Mirror Telescope Research Articles

Co-Phase Error Detection for Segmented Mirrors Based on Far-Field Information and Transfer Learning

The resolution of a telescope is closely related to its aperture size; however, the aperture of a single primary mirror telescope cannot be indefinitely enlarged due to design and manufacturing constraints. Segmented mirror technology can achieve the same resolution as a single primary mirror of equivalent aperture, provided that the segments are co-phased correctly. This paper proposes a method for high-precision detection of piston errors in segmented mirror telescope systems, based on far-field information and transfer learning. By training a ResNet-18 network model, this method can predict piston errors with high precision within 10 ms of a single-frame far-field diffraction image. Simulation results demonstrate that the method is robust to tip-tilt errors, wavefront aberrations, and noise. This approach is simple, fast, highly accurate in detection, and resistant to noise, providing a new solution for piston error detection in segmented mirror systems.

Atomic layer deposited protective coating of aluminum oxide on silver-based telescope mirror: a comparison between a pure ozone and H2O precursor

Atomic layer deposited protective coating of aluminum oxide on silver-based telescope mirror: a comparison between a pure ozone and H2O precursor

Performance comparison of the Shack-Hartmann and pyramid wavefront sensors with a laser guide star for 40 m telescopes

Context. Upcoming giant segmented mirror telescopes will use laser guide stars (LGS) for their adaptive optics (AO) systems. Two options of wavefront sensors (WFSs) are the Shack-Hartmann wavefront sensor (SHWFS) and the pyramid wavefront sensor (PWFS). Aims. In this paper, we compare the noise performance of the PWFS and the SHWFS. We aim to identify which of the two is best to use in the context of a single or tomographic configuration. Methods. To compute the noise performance, we extended a noise model developed for the PWFS to be used with the SHWFS. To do this, we expressed the centroiding algorithm of the SHWFS as a matrix-vector multiplication, which allowed us to use the statistics of noise to compute its propagation through the AO loop. We validated the noise model with end-to-end simulations for telescopes of 8 and 16 m in diameter. Results. For an AO system with only one WFS, we found that given the same number of subapertures, the PWFS outperforms the SHWFS. For a 40 m telescope, the limiting magnitude of the PWFS is around one magnitude higher than the SHWFS. When using multiple WFS and a generalized least-squares estimator to combine the signal, our model predicts that in a tomographic system, the SHWFS performs better than the PWFS (with a limiting magnitude that is higher by a 0.3 magnitude. When using sub-electron RON detectors for the PWFS, the performance quality is almost identical for the two WFSs. Conclusions. We find that when using a single WFS with LGS, PWFS is a better alternative than the SH. For a tomographic system, both sensors would give roughly the same performance.

Deformation and optical aberration prediction in ultra-precision Single Point Diamond Turning of optical components

Aluminum is the material of choice for the majority of aerospace components, and, in the past few years, its application has been extended also to the mirrors of space telescopes because of the improved thermal behavior and the possibility to build the entire telescope with the same material. However, the low elastic modulus of such material, combined with the extremely tight tolerances of optical applications, make the production of these components very challenging and, usually, based on a trial-and-error approach. This paper presents a structured methodology for the prediction of the results of manufacturing in Single Point Diamond Turning of optical components, both in terms of absolute deformation as well as optical aberrations (via Zernike polynomials). All the most significant parameters acting on the workpiece have been simulated and combined. The proposed approach has been experimental validated on an actual aluminum mirror, proving its good accuracy (<5 % rms error). While some improvement can be performed to better match the experimental data in terms of Zernike coefficients, especially for non-symmetric aberrations, this paper forms the basis for an off-machine optimization of the SPDT process, drastically reducing the trial-and-error efforts.

