Secondary Mirror Research Articles

Elimination of the field-dependent aberrations of the JWST-like space telescopes in the multi-field fine-phasing process.

This paper investigates the alignment strategies for eliminating the field-dependent aberrations of the class of large three-mirror anastigmatic (TMA) space telescopes with a segmented primary mirror (PM) like the James Webb Space Telescope (JWST) in the multi-field fine-phasing process based on the framework of nodal aberration theory. During the single-field (on-axis field) fine-phasing process, the individual segment tip, tilt, and piston errors, as well as the de-space of the secondary mirror, are well corrected, and the PM is also adjusted to compensate for those aberrations induced by the misalignments of other mirrors at the center of the science field of view. However, interrogating off-axis field points can reveal the presence of large wavefront errors due to mirror misalignments. Eliminating these field-dependent aberrations is the main goal of the multi-field fine-phasing process. This paper first presents an analytic study on an established alignment strategy used for eliminating the field-dependent aberrations. While it is demonstrated that this alignment strategy has the ability to reduce the field dependency of the wavefront errors, it will, however, also be revealed that this strategy still exhibits some problems, and its alignment efficiency is low. Then, a new alignment strategy with higher alignment efficiency is further proposed. Detailed simulations with a TMA telescope that has similar parameters with the JWST are performed to illustrate the efficiency and rationality of the proposed strategy. This work can not only contribute to an in-depth understanding of the multi-field fine-phasing process, but also present a possibility to improve the efficiency of this process.

ADRC control of a 6-DOF parallel manipulator for telescope secondary mirror

In view of the special requirements of the secondary mirror control system on large aperture telescopes, an improved 6-DOF parallel manipulator is designed and used to replace the traditional hexapod used in telescope secondary mirror position dynamic compensation. A highly robust active disturbance rejection controller (ADRC) is designed, which consists of a nonlinear tracking differentiator (NTD), an extended state observer (ESO), a nonlinear state error feedback law (NLSEF), and disturbance compensation. The ESO can track the all-order state variables, as well as estimate and compensate for unmodeled dynamics and total external disturbance of the system. The results of simulation indicate that the ADRC can improve tracking precision and control performance when it is compared with the proportion integration differentiation (PID) controller. The test results show that the absolute accuracy of the three dimensional parallel motions is about ± 4 μm, and the two dimensional tilts' is about 10 μrad. The control precision meets the system design for a telescope secondary mirror.

Laser interferometer with aspherical holographic test glass for thermal vacuum chamber

A basic optical layout of a laser interferometer with an aspherical holographic test glass is proposed. It was designed to monitor the shape of the operating convex hyperbolic surface of the secondary mirror inside the space telescope Millimetron at cryogenic temperatures. The possibility of practical implementation of this technical solution is demonstrated.

New earth system model for optical performance evaluation of space instruments.

In this study, a new global earth system model is introduced for evaluating the optical performance of space instruments. Simultaneous imaging and spectroscopic results are provided using this global earth system model with fully resolved spatial, spectral, and temporal coverage of sub-models of the Earth. The sun sub-model is a Lambertian scattering sphere with a 6-h scale and 295 lines of solar spectral irradiance. The atmospheric sub-model has a 15-layer three-dimensional (3D) ellipsoid structure. The land sub-model uses spectral bidirectional reflectance distribution functions (BRDF) defined by a semi-empirical parametric kernel model. The ocean is modeled with the ocean spectral albedo after subtracting the total integrated scattering of the sun-glint scatter model. A hypothetical two-mirror Cassegrain telescope with a 300-mm-diameter aperture and 21.504 mm × 21.504-mm focal plane imaging instrument is designed. The simulated image results are compared with observational data from HRI-VIS measurements during the EPOXI mission for approximately 24 h from UTC Mar. 18, 2008. Next, the defocus mapping result and edge spread function (ESF) measuring result show that the distance between the primary and secondary mirror increases by 55.498 μm from the diffraction-limited condition. The shift of the focal plane is determined to be 5.813 mm shorter than that of the defocused focal plane, and this result is confirmed through the estimation of point spread function (PSF) measurements. This study shows that the earth system model combined with an instrument model is a powerful tool that can greatly help the development phase of instrument missions.

