Secondary Mirror Research Articles

Linear Fresnel Collector Receiver: Heat Loss and Temperatures

For design and component specification of a Linear Fresnel Collector (LFC) cavity receiver, the prediction of temperature distribution and heat loss is of great importance. In this paper we present a sensitivity analysis for a range of geometry and material parameters. For the LFC receiver analysis we use two models developed at Fraunhofer ISE. One is a detailed model, combining the spatial distribution of reflected radiation via ray tracing with detailed convective simulations through computational fluid dynamics. The second one is a fast algorithm based on a thermal resistance model. It is applying a similar methodology as the well-known model for vacuum absorber, enhancing an absorber tube model by parameters describing the influence of the secondary mirror and cover glass. The thermal resistance model is described in detail. Obtained results indicate a significant effect of the secondary mirror temperature on heat loss for specific geometries.

Optical Design of a Reflecting Telescope for CubeSat

Space telescope optics is one of the major parts of any space mission used to observe astronomical targets or the Earth. This kind of space mission typically involves bulky and complex opto-mechanics with a long optical tube, but attempts have been made to observe a target with a small satellite. In this paper, we describe the optical design of a reflecting telescope for use in a CubeSat mission. For this design we adopt the off-axis segmented method for astronomical observation techniques based on a Ritchey-Chr $\acute{e}$ tien type telescope. The primary mirror shape is a rectangle with dimensions of $8cm{\times}8cm$ , and the secondary mirror has dimensions of $2.4cm{\times}4.1cm$ . The focal ratio is 3 which can yield a 0.383 degree diagonal angle in a $1280{\times}800$ CMOS color image sensor with a pixel size of $3{\mu}m{\times}3{\mu}m$ . This optical design can capture a ${\sim}4km{\times}{\sim}2.3km$ area of the earth's surface at 700 km altitude operation.

Optical design for divertor Thomson scattering system for JT-60SA

Optical design for divertor Thomson scattering system in JT-60SA has been conducted. The measurement system will use a Nd:YAG laser at 1064nm, and scattered photons are collected by a collection optical system. The collection optics consists of primary mirror, secondary mirror, relay optics, and fiber collection optics. The laser transmission mirror and collection optics were designed to be installed in a slender lower port of JT-60SA. The assessment of the measurement errors in temperature was conducted for the designed collection optical system. Because of spatial limitation, the solid angle from the measurement points would be small especially for the measurement points in high field side, and consequently, the temperature errors in the high field side would be considerably large. The effects of several improvements on the error are discussed. Moreover, an assessment for the in-vessel laser transmission metallic mirrors is conducted for the present design.

Design of Secondary Optical Element for a Two-Reflector Solar Concentrator

This study explores the optimum design of secondary optical element (SOE) for a non-coplanar two-reflector solar concentrator. The non-coplanar solar concentrator comprises a primary parabolic mirror and a secondary hyperbolic mirror. Ray tracing simulation is adopted to investigate the optical performance, including the acceptance angle and the optical efficiency, of the solar concentrator with various designs of SOE. Finally, preliminary optimum designs of SOE are presented and discussed.

A kinematic calibration method of a 3-DOF secondary mirror of the giant magellan telescope based on least square algorithm

Because of manufacturing and assembly errors in the geometric parameters of parallel robots, calibrating the parameters is necessary for improving the positioning accuracy. The objective of this study is a 3-DOF secondary mirror of the giant Magellan telescope, which is operated by a 3-DOF parallel robot. In this paper, a kinematic calibration method is presented. In this method, a moving platform is commanded to execute a sinusoidal movement by the cooperation of three piezoelectric actuators, and experimental data is obtained using capacitive sensors. The inverse kinematic and forward kinematic equations for the local system are illustrated in this paper. Furthermore, importantly, a least square algorithm is implemented to reduce the residual between the experimental values and expected values. Additionally, a simulation model is developed for supervising the movement of the system. Results of experiment show that the method used in this paper is effective and efficient.

An optimum structure design for Cassegrain optical system

Because the energy loss caused by the secondary mirror of Cassegrain antenna is seriously, an optimum structure is designed to improve the transmitting energy of Cassegrain optical system. It is made up of Cassegrain antenna and Double pyramidal system. The parameter of Cassegrain antenna has been analyzed, and it has been revealed that relationship between the Double pyramidal system and Cassegrain antenna. Finally, the relationship between the optimum parameters of Cassegrain and the transmitting efficiency has been researched. Through this optimum structure, the emitting power can be achieved to 91.5%.

