Mirror Telescope Research Articles

VLBI observations of GRB 201015A, a relatively faint GRB with a hint of very high-energy gamma-ray emission

Context. A total of four long-duration gamma-ray bursts (GRBs) have been confirmed at very high-energy (≥100GeV) with high significance, and any possible peculiarities of these bursts will become clearer as the number of detected events increases. Multi-wavelength follow-up campaigns are required to extract information on the physical conditions within the jets that lead to the very high-energy counterpart, hence they are crucial to reveal the properties of this class of bursts. Aims. GRB 201015A is a long-duration GRB detected using the MAGIC telescopes from ~40 s after the burst. If confirmed, this would be the fifth and least luminous GRB ever detected at these energies. The goal of this work is to constrain the global and microphysical parameters of its afterglow phase, and to discuss the main properties of this burst in a broader context. Methods. Since the radio band, together with frequent optical and X-ray observations, proved to be a fundamental tool for overcoming the degeneracy in the afterglow modelling, we performed a radio follow-up of GRB 201015A over 12 different epochs, from 1.4 days (2020 October 17) to 117 days (2021 February 9) post-burst, with the Karl G. Jansky Very Large Array, e-MERLIN, and the European VLBI Network. We include optical and X-ray observations, performed respectively with the Multiple Mirror Telescope and the Chandra X-ray Observatory, together with publicly available data, in order to build multi-wavelength light curves and to compare them with the standard fireball model. Results. We detected a point-like transient, consistent with the position of GRB 201015A until 23 and 47 days post-burst at 1.5 and 5 GHz, respectively. No emission was detected in subsequent radio observations. The source was also detected in optical (1.4 and 2.2 days post-burst) and in X-ray (8.4 and 13.6 days post-burst) observations. Conclusions. The multi-wavelength afterglow light curves can be explained with the standard model for a GRB seen on-axis, which expands and decelerates into a medium with a homogeneous density. A circumburst medium with a wind-like profile is disfavoured. Notwithstanding the high resolution provided by the VLBI, we could not pinpoint any expansion or centroid displacement of the outflow. If the GRB is seen at the viewing angle θ that maximises the apparent velocity βapp (i.e. θ ~ βapp-1), we estimate that the Lorentz factor for the possible proper motion is Гα ≤ 40 in right ascension and Гδ ≤ 61 in declination. On the other hand, if the GRB is seen on-axis, the size of the afterglow is ≤5pc and ≤16pc at 25 and 47 days. Finally, the early peak in the optical light curve suggests the presence of a reverse shock component before 0.01 days from the burst.

Design and evaluation of an active secondary mirror positioning system for a small telescope

This publication presents an active compensation system for the position of the secondary mirror of a small Ritchey–Chrétien telescope system. The goal is to maintain the optical imaging quality under varying gravitational and thermal influences, by compensation for the relative position deviations between the primary and secondary mirrors. An extensive analysis concerning the feasibility of such a system for a commercial off-the-shelf small telescope is performed and used as a basis for the design of the precision measurement and positioning system. The developed prototype uses dimensional metrology to capture relative position errors of the secondary mirror. A newly designed actuator with three degrees of freedom for the secondary mirror allows us to compensate for these deviations in a closed-loop control manner and ensures optimal positions of the two mirrors at all times. The support structure design requirements are reduced, allowing the utilization of more lightweight structures, as the artificial stiffness of the compensation system takes care of keeping the telescope mirrors in place. Furthermore, the measurement principle requires no light from the telescope, thus providing 100% of the collected light for the observation. The developed actuation and measurement principles are designed for simple scalability to larger representatives of small telescopes. The implemented setup is evaluated in various poses and temperature influences, successfully demonstrating that the calculated Strehl ratio is kept well above the diffraction limit of 80% for the used telescope system.

Phasing the Giant Magellan Telescope with the holographic dispersed fringe sensor