A novel 2-stage dish concentrator with improved optical performance for concentrating solar power plants

Here we propose a novel 2-stage dish concentrator system for concentrating solar power plants based on the so-called overlap method, which improves both the radial flux concentration ratio (β) and the intercept factor (γ) of the concentrator. The 2-stage dish is composed of paraboloid primary and hyperboloid secondary mirrors having a special hollow structure enabling the mirrors to overlap, but avoiding excess shading. Geometric and optical analyses with ray-tracing show maximum values of β = 15,498 and γ = 0.78. Also, the size of the focal spot could be reduced. The concept is applicable for solar tower concepts enabling more compact unit mirrors and reducing the receiver size.

Design and optimization for main support structure of a large-area off-axis three-mirror space camera.

To ensure excellent dynamic and static performance of large-area, off-axis three-mirror anastigmat (TMA)-space cameras, and to realize a lighter weight for the entire system, a truss support structure design is applied in this study. In contrast to traditional methods, this paper adopts topology optimization based on the solid isotropic materials with penalization method on the truss structure design. Through reasonable object function and constraint choice, optimal topology results that have concerned the effect of gravity in the X, Y, and Z axis are achieved. Subsequently, the initial truss structure is designed based on the results and manufacturing technology. Moreover, to reduce the random vibration response of the secondary mirror and fold mirror without mechanical performance decline of the whole truss, a weighted optimization of truss size is proposed and the final truss structure is achieved. Finite element analysis and experiments have confirmed the reliability of the design and optimization method. The designed truss-structure camera maintains excellent static performance with the relative optical axis angle between the primary mirror and corresponding mirrors (secondary mirror and fold mirror) being less than 5.3 in. Dynamic performances, such as random and sinusoidal vibration responses, also met the requirements that the acceleration RMS value for mount points of the fold mirror should be less than 20 g and the primary frequency reached 97.2 Hz.

Short-exposure turbulence effects on the modulation transfer function of annular aperture optical systems

The results of recent analytical work performed to characterize the short-exposure blurring effects of path integrated optical turbulence for annular aperture systems is described using the modulation transfer function. Results presented here extend the circular case to the annular aperture problem involving a circularly symmetric central obscuration associated with telescope optics secondary mirrors. To enable this extension, adjustments to procedures that compute the tilt-variance and phase-tilt correlation effects are described, and the results are illustrated. A four-dimensionally parameterized function of turbulence strength, diffraction influence, angular frequency, and fractional central obscuration is given.

Testing of a Convex Aspheric Secondary Mirror for the Cassegrain Telescope

Testing of a Convex Aspheric Secondary Mirror for the Cassegrain Telescope

1.2 m望远镜次镜支撑结构设计

1.2 m望远镜次镜支撑结构设计

First on-sky demonstration of the piezoelectric adaptive secondary mirror.

We propose using a piezoelectric adaptive secondary mirror (PASM) in the medium-sized adaptive telescopes with a 2-4m aperture for structure and control simplification by utilizing the piezoelectric actuators in contrast with the voice-coil adaptive secondary mirror. A closed-loop experimental setup was built for on-sky demonstration of the 73-element PASM developed by our laboratory. In this Letter, the PASM and the closed-loop adaptive optics system are introduced. High-resolution stellar images were obtained by using the PASM to correct high-order wavefront errors in May 2016. To the best of our knowledge, this is the first successful on-sky demonstration of the PASM. The results show that with the PASM as the deformable mirror, the angular resolution of the 1.8m telescope can be effectively improved.

Orientation of radio-telescope secondary mirror via adaptive sliding mode control

In this work a parallel manipulator (Stewart platform) is used to align and maintain the position of the secondary mirror of a radio-telescope. The six degrees of freedom platform gives the significant advantage of reaching the maximum performance for the positioning tasks. The near-singularity condition of the platform is analyzed and is handled by implementation of a new control law based on sliding mode with inner regularization procedure. Herein, the finite-time convergence of closed-loop system derived from designed control in the presence of external as well as internal disturbances/uncertainties is proved. The effectiveness of the proposed controller is verified via numerical simulation. We show that Sliding Mode Control with a gain matrix adaptation based on the Equivalent Control method can significantly reduce the undesirable chattering effect an therefore avoid the possible damages.

Automated system for reduction of observational data on RATAN-600 radio telescope

We present the automated systemfor estimating the parameters of radio sources observed on all available continuum radiometers (two receiving facilities of secondary mirrors No. 1 and No. 2 with a total of 30 radiometers) developed at RATAN-600 radio telescope and put into normal operation. The system is also used for the monitoring of the parameters of the antenna and receiving systems of RATAN-600 radio telescope, which is carried out using current measurements of calibration radio sources.