Comparison of different types of secondary mirrors for solar application

Two-stage solar concentrators make solar beams downwards providing flexible choices for energy utilization. Five types of secondary mirrors (a flat mirror, an ellipsoidal mirror, a hyperboloidal mirror with upper/lower sheet and a paraboloidal mirror) are compared. Effects of geometry parameters and concentrator precisions on the optical performance are analyzed using Advanced System Analysis Program. The results indicate that concentrators with a flat mirror or hyperboloidal mirror with lower sheet are more sensitive to rim angle or relative location. The secondary mirror is better a convex surface especially when rim angle is more than 90°. A flat mirror or hyperboloidal mirror with lower sheet performs better with higher redirect focal points. A hyperboloidal mirror with upper sheet is the best however numerical aperture changing. The intercept factors decreased with the increase of random errors or optical errors. Both the fabrication and assemblage requirements for a concentrator with a hyperboloidal mirror with lower sheet are the strictest. Experiments are carried out based on a hyperboloidal mirror with upper sheet. The experiments results are in accordance with the ray-tracing results. Therefore, further studies on optimization of the two-stage concentrators using the ray-tracing model can be conducted.

MOSE: operational forecast of the optical turbulence and atmospheric parameters at European Southern Observatory ground-based sites – II. Atmospheric parameters in the surface layer 0–30 m

This article is the second of a series of articles aiming at proving the feasibility of the forecast of all the most relevant classical atmospherical parameters for astronomical applications (wind speed and direction, temperature, relative humidity) and the optical turbulence (Cn2 and the derived astro-climatic parameters like seeing, isoplanatic angle, wavefront coherence time...). This study is done in the framework of the MOSE project, and focused above the two ESO ground-bases sites of Cerro Paranal and Cerro Armazones. In this paper we present the results related to the Meso-Nh model ability in reconstructing the surface layer atmospherical parameters (wind speed intensity, wind direction and absolute temperature, [0-30] m a.g.l.). The model reconstruction of all the atmospherical parameters in the surface layer is very satisfactory. For the temperature, at all levels, the RMSE (Root Mean Square Error) is inferior to 1{\deg}C. For the wind speed, it is ~2 m/s, and for the wind direction, it is in the range [38-46{\deg}], at all levels, that corresponds to a RMSE_relative in a range [21-26{\deg}]. If a filter is applied for the wind direction (the winds inferior to 3 m/s are discarded from the computations), the wind direction RMSE is in the range [30-41{\deg}], i.e. a RMSE_relative in the range [17-23{\deg}]. The model operational forecast of the surface layer atmospherical parameters is suitable for different applications, among others: thermalization of the dome using the reconstructed temperature, hours in advance, of the beginning the night; knowing in advance the main direction which the strong winds will come from during the night could allow the astronomer to anticipate the occurrence of a good/bad seeing night, and plan the observations accordingly; preventing adaptive secondary mirrors shake generated by the wind speed.

RING-APODIZED VORTEX CORONAGRAPHS FOR OBSCURED TELESCOPES. I. TRANSMISSIVE RING APODIZERS

The vortex coronagraph (VC) is a new generation small inner working angle (IWA) coronagraph currently offered on various 8-meter class ground-based telescopes. On these observing platforms, the current level of performance is not limited by the intrinsic properties of actual vortex devices, but by wavefront control residuals and incoherent background (e.g. thermal emission of the sky) or the light diffracted by the imprint of the secondary mirror and support structures on the telescope pupil. In the particular case of unfriendly apertures (mainly large central obscuration) when very high contrast is needed (e.g. direct imaging of older exoplanets with extremely large telescopes or space- based coronagraphs), a simple VC, as most coronagraphs, can not deliver its nominal performance because of the contamination due to the diffraction from the obscured part of the pupil. Here we propose a novel yet simple concept that circumvents this problem. We combine a vortex phase mask in the image plane of a high-contrast instrument with a single pupil-based amplitude ring apodizer, tailor designed to exploit the unique convolution properties of the VC at the Lyot-stop plane. We show that such a ring-apodized vortex coronagraph (RAVC) restores the perfect attenuation property of the VC regardless of the size of the central obscuration, and for any (even) topological charge of the vortex. More importantly the RAVC maintains the IWA and conserves a fairly high throughput, which are signature properties of the VC.

Herschel SPIRE FTS telescope model correction

Emission from the Herschel telescope is the dominant source of radiation for the majority of SPIRE Fourier transform spectrometer (FTS) observations, despite the exceptionally low emissivity of the primary and secondary mirrors. Accurate modelling and removal of the telescope contribution is, therefore, an important and challenging aspect of FTS calibration and data reduction pipeline. A dust-contaminated telescope model with time invariant mirror emissivity was adopted before the Herschel launch. However, measured FTS spectra show a clear evolution of the telescope contribution over the mission and strong need for a correction to the standard telescope model in order to reduce residual background (of up to 7 Jy) in the final data products. Systematic changes in observations of dark sky, taken over the course of the mission, provide a measure of the evolution between observed telescope emission and the telescope model. These dark sky observations have been used to derive a time dependent correction to the telescope emissivity that reduces the systematic error in the continuum of the final FTS spectra to ~0.35 Jy.