The next generation of Giant Segmented Mirror Telescopes (GSMT) will have large gaps between the segments either caused by the shadow of the mechanical structure of the secondary mirror [European Extremely Large Telescope (E-ELT) and Thirty Meter Telescope (TMT)] or intrinsically by design [Giant Magellan Telescope (GMT)]. These gaps are large enough to fragment the aperture into independent segments that are separated by more than the typical Fried parameter. This creates piston and petals modes that are not well sensed by conventional wavefront sensors such as the Shack–Hartmann wavefront sensor or the pyramid wavefront sensor. We propose to use a new optical device, the holographic dispersed fringe sensor (HDFS), to sense and control these petal/piston modes. The HDFS uses a single pupil-plane hologram to interfere the segments onto different spatial locations in the focal plane. Numerical simulations show that the HDFS is very efficient and that it reaches a differential piston root-mean-square (rms) smaller than 10 nm for GMT/E-ELT/TMT for guide stars up to 13th J + H band magnitude. The HDFS has also been validated in the lab with Magellan adaptive optics extreme and high-contrast adaptive optics phasing testbed, the GMT phasing testbed. The lab experiments reached 5-nm rms piston error on the Magellan telescope aperture. The HDFS also reached 50-nm rms of piston error on a segmented GMT-like aperture while the pyramid wavefront sensor was compensating simulated atmosphere under median seeing conditions. The simulations and lab results demonstrate the HDFS as an excellent piston sensor for the GMT. We find that the combination of a pyramid slope sensor with an HDFS piston sensor is a powerful architecture for the GMT.

Systems for Controlling Deformations of the Primary Mirror of the BTA-6 Telescope

Systems for Controlling Deformations of the Primary Mirror of the BTA-6 Telescope

Digital Holographic Positioning Sensor for a Small Deployable Space Telescope

We propose a system and a technique to measure misalignments of individual optical elements in a deployable telescope for a small satellite, and we demonstrate its application for a 2230 mm F10.3 Ritchey–Chretien type telescope. The system uses laser point sources at the edge of each of the deploying segments to form an interference pattern at the telescope image sensor. By processing the interferograms in the Fourier domain and comparing them to the computed ones, it becomes possible to measure the position of the primary mirror segments with a precision of 0.8μm and the axial displacement of the secondary mirror with the precision of 0.1 nm. Such a system would allow to build a deployable telescope fitting in a 1U CubeSat and align it in space with a sufficient accuracy, thus providing a gain in collecting power, reaching a factor of 2.49 with respect to a single mirror telescope fitting on a 1U CubeSat.

Qualification of the thermal stabilization, polishing and coating procedures for the aluminum telescope mirrors of the ARIEL mission

ARIEL, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was selected as the fourth medium-class mission in ESA’s Cosmic Vision program. ARIEL is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 and 7.8 micron and operating in cryogenic conditions. Fabrication of the 1.1 m aluminum primary mirror for the ARIEL telescope requires technological advances in the three areas of substrate thermal stabilization, optical surface polishing and coating. This article describes the qualification of the three procedures that have been set up and tested to demonstrate the readiness level of the technological processes employed. Substrate thermal stabilization is required to avoid deformations of the optical surface during cool down of the telescope to the operating temperature below 50 K. Purpose of the process is to release internal stress in the substrate that can cause such shape deformations. Polishing of large aluminum surfaces to optical quality is notoriously difficult due to softness of the material, and required setup and test of a specific polishing recipe capable of reducing residual surface shape errors while maintaining surface roughness below 10 nm RMS. Finally, optical coating with protected silver must be qualified for environmental stability, particularly at cryogenic temperatures, and uniformity. All processes described in this article have been applied to aluminum samples of up to 150 mm of diameter, leading the way to the planned final test on a full size demonstrator of the ARIEL primary mirror.

Soiling induced nano-defects on aluminum telescope mirror coatings.

Airborne contaminations on telescope mirrors are an important issue that is significantly affecting their reflectance, IR emissivity, and light scattering properties. Microscopic damage caused by environmental contaminants is of major interest for high-performance telescope mirror coatings. We have exposed unprotected Al coated mirrors under real operation conditions at the Very Large Telescope on Cerro Paranal/Chile for three years. The unique finding is that, in spite of a dry and low-dust environment, the reflective layers are damaged by interaction between dust particles on the Al mirror surface and capillary condensation. We analyze this particular damage mechanism at the microstructural level.

Experimental investigation in nodal aberration theory (NAT): separation of astigmatic figure error from misalignments in a Cassegrain telescope.

We present simulations and experimental validations for separating astigmatic figure error from misalignments in Nodal Aberration Theory (NAT) with a high-precision Cassegrain telescope. Both the primary mirror figure error and the secondary mirror misalignments induce binodal astigmatism for the telescope systems. The separation of these two aberration factors plays a crucial role in the telescope alignment process. In this study, the figure error of the aspheric primary mirror of the Cassegrain telescope induced by the mirror mounts was measured interferometrically utilizing a computer-generated hologram (CGH). According to the primary mirror figure error, the astigmatic node locations in the image plane were simulated using real raytracing. The center of the nodes was located on the field center, and the nodes were placed symmetrically with respect to the field center in the image plane. The telescope's alignment was performed using the simulation results, and the node locations were measured on a grid of field points interferometrically. Thereafter, secondary mirror misalignments around the coma-free pivot point were introduced into the optical system, and the node's center was shifted from the field center in the image plane as predicted by NAT. The simulations and interferometric field measurements were performed and compared on a grid of field points for the misaligned state in the presence of primary mirror figure error. The experimental results confirm the predictions from NAT. Statistical analysis was also performed to confirm the accuracy and stability of the measurements.

