Reflective Telescope Research Articles

Misalignment Calculation on Off-Axis Telescope System via Fully Connected Neural Network

On the process of alignment for pupil off-axis telescope system, the determination of decentering and tilt misalignments is a key step. In this paper, we proposed a new method which adopts the fully connected neural network (FCNN) as a fitting tool to establish the nonlinear mapping relation between misalignments of different fields of view (FOVs) and Zernike coefficients. Firstly, we establish a pupil off-axis reflection telescope model, then decentering and tilt misalignments are introduced to acquire corresponding aberrations that represented by Zernike coefficients. We use aberrations as the inputs of FCNN; misalignments and FOV as outputs. FCNN is trained by the combination of inputs and outputs as a dataset, and we use a new dataset to test the accuracy and effectiveness of the trained FCNN. The results show that the mean absolute error (MAE) of the X-axis decentering error, V-axis tilt error and angle of view are 0.0506 mm, 0.0204° and 0.0124°, respectively. These results demonstrate that the proposed method is effective and feasible to calculate the misalignments.

Design of a hybrid refractive-diffractive telescope for observations in UV

Telescopes are one of the common types of satellite payloads. They are used both for Earth and astronomical observations. By using space telescopes it is possible to eliminate the negative effect of the atmosphere on image quality. Additionally, observations in some spectral ranges can be performed only from space due to absorption of certain wavelengths in the atmosphere. One such range is UV below 300 nm, which is of particular interest when it comes to the investigation of hot objects. Reflective telescopes are commonly used in this spectral range, although many classical designs are limited in their useful field of view to values below 1°. In this paper a hybrid refractive-diffractive telescope design working in a 200 nm – 300 nm spectral range with a field of view 10°×10° is proposed. Its performance is compared to purely refractive and reflective systems and significant improvement in the imaging quality of the system and decrease of its size is shown. The choice of the diffractive element type is explained. Parameters of the systems are based on the requirements for a proposed Polish mission UVSat which aims to enable long-term observations of a large number of stars exhibiting UV variance.

Transformable Reflective Telescope for optical testing and education

We propose and experimentally demonstrate the Transformable Reflective Telescope (TRT) kit for educational purposes and for performing various optical tests with a single kit. The TRT kit is a portable optical bench setup suitable for interferometry, spectroscopy, measuring stray light, and developing adaptive optics, among other uses. Supplementary modules may be integrated easily due to the modular design of the TRT kit. The kit consists of five units: a primary mirror module, secondary mirror module, mounting base module, baffle module, and alignment module. Precise alignment and focusing are achieved using a precision optical rail on the alignment module. The TRT kit transforms into three telescope configurations: Newtonian, Cassegrain, and Gregorian. Students change telescope configurations by exchanging the secondary mirror. The portable design and the aluminum primary mirror of the TRT kit enable students to perform experiments in various environments. The minimized baffle design utilizes commercial telescope tubes, allowing users to look directly into the optical system while suppressing stray light down to ∼ 10 − 8 point source transmittance. The TRT kit was tested using a point source and field images. Point source measurement of the Newtonian telescope configuration resulted in an 80% encircled energy diameter of 23.8 µm.

Optimal Sphere-Sensor Ratio for Microsphere-Enhanced Photodetector Sensors in Cameras/Telescopes With and Without Central Obscuration

A microsphere lens on top of a photodetector sensor increases its sensitivity and reduces its Noise-to-Signal Ratio (NSR), as shown in our previous work. This paper discusses a methodology to determine the optimal sphere-sensor ratio, in terms of increased current sensitivity, for microsphere-lens-enhanced photodetectors, using computational analysis including a modeling, with an experimentally derived photo-detector sensor responsivity and simulation setup that emulates conditions generally found within cameras and telescopes. In particular, we study and analyze the effect that several variables have on such ratios. We focus on variables such as microsphere material type, photodetector hardware technology type, light source temperature, and in the case of reflective telescopes, central obscurations from a secondary mirror. Results of the effect of microsphere-sensor misalignment on such ratios are also presented. As we show in this paper, since the optimized ratio is dependent on a given wavenumber considered, therefore the proposed methodology uses an integral optimization over a given spectral band to derive the optimized ratio over that band. Some of our findings show, for the first time, that the optimal ratio is virtually independent of a given central obscuration. Finally, we demonstrate here the flexibility of the proposed methodology in showing performances of microsphere-enhanced photodetector sensors.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ СКОРОСТИ ПНЕВМАТИЧЕСКОГО НАГРУЖЕНИЯ ЦИЛИНДРИЧЕСКОЙ КРУПНОГАБАРИТНОЙ ОБОЛОЧКИ НА ЕЕ ДИНАМИЧЕСКИЕ ПАРАМЕТРЫ

