secondary reflection before arriving to the absorber and a sharp angle transmission curve. The new concentrator is proposed for application in solar thermal energy (STE) trough power plants.

Fabrication of petal-shaped masks for suppression of the on-axis Poisson spot in telescope systems.

The presence of a bright (Poisson) spot in the geometrical shadow of circular/spherical shapes has been known for the past two centuries. A broad class of telescopes that involve simultaneous transmit and receive require suppression of the reflected light from the secondary mirror on the detector. For instance, the on-axis design of optical telescope for the evolved Laser Interferometric Space Antenna (eLISA), a re-scoped version of the baseline LISA mission concept, requires suppression of reflected laser light from the secondary mirror on the detector. In the past few years, the hypergaussian functions with petal-shaped realization have been shown to significantly suppress intensity along the optical axis. This work reports on fabrication of a series of petal-shaped masks using a variety of techniques such as 3D printing, photolithography, and wire Electro Discharge Machining. These masks are designed and fabricated to operate in the range of Fresnel numbers between 4 and 120. This paper discusses the challenges, successes, and failures of each fabrication technique and the optical performance of typical masks with suggestions for potential follow up work.

Miniature Schwarzschild objective as a micro-optical component free of main aberrations: concept, design, and first realization with silicon-glass micromachining.

This paper presents the conception of a new micro-optical component fabricated within the wafer-level approach: a micromachined reflective objective, the so-called micro-Schwarzschild objective, characterized by superior optical performances than widespread microlenses. The system, made of two vertically integrated mirrors, works in transmission similarly as microlenses. While the specific geometric configuration of the two-mirrors allows elimination of most common optical aberrations, the reflective architecture provides inherent achromaticity. This paper presents in detail the optical design and analyzes fabrication tolerances. It also describes a fabrication flow chart based on silicon micromachining done at the wafer level that could allow production of thousands of such micro-optical devices within a single fabrication run. The realized prototype employs the two-step KOH etching process to generate the micromirror pairs followed by glass reflow for the secondary mirror generation and selective metallic deposition. Despite an insufficient mirror quality attributed to this specific silicon etching technique and highlighted by the reflective configuration, the objective fabrication in terms of alignment, bonding, and coating is shown as feasible.

Qualification and Testing of a Large Hot Slumped Secondary Mirror for Schwarzschild–Couder Imaging Air Cherenkov Telescopes

Dual-mirror Schwarzschild–Couder (SC) telescopes are based on highly aspherical optics, and they represent a novel design in the world of very high energy astrophysics. This work addresses the realization and the qualification of the secondary mirror for an SC telescope, named ASTRI, developed in the context of the Cherenkov Telescope Array Observatory. The discussion surveys the overall development from the early design concept to the final acceptance optical tests.

Online refocusing algorithm for a satellite camera using stellar sources.

In this study, an online refocusing algorithm is proposed for a satellite camera performing an Earth observation mission. Satellite cameras are vulnerable to misalignment in orbit because of their severe launching environments and the thermal vacuum environment in space. The proposed online refocusing algorithm is able to guarantee high quality images by aligning the satellite camera in real time. This alignment is achieved by precisely adjusting the movement mechanism of the secondary mirror (M2) and the focal plane. The target optical system used in this study was originally designed for the purposes of algorithm development. The system uses a Schmidt-Cassegrain-type satellite camera with a 200-mm diameter primary mirror (M1). The ground sampling distance (GSD) is 3.8 m from an altitude of 700 km. A fourth-order equation model is derived for the modulation transfer function (MTF) variation tendency for M2 de-spacing. Following this, the proposed online refocusing algorithm for the target optical system is developed. The algorithm is able to assess the de-space position from the MTF measurements using stellar sources. It is determined from the simulation that any misaligned satellite camera can be refocused within a ± 0.5μm M2 de-space error by applying the proposed refocusing algorithm in real time.

A Coaxial and Off-axial Integrated Three-mirror Optical System with High Resolution and Large Field of View

A novel optical design for high resolution, large field of view (FOV) and multispectral remote sensing is presented. An f/7.3 Korsch and two f/17.9 Cook three-mirror optical systems are integrated by sharing the primary and secondary mirrors, bias of the FOV, decentering of the apertures and reasonable structure arrangement. The aperture stop of the Korsch system is located on the primary mirror, while those of the Cook systems are on the exit pupils. High resolution image with spectral coverage from visible to near-infrared (NIR) can be acquired through the Korsch system with a focal length of 14 m, while wide-field imaging is accomplished by the two Cook systems whose focal lengths are both 13.24 m. The full FOV is 4°×0.13°, a coverage width of 34.9 km at the altitude of 500 km can then be acquired by push-broom imaging. To facilitate controlling the stray light, the intermediate images and the real exit pupils are spatially available. After optimization, a near diffraction-limited performance and a compact optical package are achieved. The sharing of the on-axis primary and secondary mirrors reduces the cost of fabrication, test, and manufacture effectively. Besides, the two tertiary mirrors of the Cook systems possess the same parameters, further cutting down the cost.