Method for adaptation of segmented primary telescope mirror

Method for adaptation of segmented primary telescope mirror

Redundant apodization for direct imaging of exoplanets

Context. Direct imaging and spectroscopy of Earth-like planets and young Jupiters require contrast values up to 106−1010 at angular separations of a few dozen milliarcseconds. To achieve this goal, one of the most promising approaches consists of using large segmented primary mirror telescopes with coronagraphic instruments. While they are able to reach high contrast at small angular separations, coronagraphs are highly sensitive to wavefront errors, however. The segmentation itself is responsible for phasing errors and segment-level vibrations that have to be controlled at a subnanometric accuracy. Aims. We propose an innovative method for a coronagraph design that allows a consequent relaxation of the segment phasing and stability constraints for low segment-count mirrors and generates an instrument that is more robust to segment-level wavefront errors. Methods. This method is based on an optimization of the coronagraph design that includes a segment-level apodization. This is repeated over the pupil to match the segmentation redundancy and improves the contrast stability beyond the minimum separation set by the single-segment diffraction limit. Results. We validate this method on a Giant Magellan Telescope (GMT)-like pupil (seven circular segments) for two coronagraph types: apodized pupil Lyot coronagraphs, and apodizing phase plate coronagraphs. For the apodized pupil Lyot coronagraphs, redundant apodization enables releasing the piston phasing constraints by a factor of 5-20 compared to classical designs. For the apodizing phase plate coronagraphs, the contrast remains almost constant up to 1 radian RMS of the phasing errors. We also show that redundant apodizations increase the robustness of the coronagraph to segment tip-tilt errors, as well as to missing segment errors. Conclusions. Redundant apodization enables reducing or even removing any constraints on the primary mirror segment phasing at the price of larger angular separations and lower throughputs. This method cannot be applied to higher-segment count mirrors such as the ELT or the TMT, but it is particularly suitable for low segment-count mirrors (fewer than ~20 segments) such as the GMT aperture. These mirrors aim for high-contrast imaging of debris disks or exoplanets down to 100 mas.

Upcoming 4m ILMT facility and data reduction pipeline testing

The 4m international liquid mirror telescope (ILMT) installation activities have recently been completed at the Devasthal observatory (Uttarakhand, India). The ILMT will perform continuous observation of a narrow strip of the sky (\(\sim\)27\('\)) passing over the zenith in the SDSS \(g'\), \(r'\) and \(i'\) bands. In combination with a highly efficient \(4{\rm k}\times 4{\rm k}\) CCD camera and an optical corrector, the images will be secured at the prime focus of the telescope using the time delayed integration technique. The ILMT will reach \(\sim\)22.5 mag (\(g'\)-band) in a single scan and this limiting magnitude can be further improved by co-adding the nightly images. The uniqueness of the one-day cadence and deeper imaging with the ILMT will make it possible to discover and study various galactic and extra-galactic sources, specially variable ones. Here, we present the latest updates of the ILMT facility and discuss the preparation for the first light, which is expected during early 2022. We also briefly explain different steps involved in the ILMT data reduction pipeline.

Eigenvectors-guided topology optimization to control the mode shape and suppress the vibration of the multi-material plate

Although the structural vibration can be suppressed by adjusting eigenvalues outside the excitation frequency band, it becomes increasingly difficult to suppress vibration as the width of the excitation frequency band increases. Besides, for some high-precision equipment, e.g. space telescope mirror substrate and rocket motor casings, their complex and diverse modals directly affect the performance. Thus, a novel optimization algorithm was proposed, which applies to achieve eigenvector-based modal control and vibration suppression. The eigenvectors were defined as the objective function, and Nelson’s method without truncation error was used to calculate the sensitivity information. Due to the introduction of dynamic equations, the non-linearity of the objective function is prominent. Then, an improved solver that can handle this non-linear topology optimization problem was proposed. The optimization was performed under a multi-material framework, and the extended multi-material interpolation scheme was proposed to readily realize the optimization with three or more materials. In addition, the consistent mass matrix constructed by multi-material interpolation was used to describe the mass matrix without concentrated mass in the dynamic equation. Moreover, the modal assurance criterion was used to track jumping modals. Finally, the modal controllability was achieved through several numerical examples, which verify the effectiveness of the proposed method.