Optical System Design for a Head-up Display Using Aberration Analysis of an Off-axis Two-mirror System

This study presents a new optical system for a combiner-type head-up display (HUD) with a cylindrical lens as an asymmetrical aberration corrector, instead of a freeform mirror. In the initial design process based on off-axial aberration analysis, we obtain an off-axis two-mirror system corrected for linear astigmatism and spherical aberration by adding a conic secondary mirror to an off-axis paraboloidal mirror. Thus, since the starting optical system for an HUD is corrected for dominant aberrations, it enables us to balance the residual asymmetrical aberrations with a simple optical surface such as a cylinder, not a complex freeform surface. From this design process, an optical system for an HUD having good performance is finally obtained. The size of the virtual image is 10 inches at 2 meters away from a combiner, and the area of the eye box is 130×50 mm2.

A novel free-form Cassegrain concentrator for PV/T combining utilization

The conventional imaging solar concentrators normally obtain a Gaussian distribution on the receiving surface. The aperture size of a thermal receiver (TR) is always limited primarily to the high concentration area, discarding the edge part to be lost. The current study proposes a novel free-form Cassegrain concentrator (FFCC) for PV/T combining utilization. The remaining heat flux by the edge of focal spot is directed by the secondary mirror for electric generation. The profile of secondary reflector is constructed by the geometric construction method (GCM), aiming at achieving high concentration for the TR and uniform flux distribution for the PV receiver (PVR). The energy transmission characteristics of FFCC, including the incident vectors and heat flux distribution on the TR/PVR are both investigated. The effects of changing the receiving locations, PV/T dividing ratio, etc. are analyzed. The sun size/solar conic angular is also considered for practical use. The GCM method and simulation results in this article can provide a reference for the PV/T solar energy system and free form surface design.

Laboratory demonstration of a dual-stage vortex coronagraph

While an ideal optical vortex coronagraph operating behind a clear, circular, unaberrated telescope aperture can theoretically provide perfect rejection of the incident plane wave from an unresolved star, use of a telescope with an on-axis secondary mirror limits the rejection. In theory, a dual-stage vortex coronagraph can provide improved starlight rejection for an on-axis telescope, and here we provide experimental confirmation of the predicted distribution of the residual light in the output pupil plane of a dual-stage vortex coronagraph. In addition, a simple method of further improving the rejection of such a coronagraph is suggested: by slightly oversizing the first Lyot stop and phase-shifting the light within the exposed annulus by half a wave, the residual starlight within the pupil can be canceled to deeper levels.

Reflective Schmidt-Cassegrain system for large-aperture telescopes.

A reflective modification of the Schmidt-Cassegrain system was built and tested. Ultraviolet (UV) and soft x-ray applications are discussed. The system consists of a planoid mirror with an aspheric profile and prime concave and secondary convex spherical mirrors. Spherical aberration in a wide field of view and astigmatism are compensated by the aspheric profile of the planoid. The main parameters of the scheme are as follows: an entrance aperture of 180 mm, a focal ratio F/3.2, an angular resolution better than 3'' (corresponding to a pixel size of a back-side illuminated CCD), a field of view of ±1.5° (2ω=3°) and a flat image field with a diameter of 30.4 mm. Due to the absence of chromatic aberrations and wide field of view, the scheme is of considerable interest for hyperspectral instruments. In particular, the operating range of the instruments can be expanded into vacuum UV and UV regions.

Modeling pyramidal sensors in ray-tracing software by a suitable user-defined surface

Following the unprecedented results in terms of performances delivered by the first light adaptive optics system at the Large Binocular Telescope, there has been a wide-spread and increasing interest on the pyramid wavefront sensor (PWFS), which is the key component, together with the adaptive secondary mirror, of the adaptive optics (AO) module. Currently, there is no straightforward way to model a PWFS in standard sequential ray-tracing software. Common modeling strategies tend to be user-specific and, in general, are unsatisfactory for general applications. To address this problem, we have developed an approach to PWFS modeling based on user-defined surface (UDS), whose properties reside in a specific code written in C language, for the ray-tracing software ZEMAX™. With our approach, the pyramid optical component is implemented as a standard surface in ZEMAX™, exploiting its dynamic link library (DLL) conversion then greatly simplifying ray tracing and analysis. We have utilized the pyramid UDS DLL surface—referred to as pyramidal acronyms may be too risky (PAM2R)—in order to design the current PWFS-based AO system for the Giant Magellan Telescope, evaluating tolerances, with particular attention to the angular sensitivities, by means of sequential ray-tracing tools only, thus verifying PAM2R reliability and robustness. This work indicates that PAM2R makes the design of PWFS as simple as that of other optical standard components. This is particularly suitable with the advent of the extremely large telescopes era for which complexity is definitely one of the main challenges.