The solid mechanics problem on the packaging and subsequent pneumatic deployment of large-sized shells is relevant due to the need to operate such structures in hard-to-reach places. In particular, such a place is outer space. The paper considers the elastic cylindrical shell deployment from a packed state by internal pressure in a finite time interval. The packaged shell consists of two rectangular plates joined by ideal (with no bending resistance) joint-hinges arranged at the edges. Both static and dynamic problems are considered. Solutions are obtained on the basis of a geometrically nonlinear model of an elastic body using the ANSYS engineering package. It is shown that the pneumatic loading of an elastic structure is accompanied by its oscillations with increasing frequency and the amplitude decreasing in time. The period of shell oscillations is found to be dependent on the operating loading pressure and independent of the pressure supply rate, and it almost equals the period of oscillations of the statically loaded shell under the same pressure. The wide range variation in the natural oscillation frequencies of the structure during pneumatic deployment, which is revealed in the study, is of great importance since it makes possible to predict resonant modes accompanying the process. The results of the study will be used in problems of mechanics related to the pneumatic deployment of shell structures (for example, telescope reflectors, antennas, etc.) in space.

Design of a high-spectral-resolution lidar for atmospheric temperature measurement down to the near ground.

In this paper, a high-spectral-resolution lidar (HSRL) for profiling atmospheric temperature from the ground to 10 km is proposed. A double Nd:YAG laser produces the transmitted laser at 532 nm. The backscattering lidar signal is passed through two different saturated iodine-vapor filters and thus obtains molecular scattering signals that can be employed to determine the temperature. A coaxial postposition transceiver is constructed with an off-axis aspheric reflective telescope (OART). The design of the transceiver and that of the OART are demonstrated. With this transceiver, the lidar blind zone where the overlap factor is zero can be reduced greatly, and accurate temperature measurement for full elevation can be achieved. The whole system is optimized with theoretical models based on geometrical optics and statistical error analyses. Monte Carlo simulations display the performance of the designed HSRL, showing that the all-day temperature retrieval error is better than 1.4 K from the ground to 10 km. The proposed HSRL is expected to provide more accurate atmospheric auxiliary parameters for the detection of aerosols' optical characteristics.

Revisiting RV Tauri stars: first ionization potential (FIP) effect

We have derived elemental abundances of eight RV Tauri-type post-asymptotic giant branch stars: AR Sgr, CE Vir, DY Aql, HP Lyr, SS Gem, SZ Mon, TT Oph, and UZ Oph. The abundance analysis of these luminous stars is based on the high-resolution (R ≈ 55 000) optical echelle spectra obtained with the 2.1 m Struve reflector telescope and the CCD-equipped Sandiford Cassegrain echelle spectrograph at the McDonald Observatory. Standard 1D local thermodynamic equilibrium analysis provides a fresh determination of the atmospheric parameters and elemental abundances for 17 species including C, Na, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Y, and Nd. Then, the computed up-to-date abundances are used to investigate FIP in the program stars, as seen in the Sun and sun-like stars. Also, we scrutinize their behavior for dust–gas separation as an alternative mechanism to the FIP. For the first time, we provide atomic line lists with up-to-date atomic data employed in the analysis for AR Sgr, HP Lyr, SZ Mon, TT Oph, and UZ Oph.