Enhanced mechanical properties in transparent mullite glass-ceramics synthesized from EMT-type zeolites via spark plasma sintering

Transparent glass-ceramics offer a unique combination of optical clarity and the mechanical strength of ceramics, making them ideal for various advanced applications such as cell phone screens and telescope mirrors. However, traditional fabrication methods involve high temperatures and long processing times, which result in high energy consumption and limitations in material integration. In this study, we explore the fabrication of transparent mullite glass-ceramics from EMT-type zeolites using the spark plasma sintering (SPS) technique to address these challenges. By varying the concentrations of ammonium salts to replace Na+ in the zeolite, we modified the SiO2+Al2O3 content and successfully synthesized the mullite glass-ceramics at 950 °C. The resulting glass-ceramics exhibited high transmittance (>65 % in the visible region), improved mechanical properties with a Young's modulus up to 103 GPa, a Vickers hardness of 8.0 GPa, and an indentation fracture toughness of 1.8 MPa m1/2. X-ray diffraction and Raman spectroscopy confirmed the formation of mullite phases, with increased crystallinity and a strengthened glass structure, correlating with higher SiO2+Al2O3 content. The study demonstrates the potential of using high Al/Si ratio molecular sieves as precursors for high-performance glass-ceramics, offering significant advancements in both optical and mechanical properties.

High fidelity adaptive mirror simulations with reduced order models

In the design process of large adaptive mirrors numerical simulations represent the first step to evaluate the system design compliance in terms of performance, stability and robustness. For the next generation of Extremely Large Telescopes increased system dimensions and bandwidths lead to the need of modeling not only the deformable mirror alone, but also all the system supporting structure or even the full telescope. The capability to perform the simulations with an acceptable amount of time and computational resources is highly dependent on finding appropriate methods to reduce the size of the resulting dynamic models. In this paper we present a framework developed together with the company Microgate to create a reduced order structural model of a large adaptive mirror as a preprocessing step to the control system simulations. The reduced dynamic model is then combined with the remaining system components allowing to simulate the full adaptive mirror in a computationally efficient way. We analyze the feasibility of our reduced models for Microgate’s prototype of the adaptive mirror of the Giant Magellan Telescope.

The Optimization Design of a Lightweight 2 m SiC Mirror for Ground-Based Telescopes

The Optimization Design of a Lightweight 2 m SiC Mirror for Ground-Based Telescopes

Method for LAMOST cofocus maintenance based on an Eddy current edge sensor

The current cofocus maintenance strategy of the primary mirror of the Large Sky Area Multi-Object Spectroscopic Telescope (Mb) using Shack–Hartmann Wavefront Sensor (SHWFS) interrupts the observation mission. This study proposed an Eddy current edge sensor with high precision and accuracy to achieve closed-loop cofocus control. Through structural design, interaction matrix deduction, installation parameter analysis, and performance calibration, the edge sensor was validated for use in detecting segment attitudes. The minimum cofocus maintenance system comprising three segments was separated from Mb and established to validate the effectiveness of the closed-loop cofocus control with the novel edge sensor. The closed-loop control system was effective within less than 1.5°C environmental temperature changes after 13 h, which is considerably longer than the maintenance time with the SHWFS. Moreover, continuous observation without interruption is possible during one observation night.

Expected performance of the Pyramid wavefront sensor with a laser guide star for 40 m class telescopes

Context. The use of artificial laser guide stars (LGS) is planned for the new generation of giant segmented mirror telescopes in order to extend the sky coverage of their adaptive optics systems. The LGS, being a 3D object at a finite distance, will have a large elongation that will affect its use with the Shack–Hartmann (SH) wavefront sensor. Aims. In this paper, we compute the expected performance for a Pyramid WaveFront Sensor (PWFS) using an LGS for a 40 m telescope affected by photon noise, and also extend the analysis to a flat 2D object as reference. Methods. We developed a new way to discretize the LGS, and a new, faster method of propagating the light for any Fourier filtering wavefront sensors (FFWFS) when using extended objects. We present the use of a sensitivity model to predict the performance of a closed-loop adaptive optic system. We optimized a point-source-calibrated interaction matrix to accommodate the signal of an extended object by computing optical gains using a convolutional model. Results. We find that the sensitivity drop, given the size of the extended laser source, is large enough to make the system operate in a low-performance regime given the expected return flux of the LGS. The width of the laser beam is identified as the limiting factor, rather than the thickness of the sodium layer. Even an ideal, flat LGS will have a drop in performance due to the flux of the LGS, and small variations in the return flux will result in large variations in performance. Conclusions. We conclude that knife-edge-like wavefront sensors, such as the PWFS, are not recommended for use with LGS for a 40 m telescope, as they will operate in a low-performance regime, given the size of the extended object.