Alternative design for extremely large telescopes and options to use the VATT for ELT design demonstration

Abstract A variety of optical designs for extremely large telescopes (ELTs) can be found throughout the technical literature. Most feature very fast primary mirrors of either conic or spherical figure. For those designs with conic primary mirrors, many of the optical approaches tend to be derivatives of either the aplanatic Cassegrain or Gregorian systems. The Cassegrain approach is more common as it results in a shorter optical system, but it requires a large convex aspheric secondary mirror, which is extremely difficult and expensive to test. The Gregorian approach is physically longer and suffers from greater field curvature. In some design variations, additional mirrors are added to reimage and possibly flatten a Cassegrain focus. An interesting alternative ELT design uses a small Cassegrain system to image the collimated output of a Gregorian-Mersenne concentrator. Another alternative approach, currently in favor for use on the European ELT, uses three powered mirrors and two flat mirrors to reimage a Cassegrain focus out the side similar to a Nasmyth system. A preliminary examination suggests that a small, fast primary mirror, such as that used on the VATT, might be used for a subscale prototype of current ELT optical design options.

Optical designs of reflection and refraction collection optics for a JT-60SA core Thomson scattering system

Collection optics for core measurements in a JT-60SA Thomson scattering system were designed. The collection optics will be installed in a limited space and have a wide field of view and wide wavelength range. Two types of the optics are therefore suggested: refraction and reflection types. The reflection system, with a large primary mirror, avoids large chromatic aberrations. Because the size limit of the primary mirror and vignetting due to the secondary mirror affect the total collection throughput, conditions that provide the high throughput are found through an optimization. A refraction system with four lenses forming an Ernostar system is also employed. The use of high-refractive-index glass materials enhances the freedom of the lens curvatures, resulting in suppression of the spherical and coma aberration. Moreover, sufficient throughput can be achieved, even with smaller lenses than that of a previous design given in [H. Tojo, T. Hatae, T. Sakuma, T. Hamano, K. Itami, Y. Aida, S. Suitoh, and D. Fujie, Rev. Sci. Instrum. 81, 10D539 (2010)]. The optical resolutions of the reflection and refraction systems are both sufficient for understanding the spatial structures in plasma. In particular, the spot sizes at the image of the optics are evaluated as ~0.3 mm and ~0.4 mm, respectively. The throughput for the two systems, including the pupil size and transmissivity, are also compared. The results show that good measurement accuracy (<10%) even at high electron temperatures (<30 keV) can be expected in the refraction system.

Modeling and Simulation of Chinese SONG Telescope

Telescope structure is used to support and position the optical mirrors in the right place during the period of telescope observation, so it will have such capability to withstand a certain load. In order to ensure SONG telescope has such capability, FEM analysis and calculation are finished in this paper. With the help of ANSYS, firstly the finite element model of SONG telescope is built. Then using this model, the analysis and calculation of static and dynamic are performed, including the deformation under the gravity load with the tube in different directions, eigenfrequency calculation of the whole system and the responses under the seismic acceleration spectra. The results show that the max. offsets and max. relative tilt angles between primary mirror and secondary mirror are very little under the gravity load. The first natural frequency is 20.083Hz, larger than the design requirement 8Hz. The results of seismic load analysis show that the deformation in the telescope structure is 0.42mm and the stress is 33.8MPa, less than the allowable stress of the structure material. The above analyses verified that the structure design of SONG telescope is reliable and acceptable.

Sensitivity analysis of a wide-field telescope

We are developing three ground-based wide-field telescopes. A wide-field Cassegrain telescope consists of two hyperbolic mirrors, aberration correctors and a field flattener for a 2-degree field of view. The diameters of the primary mirror and the secondary mirror are 500 mm and 200 mm, respectively. Corrective optics combined with four lenses, a filter and a window are also considered. For the imaging detection device, we use a charge coupled device (CCD) which has a 4096 × 4096 array with a 9-µm2 pixel size. One of the requirements is that the image motion limit of the opto-mechanical structure be less than 1 pixel size of the CCD on the image plane. To meet this requirement, we carried out an optical design evaluation and a misalignment analysis. Line-of-sight sensitivity equations are obtained from the rigid-body rotation in three directions and the rigid-body translation in three directions. These equations express the image motions at the image plane in terms of the independent motions of the optical components. We conducted a response simulation to evaluate the finite element method models under static load conditions, and the result is represented by the static response function. We show that the wide-field telescope system is stiff and stable enough to be supported and operated during its operating time.