Initial pointing model of the 40m Thai National Radio Telescope with an 8-inch optical telescope

As a part of the mega-project, known as “Radio Astronomy Network and Geodesy for Development (RANGD)” by National Astronomical Research Institute of Thailand (NARIT), the fully steerable 40m Thai National Radio Telescope (TNRT) is under construction in the eastern region of Chiang Mai. From December, 2019, commissioning stage of TNRT includes the determination of pointing and aperture efficiencies, which are expected at no wind condition as 2 arcsec (rms) and 60 percent at K-band, respectively. Perfect alignment between the antenna pointing system with on-sky coordinates system, e.g. equatorial, is very crucial in obtaining such high pointing accuracy. In the initial step without the K-band receiver, this can be partially achieved with the installation of an optical telescope with an imaging system on the antenna structure and the initial pointing offset can be determined by tracking planets or other celestial sources in the optical. Here, we present the complete design of the system consisted of an 8-inch reflective telescope, a focuser and 20 MP camera, which results in the field of view of 22.67 by 15.15 sq. arcmin and the resolution of 0.25 arcsec. The complete measurement procedure for such approach and discussion on the advantages and limitations are also included.

Book Review: "Radio Telescope Reflectors: Historical Development of Design and Construction by Jacob W. M. Baars and Hans J. Kärcher"

Book Review: "Radio Telescope Reflectors: Historical Development of Design and Construction by Jacob W. M. Baars and Hans J. Kärcher"

Real-time position calculation method for large-diameter radio telescope panel using angle sensor

<p indent="0mm">With diameter enlarging and frequency upgrading of the radio telescope, the telescope structure is more complex, and the telescope efficiency and steering are more and more sensitive to structural distortion. It is difficult if we adopt traditional compensation method to satisfy the telescope electrical performance requirements, so taking the initiative to adjust the main reflector telescope to compensate telescope performance is essential, but the common active surface adjustment technology does not well meet the important instantaneity index. To this end, this paper proposes a real-time position calculation method for large-diameter radio telescope panel using the angle sensor. The physical position and attitude of the telescope panel can be computed quickly through using the reading of the angle sensors, which can make the active adjustment of the telescope panel more efficient and the telescope electrical performance being better.

Wide-field off-axis telescope for the Mesospheric Airglow/Aerosol Tomography Spectroscopy satellite.

We present the development of a compact f/7.3 (D=35 mm) three-mirror reflective telescope for the atmospheric-research microsatellite Mesospheric Airglow/Aerosol Tomography Spectroscopy (MATS). The telescope design was driven by the end users' need for a reflective wide-field (5.67°×0.91°) optic with high stray light rejection and six detection channels with separate image sensors, operating at wavelengths 270-772nm. For the first time, a design method for wide-field off-axis telescopes-in which linear astigmatism is eliminated-was applied and tested in practice. Single-point diamond turning was used to produce two sets of 37-110mm large free-form aluminum mirrors with surface figure errors and roughness values of 34-62nm (RMS)/193-497nm (PV) and 2.8-3.5nm (RMS), respectively. A method that combines precise machining and geometry measurements (using a coordinate measuring machine) was employed to fabricate an aluminum structure to accurately position the mirrors without the need for manual alignment. The telescope was tested with a network of plate beamsplitters and filters, which define the spectral selection for the six detection channels. Imaging performance measurements were carried out using a reflective off-axis collimator, which projects imaging targets at infinite focus. A modulation transfer function (MTF) value of 0.45 at 20lp/mm was measured at ∼760 nm (diffraction limit: 0.85) using a slanted edge target. By modeling the measured mirror surfaces in optical design software, a reoptimization of the mirror positions could be performed and an improved MTF of ∼0.75 at 20lp/mm was predicted. The results demonstrate design- and building methods that can be utilized to make off-axis telescopes for a vast range of applications.

Mapping distortion correction in freeform mirror testing by computer-generated hologram.

Distortion can be introduced in null testing using computer-generated holograms for off-axis aspheres or freeforms with significant deviation. It leads to the failure of testing results to guide deterministic optical processing. In this paper, based on ray tracing and calibration marks applied to a mirror surface, a high-accuracy method is proposed to correct the distortion. The correction error is less than 1mm. Second and fourth mirrors of a reflective telescope prototype with an F number of 6.5 and field of view of 76° are polished. In the process, distortion is corrected, and the position misalignment error is as small as 0.783mm. For the sake of alignment, the two mirrors are fixed on a 790 mm×390 mm SiC substrate. The root-mean-square value of the mirror surface error is 0.0433λ (λ=0.6328 μm) after ion beam finishing.