Field-balancing alignment of the secondary mirror of space telescope using a self-calibrated model

In-orbit alignment of the secondary mirror (SM) of deployable space telescopes is crucial for high-resolution imaging. Traditional alignment methods, which typically involve dedicated wavefront sensors or numerous iterations to optimize a metric function, are not well-suited for space applications. In this paper, an improved model-based SM alignment method is proposed. The term model refers to the relationship between misalignments and a metric function. The improved method offers two advantages over the previous model-based method in [12]. Firstly, our method allows for self-calibration of the model without the need for an additional wavefront sensor, which greatly simplifies the calibration process. Secondly, our method enables field-balancing alignment of the SM with several correction cycles. The exposure number is only six in one correction cycle and is regardless of the number of FOV sampling points, making it highly efficient for telescopes with a large FOV. The feasibility and performance of our method are verified by simulation. A comparison with existing methods is also given to show the advantages of our method in convergence speed and accuracy.

Double light source Ronchi Tester for detection of ruling rotations

Abstract In this work, a modified Ronchi tester with a double light source has been implemented for evaluating and minimizing the rotation angle of the Ronchi ruling. The ruling rotations produced by positioning errors can become very small and could be going unnoticed (0.1°), however we have demonstrated that for these angle sizes the aberrations in the wavefront can not be completely neglicted. Accordingly with a rotatory mechanical device for correctioning the Ronchi ruling has been also added with this proposals. With our aim, in spite of non-straight shape that the ronchigrams can present in the patterns, the process of the ruling rotation degree measuring has hurled some experimental results when three concave mirrors: a concave spherical mirror, a concave parabolic mirror (the primary mirror of a Newtonian telescope) and a spherical concave mirror (the primary mirror of a Maksutov-Cassegrain telescope) have been tested.

Contactless active mirror for space telescopes

The SPLATT project is a technological research activity funded by INAF, in the field of primary active mirrors for next generation space telescopes. The goal is to demonstrate that active mirrors with contactless actuators are insensitive to disturbance from the telescope and can therefore be key-elements in the reduction of complexity and cost.&#x0D; Keywords: space telescopes, active mirror, active optics, LUVOIR.

Principle and application of holographic modal wavefront sensor for extended objects and general aberration modes

By employing a computer-generated hologram (CGH) to modulate the incident wavefront, the holographic modal wavefront sensor (HMWFS) can efficiently measure multiple aberration modes in a single shot, providing significant advantages such as computational simplicity, rapid measurement speed, and customizable assessment of aberration modes. However, the current HMWFS is confined to point-like objects and aberrations with specific orthogonality, thereby limiting its application scope. In this paper, we introduce a novel HMWFS that, to the best of our knowledge, is compatible with extended objects and general aberration modes. We establish the theoretical relationship between the second moment of the image intensity and the coefficients of general aberration modes for extended objects. Simulation and experimental results confirm the feasibility and performance of the proposed HMWFS. Additionally, we demonstrated its practical use in measuring misalignments of the secondary mirror of a space telescope. Simulations show that the misalignments can be effectively corrected within 2∼3 iterations.

Development of an alignment device for the Prototype Segmented Mirror Telescope

Development of an alignment device for the Prototype Segmented Mirror Telescope

Analysis of Threshold Conditions for Cementation of Soiling on PV Modules and Telescope Mirrors

Analysis of Threshold Conditions for Cementation of Soiling on PV Modules and Telescope Mirrors

Phasing segmented telescopes via deep learning methods: application to a deployable CubeSat.