Passive focusing techniques for piezoelectric air-coupled ultrasonic transducers

This paper proposes a novel passive focusing system for Air-Coupled Ultrasonic (ACU) piezoelectric transducers which is inspired by the Newtonian–Cassegrain (NC) telescope concept. It consist of a primary spherical mirror with an output hole and a flat secondary mirror, normal to the propagation axis, that is the transducer surface itself. The device is modeled and acoustic field is calculated showing a collimated beam with a symmetrical focus. A prototype according to this design is built and tested with an ACU piezoelectric transducer with center frequency at 400kHz, high-sensitivity, wideband and 25mm diameter flat aperture. The acoustic field is measured and compared with calculations. The presented prototype exhibit a 1.5mm focus width and a collimated beam up to 15mm off the output hole. In addition, the performance of this novel design is compared, both theoretically and experimentally, with two techniques used before for electrostatic transducers: the Fresnel Zone Plate – FZP and the off-axis parabolic or spherical mirror.The proposed NC arrangement has a coaxial design, which eases the transducers positioning and use in many applications, and is less bulky than off-axis mirrors. Unlike in off-axis mirrors, it is now possible to use a spherical primary mirror with minimum aberrations. FZP provides a more compact solution and is easy to build, but presents some background noise due to interference of waves diffracted at out of focus regions. By contrast, off-axis parabolic mirrors provide a well defined focus and are free from background noise, although they are bulky and more difficult to build. Spherical mirrors are more easily built, but this yields a non symmetric beam and a poorly defined focus.

Construction and Testing of Lightweight and Low-Cost Pneumatically Inflated Solar Concentrators

Design, construction, and evaluation of a cylindrical-trough solar concentrator with 1.3 m aperture, 2.15 m length, and 0.54 m focal length, with heat-pipe or vacuum tube receiver and one axis tracking system, are presented. Design performance was tested under ASHRAE standard 93-1986 (RA 91). The concentrator system is lightweight and inexpensive since it was made of polymeric membranes and was pneumatically inflated to acquire its cylindrical shape achieving good optical quality. Further implementation of a flat and a cylindrical extension of the concentrating mirror as secondary mirrors was incorporated into the concentrator design in order to compensate for seasonal variations of collected radiation. Total initial investment of $163.30 or $58.5/m2and efficiencies ranging from 33 to 25% for 25 up to 65°C show an excellent cost-performance ratio. Construction, costs, and efficiencies obtained by us and developed by other groups are compared to emphasize the high cost/benefit ratio and efficiencies of this approach.

基于传递矩阵的宽视场离轴三反光学系统设计

To the problem that off-axis three-mirror optical system has narrow field and complicating initial structure solving process, a method by using optical transfer matrix to solve initial structure was proposed, the focus length and rear cut-off distance of three-mirror system was deduced, then an initial structure parameter of three-mirror system with aperture stop on secondary mirror was given. High order aspheric surface was used to increase system design variance, and making off-axial optimization design to coaxial system based on optical design software ZEMAX, finally got an off-axis three-mirror optical system with wide rectangle field of view 172, focus length of 1 440 mm, F number of 4.8. The system has three high order aspheric optical elements, it can satisfy the requirement of high resolution and large field, the MTF is more than 0.6 at 50 lp/mm, approaches to the diffraction limitation. The designed result shows that the system can be used in space of imaging system with Timed Delay and Integrated Charge-couple Device(TDI-CCD), can expand the ground coverage of system obviously, and achieves high information retrieval efficiency.