Techno-economic appraisal of a dish/stirling solar power plant in Greece based on an innovative solar concentrator formed by elastic film

As the environmental problems caused by the use of conventional fuels have risen significantly and due to the increase in fossil fuel prices, the importance of reducing society dependence on non-renewable energy sources becomes more and more urgent. It is clear that this could only happen by switching to extensive use of clean energy sources such as renewable energy sources. In this paper a technical feasibility and economic viability study of a dish/Stirling solar power plant in Greece is presented. The proposed power plant uses Dish/Stirling technology and has nominal power 10 MW. The proposed solar concentrator system consists of a primary mirror, formed by elastic film, used to focus the sun's rays onto a secondary mirror which then deviates this radiation into the receiver (Stirling engine). TRNSYS program was developed to simulate Dish/Stirling power plant operation in order to investigate its performance in Greece. The simulation predicted that the proposed power plant could produce 11.19 GWh annually. Project investment cost is approximately 27,000,000 €, while payback is achieved after 16 years of operation. The total profit at the end of the analysis period of 25 years is estimated to be 25,500,000 €.

The Large Binocular Telescope Interferometer &amp; Adaptive Optics System: On-sky Performance and Results

AbstractIncreasing spatial resolution and contrast capabilities will make possible new direct detections of exoplanets, exozodis, and circumstellar disks. The Large Binocular Telescope Interferometer (LBTI) has been engineered to sit at the combined focus of the Large Binocular Telescope's two 8.4m apertures. Both apertures are equipped with 672-actuator deformable secondary mirrors, the first of the next generation of “extreme” adaptive optics (AO) systems. We present an overview of the LBTI AO instrument suite and detail current on-sky performance.

Structure Design and Finite Element Analysis of Receiving System for Optical Communication

Along with the rapid development of science and technology, and due to the special advantage of laser communication, such as big traffic and good secrecy, the application of optical communication is more and more wide. One of key technologies is the structure design of receiving system. In this paper, a structure of receiving system for optical communication based on Cassegrain system is introduced. Consist of the support structure of primary mirror and secondary mirror, adjustment structure of secondary mirror, structure design of reflectors, etc. FEM is used to analyze the distortion of primary mirror which is based on this structure. The distortion is caused by pressure, self-weight and temperature change, etc. And the stress deformation of primary mirror tube is analysed particularly. Analysis and test results demonstrate that the design project is reasonable.

Design method of non-imaging secondary (NIS) for CPV usage

Analytical method is used to obtain the geometrical shape for non-imaging secondary (NIS) mirror in the application of concentrated photovoltaic (CPV). It is discussed in beam down optical system, NIS presents advantages to eliminate dark image, to reduce the solar disc effect, to improve the uniformity of the illumination, etc. comparing to the imaging secondary. The authors have emphasized that the geometrical shape of optical components in non-imaging optics can generally be high order surfaces not restricted to the quadratic surface in imaging optics and mostly “CPC” in early developed non-imaging optics. This paper has discussed the general criteria and two practical approaches for designing NIS. We listed some of the examples in this paper to form NIS by rotating a segment of linear or quadratic or cubic curve around the central axis of primary reflector. The method to calculate the parameters of such segment is described. Although in present discussion, we are referring the usage in PV concentrator, the method is generally applied in other optical fields.

A Design of Mid-wave Infrared Integral Catadioptric Optical System with Wide FOV

In order to deduce the difficulty of fixing the Ritchey-Chretien (R-C) dual reflective optical system and enhance the stability of the secondary mirror, a compact integral structure is presented here composed of two transmitting and two reflective aspheric surfaces. The four surfaces were manufactured from a single germanium lens and integrated together. The two reflective surfaces formed by coating the inner reflecting films were assembled in one lens. It makes the installation of the two mirrors easier and the structure of the secondary mirror more stable. A design of mid-wave infrared (MWIR) compact imaging system is presented with a spectral range chosen as <TEX>$3.7-4.8{\mu}m$</TEX>. The effective focal length is f=90 mm. The field of view (FOV) for the lens is <TEX>$4.88^{\circ}$</TEX>. It has good imaging capability with Modulation Transfer Function (MTF) of all field of view more than 0.55 close to the diffraction limitation. Outdoor experiments were carried out and it is shown that the integral catadioptric optical system performs well on imaging.

Optimizing the shaping technology and test methods for convex aspheric surfaces of large optical items

This paper discusses the automatic shaping technology for the convex hyperbolic surfaces of the secondary mirrors of telescopes with asphericity to 400 ?m, 1 m or more in diameter. It describes a method of testing a convex mirror with an auxiliary Hindle sphere and of optimizing this method by using two Hindle spheres of smaller diameter for one item instead of one sphere of greater diameter. A method is proposed of joining the topographical maps obtained from the results of testing the surface of a hyperboloid with two spheres. The features of fabricated auxiliary equipment (load-bearing fixtures, containers, devices for cementing interface elements) are presented.