BVRI Photometric Observations and Study of the Newly Discovered W UMa Type Eclipsing Binary of Multiple System 1SWASP J093010.78+533859.5

New BVRI light curves of the W UMa type eclipsing binary of multiple system 1SWASP J093010.78+533859.5 (GSC 0380-00759) have been constructed based on CCD observations obtained using 1.88 m reflector telescope of KAO, (Kottamia Astronomical Observatory), Egypt on March, 20th in 2013. New times of minima and new ephemeris have been determined from these light curves. Using the Binary Maker 3.0 (BM3) and PHOEBE program, the geometric and photometric element parameters of this system have been derived. Light curve analysis results indicate that J093010B is an overcontact binary with third body shared with 0.056 ± 0.006 of its total luminosity. The filling factor of the system is approximately equal to 17%. According to all available eclipse timings, the preliminary rate of period change has been determined to be -6.35 x 10-5 day/yr. The semi-major axis has been determined as 1.6 ± 0.1 R ⊙.

Polarization study about a telescope-based transmitter for quantum communication.

We studied the polarization evolution of a reflective telescope designed for the quantum satellite Micius. The change in polarization extinction ratio (PER) of quantum light was derived and calculated. The PER deterioration caused by increase of incidence angle was calculated to determine the boundary conditions for the system design. The performance of the Micius prototype was evaluated both theoretically and experimentally to verify the viability of our optical design. Minimum and maximum PERs of 38 and 55dB, respectively, were recorded, which were mostly in good agreement with the numerical calculations. Our investigations have contributed to the success of Micius, which is a significant milestone for building a global security network.

Space-to-Ground Quantum Key Distribution Using a Small-Sized Payload on Tiangong-2 Space Lab**Supported by China Manned Space Program, Technology and Engineering Center for Space Utilization Chinese Academy of Sciences, Chinese Academy of Sciences, and the National Natural Science Foundation of China.

Quantum technology establishes a foundation for secure communication via quantum key distribution (QKD). In the last two decades, the rapid development of QKD makes a global quantum communication network feasible. In order to construct this network, it is economical to consider small-sized and low-cost QKD payloads, which can be assembled on satellites with different sizes, such as space stations. Here we report an experimental demonstration of space-to-ground QKD using a small-sized payload, from Tiangong-2 space lab to Nanshan ground station. The 57.9-kg payload integrates a tracking system, a QKD transmitter along with modules for synchronization, and a laser communication transmitter. In the space lab, a 50 MHz vacuum + weak decoy-state optical source is sent through a reflective telescope with an aperture of 200 mm. On the ground station, a telescope with an aperture of 1200 mm collects the signal photons. A stable and high-transmittance communication channel is set up with a high-precision bidirectional tracking system, a polarization compensation module, and a synchronization system. When the quantum link is successfully established, we obtain a key rate over 100 bps with a communication distance up to 719 km. Together with our recent development of QKD in daylight, the present demonstration paves the way towards a practical satellite-constellation-based global quantum secure network with small-sized QKD payloads.

Design and optimization of the CFRP mirror components

As carbon fiber reinforced polymer (CFRP) material has been developed and demonstrated as an effective material in lightweight telescope reflector manufacturing recently, the authors of this article have extended to apply this material on the lightweight space camera mirror design and fabrication. By CFRP composite laminate design and optimization using finite element method (FEM) analysis, a spherical mirror with φ316 mm diameter whose core cell reinforcement is an isogrid configuration is fabricated. Compared with traditional ways of applying ultra-low-expansion glass (ULE) on the CFRP mirror surface, the method of nickel electroplating on the surface effectively reduces the processing cost and difficulty of the CFRP mirror. Through the FEM analysis, the first order resonance frequency of the CFRP mirror components reaches up to 652.3 Hz. Under gravity affection coupling with +5°C temperature rising, the mirror surface shape root-mean-square values (RMS) at the optical axis horizontal state is 5.74 nm, which meets mechanical and optical requirements of the mirror components on space camera.