Design of athermalized mounting structure for the sub-aperture primary mirror of a synthetic aperture telescope

The imaging quality of the synthetic aperture system is sensitive to the positioning accuracy of the sub-aperture primary mirror. A novel flexible mounting structure of bimetallic material is proposed for the athermalization of the sub-aperture primary mirror of the Fizeau Synthetic Aperture Telescope – which is composed of seven sub-aperture. The axial position accuracy of the sub-aperture primary mirror must be less than 5 µm under 10°C temperature rise to meet the requirements of the optical system. Firstly, a single mounting unit is analyzed theoretically, and the initial parameters are determined. The conceptual design of the mounting structure is carried out by using initial parameters. The orthogonal optimization algorithm and range analysis are used to optimize the structural parameters. The finite element model of the flexible mounting structure is established and the coupled thermal-mechanical simulation analysis is performed. Then the thermal sensitivity test of the sub-aperture primary mirror mounting structure was carried out. Under the effect of a temperature rise of 10°C, the axial displacement of the sub-aperture primary mirror mounting surface is less than 3 µm. Finally, the synthetic aperture system is assembled, and the optical test verifies that the synthetic aperture system has good imaging capabilities.

Overview of key technologies for segmented mirrors of large-aperture optical telescopes

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

Interferometer for Testing of Convex Mirrors of Optical and Radio Telescopes Constructed by the Scheme of Orthogonal Rays

Interferometer for Testing of Convex Mirrors of Optical and Radio Telescopes Constructed by the Scheme of Orthogonal Rays

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

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Solar VUV telescope for nanosatellites

Within the Universat program, a set of solar vacuum ultraviolet (VUV) telescopes has been developed for deployment on 6U nanosatellites. Telescopes are designed to get images of the solar corona. The spectral ranges of observations is considered, the characteristics of the nanosatellite from the point of view of the observations feasibility are opmized, the optical scheme of the telescope and VUV multilayer mirrors coatings and thin-film filters are modelled. Keywords: Nanosatellite, VUV, telescope, solar corona.

High-precision co-phase method for segments based on a convolutional neural network

In order to achieve the resolution comparable to the resolution of a monolithic primary mirror telescope and make the imaging quality of the imaging system reach or approach to the diffraction limit, the submirrors of the segments telescope should ensure co-phase splicing. To solve the problem of phase error detection, a high-precision piston error detection method is proposed based on convolutional neural network (CNN). By setting a mask with a sparse multi-subpupil configuration on the exit pupil of the imaging system, a point spread function (PSF) image dataset that is extremely sensitive to the piston error is constructed. According to the characteristics of this dataset, a high-performance CNN model is built. And the best detection range of CNN is tested. The simulation results show that a single network can accurately output the piston error of one or more submirrors in the capture range slightly less than one wavelength. When the single network is applied to the six-submirror imaging system, the detection precision of the piston error reaches an RMS value of 0.0013<i>λ</i> (here, RMS stands for root mean square). And the method has good robustness to residual tip-tilt error, wavefront aberration, and CCD noise, light source bandwidth. The method is simple and fast, and can be widely used to detect the piston error of the segments.

Fracture toughness and hardness of transparent MgO–Al2O3–SiO2 glass-ceramics

Hard and strong transparent glass-ceramics (TGCs) can be used as cooktop plates, telescope mirrors, armor materials, and screens for smartphones and tablets. In this work, transparent glass-ceramics from the MgO–Al2O3–SiO2 (MAS) system were obtained after a refined heat-treatment protocol. The evolution in fracture toughness, KIC (Single Edge Precracked Beam), and Vickers hardness (HV) were characterized as a function of the crystallization progress. The KIC of the parent glass was 0.6 MPa m1/2, whereas the TGC samples present values around 1.1 MPa m1/2. The average glass hardness was 8.5 ± 0.8 GPa for loads of 0.6–5 N, whereas the HV of the TGC samples varied from 9 to 10 GPa, for indentation loads ranging from 4.9 to 9.8 N. These results are discussed in light of our previous findings on the evolution of elastic modulus in the same glass-ceramics.

Forming of Bessel beams from annular fields on large distances

Bessel light beam formation from annular fields is important for many applications related to laser remote sensing and also for the free-space laser communication. Annular beams have a few advantages, such as optimal combination with optical projectors based on mirror telescopes, self-healing property after passing around obstacles including stability in the atmospheric turbulence in some cases, and also large focal range. In the paper a process of annular beam transformation into a Bessel-type field by the free-space propagation diffraction is investigated. The transverse Bessel beam profile at large propagation distances is calculated. It is shown that the profile contains small number of rings, the on-axis maximum and the first ring contain more than 90% of light power.