Simulating the optical performance of a small-sized telescope with secondary optics for the Cherenkov Telescope Array

The Gamma-ray Cherenkov Telescope (GCT) is a small-sized telescope (SST) that represents one of three novel designs that are based on Schwarzschild–Couder optics and are proposed for use within the Cherenkov Telescope Array (CTA). The GAmma-ray Telescope Elements (GATE) program has led an effort to build a prototype of the GCT at the Paris Observatory in Meudon, France. The mechanical structure of the prototype, known as the SST-GATE prototype telescope, is now complete along with the successful installation of the camera. We present the results of extensive simulation work to determine the optical performance of the SST-GATE prototype telescope. Using the ROBAST software and assuming an ideal optical system, we find the radius of the encircled point spread function (θ80) of the SST-GATE to be ∼1.3 arcmin (∼0.02°) for an on-axis (θfield=0∘) observation and ∼3.6 arcmin (∼0.06°) for an observation at the edge of the field of view (θfield=4.4∘). In addition, this research highlights the shadowing that results from the stopping of light rays by various telescope components such as the support masts and trusses. It is shown that for on-axis observations the effective collection area decreases by approximately 1 m2 as a result of shadowing components other than the secondary mirror. This is a similar loss (∼11%) to that seen with the current generation of conventional Davies–Cotton (DC) Cherenkov telescopes. An extensive random tolerance analysis was also performed and it was found that certain parameters, especially the secondary mirror z-position and the tip and tilt rotations of the mirrors, are critical in order to contain θ80 within the pixel limit radius for all field angles. In addition, we have studied the impact upon the optical performance of introducing a hole in the center of the secondary mirror for use with pointing and alignment instruments. We find that a small circular area (radius < 150 mm) at the center of the secondary mirror can be used for instrumentation without any significant impact upon optical performance. Finally, we studied the impact of reducing the size of the primary mirror for the prototype telescope and found that this comes at the cost of poorer image quality and light collection efficiency for all field angles, but at a significant cost saving for a one-off prototype.

Thermal refocusing method for spaceborne high-resolution optical imagers.

We describe the design of a thermal refocusing method for spaceborne high-resolution imagers where Korsch optical design is usually implemented. The secondary mirror is made of aluminum, a high thermal expansion coefficient material, instead of conventional zero-expansion glass ceramics. In this way, the radius of the curvature can be controlled by means of temperature change of the mirror. Change in the radius of curvature also changes the effective focal length of the camera which is used for compensation of the defocus that occurred in space. We show that the 30 μm despace of the secondary mirror in the optical system can be compensated by an ∼10°C temperature change of the mirror while the image quality is maintained.

A suggested 12-m telescope optical system configuration for China

This is an optical/infrared telescope. It is a general purpose telescope. A segmented primary mirror is adopted with diameter 12 m and f-ratio 1.6. This telescope has prime focus, Cassegrain, Nasmyth and coud´e systems. The prime focus system has a 1.◦5 field-of-view corrector with excellent image quality. It will be used for multi-object fibre spectroscopic observation and CCD photography. In this 12-m telescope, all systems except the prime focus system share the same secondary mirror; the Nasmyth and coud´e systems are formed byadding relay mirrors; and the method of moving a secondary mirror is used to enhance image quality. All these features originate from the innovative optical system of the Chinese 2.16-m telescope. At the Cassegrain focus, a dark object spectrograph, for example, can be installed without any corrector; and in case a field of view is needed, a corrector may be added. Both the Nasmyth and coud´e systems have exceptionally excellent image quality and the exchange between them occurs very conveniently. Many instruments in visible and infrared wavebands, some with an adaptive optics system, will be installed at the Nasmyth platform. Coud´e system will be used for interferometry and for those instruments which require high stability. This configuration has a nearly full range of systems but it is not complex, and it can even be adopted by 20–40-m class telescopes.