On the discovery of K I 7699 Å line strength variation during the 1982–1984 eclipse of ε Aurigae

The discovery of K I 7699Å line strength variations during the 1982–1984 eclipse of ε Aurigae is described. The equivalent widths and radial velocities of the K I 7699Å line derived from spectra obtained during 1981 November–1983 July with the 2.1m Otto Struve reflector telescope of the McDonald observatory are presented.

Reconfigurable Near-Field Beam Pattern Measurement System From 0.03 to 1.6 THz

With the discovery and utilization of great observation sites, on-ground radio astronomical observations have been extended from the millimeter-wave (mm-wave) range to around 1.6 THz. The radiation collected by the telescope reflector antenna is coupled to the receiver by means of the receiver optics. These optics must be designed to match the in-coming fields from the telescope and properly characterized by amplitude and phase measurements. This paper presents a reconfigurable near-field beam pattern measurement system which can characterize magnitude and phase patterns of antennas and optics from the mm-wave to the THz region. Measurements at 900 GHz and 1.37 THz are presented for two different configurations, using different phase-lock components, and measured under cryogenic and room-temperature conditions, respectively.

Femtosecond laser cleaning the surface of reflective mirror in telescope

Metallic mirror is the core component of the imaging system in the reflective astronomical telescope. However, because the telescopesare usually located in the wild, they are often contaminated by dust and other pollutants which will greatly reduce the imaging quality. Therefore, the surface of reflective mirror in telescope should be cleaned routinely in order to recover high performance of telescope. Currently, the artificial ice cleaning is the most common method employed in the cleaning of mirror surface, which is complicated and costly. Besides, the micron particles adhered on the surface of the mirror are especially difficult to be removed by dry ice flow when their diameters are less than 20 μ m because of the Van der Waals force between the particles and metal mirror. Moreover, an ordinary method with wipe and cleanser is also used in which the friction between wipe and mirror may damage the telescope. To overcome these shortcomings, this work proposed a laser cleaning strategy in which amplified femtosecond pulsed lasers was used to clean the reflecting mirror surface. As a new cleaning method, laser cleaning technology which is non-contact and frictionless has been applied successfully in the cleaning of industrial molds, buildings, precision machinery components. For the advantages of femtosecond laser, such as short pulse duration, small thermal effects, high pulse energy, it has been used in the cleaning of cultural relics and has good achievement in recent years. This article firstly studied the interaction between the femtosecond laser and the reflective aluminum mirror. The laser damage threshold of aluminum mirror was measured about 60 mJ/cm2, which would reduce to 57 mJ/cm2 for dirty mirror because of the microparticles adhered on the surface of reflector mirror. Then, by optimizing laser parameters including the scan rate, the scan pitch, and the laser energy, it could be found that when the laser energy density ranges from 30 to 55 mJ/cm2, micron dust particles with diameters less than 20 μ m on mirror surface could be cleaned with excellent clean effect and the reflectivity in the visible light region has been greatly improved. Finally, through the dust particles and aluminum mirror substrate spectrum analysis, we ruled out that the removed microparticles were thermal melted by laser energy. The results indicated that thermal expansion plays the key role in laser cleaning of metal mirror. This work shows that femtosecond laser cleaning has a good effect on the removing of micron particles. As a simple and easy way, femtosecond laser cleaning has a good prospect in cleaning of astronomical telescope.

Visual Measurement for On-Orbit Reflector Assembly

&lt;div class=""abs_img""&gt; &lt;img src=""[disp_template_path]/JRM/abst-image/00270005/03.jpg"" width=""300"" /&gt; Artist concept of in-orbit assembly&lt;/div&gt;In Earth orbit, a space telescope is free from any absorption or disturbances from the Earth's atmosphere. Therefore, a large space telescope is planned for the future. We discuss the assembly of a large telescope reflector performed by a robot arm. For the assembly work, positioning of the arm using visual servo control to colored markers is proposed. A ground test-bed is used to test the characteristics of the new image processing method for colored markers. The results of the tests are also given.