基于主动约束阻尼层的次镜支撑结构设计

基于主动约束阻尼层的次镜支撑结构设计

大型光学载荷次镜调整机构优化设计及误差分配

大型光学载荷次镜调整机构优化设计及误差分配

基于主动约束阻尼层的次镜支撑结构设计

基于主动约束阻尼层的次镜支撑结构设计

Design of an Optical Fibre-based Receiver System for Atmospheric Sensing Lidar

In this paper we report on the design of an optical fiber-based light receiver which is meant for replacing the conventional beam steering secondary mirror of the Newtonian telescope of a biaxial Raman lidar. The influence of using optical fibre, acting as an aperture stop of constant diameter on the geometrical compression form factor; the transitional altitude and the resulting blur disk diameter have been determined. The viability of using a commercially available optical fibre as a receiver and the associated trade-offs have been discussed. A hot-fused fibre bundle-based receiver is proposed as a potential alternative for low altitude profiling to be enabled. These theoretical investigations have formed the basis for our current experimental work aimed at the development of all fibre-based reconfigurable beam delivery systems for mobile lidars. Full Text: PDF References A.M. Melesse et al., "Remote Sensing Sensors and Applications in Environmental Resources Mapping and Modelling", Sensors 51, 3209 (2007). CrossRef P. Wang et al., "A macrobending singlemode fiber refractive index sensor for low refractive index liquids", Phot. Lett Poland 2, 67 (2010). CrossRef J.R. Jenness et al., "Design of a lidar receiver with fiber-optic output", Appl. Optics 36, 4278 (1997). CrossRef J.V. Hey et al., "Determination of overlap in lidar systems", Appl. Optics 50, 5791 (2011). CrossRef K. Stelmaszczyk et al., "Analytical function for lidar geometrical compression form-factor calculations", Appl. Optics 44, 1323 (2005). CrossRef A.A.P. Boechat et al., "Bend loss in large core multimode optical fiber beam delivery systems", Appl. Optics 30, 321 (1991). CrossRef http://www.schott.com/china/chinese/download/284345_fo_light_it_right_e_without_tecspec.pdf DirectLink https://www.thorlabs.com/thorproduct.cfm?partnumber=APCH1500 DirectLink

Cassegrian Telescope Design Based on Root-Finding Optimization

A two-mirror Cassegrian telescope design is presented. The primary mirror is paraboloid with an aperture of 1m in diameter. The secondary mirror is hyperboloid with an aperture that makes a light obstruction of merely 0.25%. The telescope resolving power is twofold the one a spherical primary mirror produces with the same aperture. Aberration elimination is obtained by modifying the radius of curvature of the secondary mirror and also its asphericity factor. Achieving this work demanded constructing a programming code for tracing light rays. This code not only used to tracing the general type of light rays (skew ray) through all Cartesian surfaces but it is also used to exhibit the telescope performance when varying the secondary mirror radius of curvature and its asphericity factor.

On-axial defocused characteristic analysis for Cassegrain antenna in optical communication

In order to reduce the chromatic aberration problem of the multiple wavelength, reflex cassegrain antenna is widely used in space optical communication system. There are many external factors such as temperature and vibration that may cause defocusing in practice which has influence on Cassegrain antenna. That is to say, the primary mirror's and the secondary mirror's focus cannot be coincided, so it will bring impact greatly on transmission efficiency. This paper studies the effects on the transmission characteristics of the defocused Cassegrain antenna system. While defocused 2.5cm, the transmission efficiency of Cassegrain antenna will decrease 80% of the initial efficiency. So it is very important to maintain optimum position for high transmission efficiency.

Self-Tuning Mechanism for the Design of Adaptive Secondary Mirror Position Control

Deformable mirrors (DMs) are electromechanical devices used in ground-based telescopes to compensate for the distortions caused by the atmospheric turbulence, the main factor limiting the resolution of astronomical imaging. Adaptive secondary mirrors (ASMs) represent a new type of DMs; two of them have been recently installed on the 8-m-class large binocular telescope (LBT). ASMs are able to jointly correct rigid and nonrigid wave-front distortions thanks to the use of force actuators distributed on the overall mirror surface. As an offset, each actuator needs to be piloted by a dedicated controller, whose parameters must be accurately tuned to obtain the desired mirror shape. At the present time, the calibration of the controller parameters is executed manually. This paper presents a novel automatic controller tuning procedure that does not rely on the modeling of the mirror dynamics. The experimental validation on a prototype reproducing the three innermost rings of the LBT ASM is reported.

Lyot-plane phase masks for improved high-contrast imaging with a vortex coronagraph

The vortex coronagraph is an optical instrument that precisely removes on-axis starlight allowing for high contrast imaging at small angular separation from the star, thereby providing a crucial capability for direct detection and characterization of exoplanets and circumstellar disks. Telescopes with aperture obstructions, such as secondary mirrors and spider support structures, require advanced coronagraph designs to provide adequate starlight suppression. We introduce a phase-only Lyot-plane optic to the vortex coronagraph that offers improved contrast performance on telescopes with complicated apertures. Potential solutions for the European Extremely Large Telescope (E-ELT) are described and compared. Adding a Lyot-plane phase mask relocates residual starlight away from a region of the image plane thereby reducing stellar noise and improving sensitivity to off-axis companions. The phase mask is calculated using an iterative phase retrieval algorithm. Numerically, we achieve a contrast on the order of $10^{-6}$ for a companion with angular displacement as small as $4~\lambda/D$ with an E-ELT type aperture. Even in the presence of aberrations, improved performance is expected compared to either a conventional vortex coronagraph or optimized pupil plane phase element alone.