Capturing high-resolution imagery of the Earth's surface often calls for a telescope of considerable size, even from low Earth orbits (LEOs). A large aperture often requires large and expensive platforms. For instance, achieving a resolution of 1m at visible wavelengths from LEO typically requires an aperture diameter of at least 30cm. Additionally, ensuring high revisit times often prompts the use of multiple satellites. In light of these challenges, a small, segmented, deployable CubeSat telescope was recently proposed creating the additional need of phasing the telescope's mirrors. Phasing methods on compact platforms are constrained by the limited volume and power available, excluding solutions that rely on dedicated hardware or demand substantial computational resources. Neural networks (NNs) are known for their computationally efficient inference and reduced onboard requirements. Therefore, we developed a NN-based method to measure co-phasing errors inherent to a deployable telescope. The proposed technique demonstrates its ability to detect phasing errors at the targeted performance level [typically a wavefront error (WFE) below 15nm RMS for a visible imager operating at the diffraction limit] using a point source. The robustness of the NN method is verified in presence of high-order aberrations or noise and the results are compared against existing state-of-the-art techniques. The developed NN model ensures its feasibility and provides a realistic pathway towards achieving diffraction-limited images.

Topology Optimization of a Single-Point Diamond-Turning Fixture for a Deployable Primary Mirror Telescope

CubeSats, known for their compact size and cost effectiveness, have gained significant popularity. However, their limited size imposes restrictions on the optical aperture and, consequently, the Ground Resolution Distance in Earth Observation missions. To overcome this limitation, the concept of deployable optical payloads with segmented primary mirrors which can unfold like petals has emerged, enabling larger synthetic apertures and enhanced spatial resolution. This study explores the potential benefits of leveraging Additive Manufacturing (AM) and Topology Optimization (TO) in the realm of ultra-precision machining, specifically single-point diamond machining. The goal is to reduce fixture weight while improving stiffness to minimize deformations caused by rotational and cutting forces which compromise optical performance. Through Finite Element Analysis, this research compares conventionally machined fixtures with those produced using AM and TO techniques. The results reveal that concept designs created via TO can achieve a remarkable 68% reduction in weight. This reduction makes the assembly, including the machining fixture and 12 U deployable segments, manageable by a single operator without the need for specialized lifting equipment. Moreover, these innovative designs lead to substantial reductions of up to 86% and 51% in deformation induced by rotational and cutting forces, respectively.

Summary of the 3rd BINA Workshop

The third BINA workshop (BINA-3) was the third workshop of this series involving scientists from India and Belgium aimed at fostering future joint research in the view of cutting-edge observatories and advances in theory. BINA-3 was held at the Graphic Era Hill University, 22-24 March 2023 at Bhimtal (near Nainital), Uttarakhand, India. A major event was the inauguration of the International Liquid-Mirror Telescope (ILMT), the first liquid mirror telescope devoted exclusively to astronomy. BINA-3 provided impressive highlights encompassing topics of both general astrophysics and solar physics. Research results and future projects have been featured through invited and contributed talks, and poster presentations.

Observation of Multiply Imaged Quasars with the 4-m ILMT

Gravitationally lensed quasars (GLQs) are known to potentially provide an independent way of determining the value of the Hubble-Lemaître parameter H0, to probe the dark matter content of lensing galaxies and to resolve tiny structures in distant active galactic nuclei. That is why multiply imaged quasars are one of the main drivers for a photometric monitoring with the 4-m International Liquid Mirror Telescope (ILMT). We would like to answer the following questions: how many multiply imaged quasars should we be able to detect with the ILMT? And how to derive accurate magnitudes of the GLQ images? Our estimation of the possible number of multiply imaged quasars is 15, although optimistic forecasts predict up to 50 of them. We propose to use the adaptive PSF fitting method for accurate flux measurements of the lensed images. During preliminary observations in spring 2022 we were able to detect the quadruply imaged quasar SDSS J1251+2935 in the i’ and r’ spectral bands.

Surface Brightness Properties of LSB Galaxies with the International Liquid Mirror Telescope

Low surface brightness (LSB) galaxies make up a significant fraction of the luminosity density of the local universe. Their low surface brightness suggests a different formation and evolution process compared to more-typical high-surface-brightness galaxies. This study presents an analysis of LSB galaxies found in images obtained by the International Liquid Mirror Telescope during the observation period from October 24 to November 1, 2022. 3,092 LSB galaxies were measured and separated into blue and red LSB categories based on their g′−i′ colours. In these samples, the median effective radius is 4.7 arcsec, and the median value of the mean surface brightness within the effective radius is 26.1 mag arcsec−2. The blue LSB galaxies are slightly brighter than the red LSB galaxies. No significant difference of ellipticity was found between the blue and the red LSB galaxies.