Ground-based Telescopes Research Articles

Augmenting the power of time-delay cosmography in lens galaxy clusters by probing their member galaxies

We present a simple and promising new method to measure the expansion rate and the geometry of the universe that combines observations related to the time delays between the multiple images of time-varying sources, strongly lensed by galaxy clusters, and Type Ia supernovae, exploding in galaxies belonging to the same lens clusters. By means of two different statistical techniques that adopt realistic errors on the relevant quantities, we quantify the accuracy of the inferred cosmological parameter values. We show that the estimate of the Hubble constant is robust and competitive, and depends only mildly on the chosen cosmological model. Remarkably, the two probes separately produce confidence regions on the cosmological parameter planes that are oriented in complementary ways, thus providing in combination valuable information on the values of the other cosmological parameters. We conclude by illustrating the immediate observational feasibility of the proposed joint method in a well-studied lens galaxy cluster, with a relatively small investment of telescope time for monitoring from a 2 to 3 m class ground-based telescope.

Deconvolution of Astronomical Images using Wiener Deconvolution: Mitigating Ground-Based Telescope Effects and Cosmic Noise

This study focuses on the deconvolution of astronomical images employing the Wiener deconvolution method to overcome the challenges posed by the ground-based telescope’s seeing effect and cosmic noise. Utilizing the Hubble Space Telescope, which is free from seeing effects, we intentionally introduce a simulated seeing effect through Airy Point Spread Function (PSF) convolution. Additionally, Gaussian noise is incorporated to emulate cosmic noise commonly encountered in astronomical observations. The proposed deconvolution method is validated using key metrics such as Structural Similarity Index (SSIM), Peak Signal-to-Noise Ratio (PSNR), and Mean Squared Error (MSE). The Hubble Space Telescope’s unique characteristics, combined with the intentional introduction of seeing effects and cosmic noise, contribute to a comprehensive evaluation of the deconvolution process. Our results showcase the efficacy of the Wiener deconvolution method in enhancing the resolution of astronomical images affected by ground-based telescope limitations and cosmic noise. The validation metrics demonstrate the capability of the proposed approach to restore details and improve the overall quality of astronomical imagery. This research offers valuable insights for astronomers and researchers seeking to optimize image processing techniques for enhanced astronomical observations and analyses.

The Elusive Nature of “Seeing”

Atmospheric image blur, “seeing”, is one of the key parameters that influences the selection of observatory sites and the performance of ground-based telescopes. In this review, the common definition of seeing based on the Kolmogorov turbulence model is recalled. The ability of this model to represent real, non-stationary fluctuations of the air refractive index is discussed. Even in principle, seeing (a model parameter) cannot be measured with arbitrary accuracy; consequently, describing atmospheric blur by a single number, seeing, is a crude approximation. The operating principles of current seeing monitors are outlined. They measure optical effects caused by turbulence, sampling certain regions of spatial and temporal spectrum of atmosphreic optical disturbances, and interpret their statistics in the framework of the standard model. Biases of seeing monitors (measurement noise, propagation, finite exposure time, optical defects, wind shake, etc.) should be quantified and corrected using simulations, while instrument comparison campaigns serve as a check. The elusive nature of seeing follows from its uniqueness (a given measurement cannot be repeated or checked later), its non-stationarity (dependence on time, location, and viewing direction), a substantial role of the highly variable surface layer, and a potential bias caused by the air flow in the immediate vicinity of the seeing monitors. The results of seeing measurements are outside the scope of this review.

Rotational spectroscopic characterisation of the [D2,C,S] system: an update from the laboratory and theory

The synergy between high-resolution rotational spectroscopy and quantum-chemical calculations is essential for exploring future detection of molecules, especially when spectroscopy parameters are not available yet. By using highly correlated ab initio quartic force fields (QFFs) from explicitly correlated coupled cluster theory, a complete set of rotational constants and centrifugal distortion constants for D 2 CS and cis/trans-DCSD isomers have been produced. Comparing our new ab initio results for D 2 CS with new rotational spectroscopy laboratory data for the same species, the accuracy of the computed B and C rotational constants is within 0.1% while the A constant is only slightly higher. Additionally, quantum chemical vibrational frequencies are also provided, and these spectral reference data and new experimental rotational lines will provide additional references for potential observation of these deuterated sulfur species with either ground-based radio telescopes or space-based infrared observatories.

Comparative Analysis of Image-shift Measurement Algorithms for Solar Shack–Hartmann Wavefront Sensors

Observations of the Sun provide unique insights into its structure, evolution, and activity, with significant implications for space weather forecasting and solar energy technologies. Ground-based telescopes offer cost-effective and flexible solutions for high-resolution solar observations, but image quality can be affected by atmospheric turbulence. Adaptive optics (AO) systems equipped with Shack–Hartmann wave front sensors (SH-WFS) enable real-time image correction to mitigate these effects. The accuracy of SH-WFS relies on correlation algorithms that measure wave front shifts, but reaching consistent conclusions regarding their accuracy remains challenging. In this study, we conducted an evaluation and comparison of standard correlation algorithms (the Square Difference Function, Normalized Cross-Correlation, Absolute Difference Function, Absolute Difference Function-Squared, and the Covariance Function in the frequency domain (CFF)) using simulated and authentic solar images. We optimized the algorithms through pre-processing techniques and carefully selected the most suitable window function for the CFF algorithm. Additionally, we analyzed the influence of various factors, such as shift ranges, bias, and the size of live images on the accuracy of algorithms. The consistent findings revealed that the CFF algorithm demonstrates superior measurement accuracy and robustness compared to the others. Choosing the CFF algorithm for solar observations can significantly enhance measurement accuracy, AO system performance, and the overall quality of solar research findings, thereby providing crucial support for space weather forecasting and other related scientific fields.

Physical Characterization of Moon Impactor WE0913A

On 2022 March 4, the object known as WE0913A crashed into the Moon after several close flybys of the Earth and the Moon in the previous three months. Leading up to impact, the identity of the lunar impactor was up for debate, with two possibilities: the Falcon 9 from the DSCOVR mission or the Long March 3C from the Chang’e 5-T1 mission. In this paper, we present a trajectory and spectroscopic analysis using ground-based telescope observations to show conclusively that WE0913A is the Long March 3C rocket body (R/B) from the Chang’e 5-T1 mission. Analysis of photometric light curves collected before impact give a spin period of 185.221 ± 6.540 s before the first close Earth flyby on 2022 January 20 and a period of 177.754 ± 0.779 s, both at a 1σ confidence level, before the second close Earth flyby on 2022 February 8. Using Markov Chain Monte Carlo sampling and a predictive light curve simulation based on an anisotropic Phong reflection model, we estimate both physical and dynamical properties of the Chang’e 5-T1 R/B at the start of an observation epoch. The results from the Bayesian analysis imply that there may have been additional mass on the front of the rocket body. Using our predicted impact location, the Lunar Reconnaissance Orbiter was able to image the crater site approximately 7.5 km from the prediction. Comparing the pre- and post-impact images of the location shows two distinct craters that were made, supporting the hypothesis that there was additional mass on the rocket body.

Solar Observation with the Fourier Transform Spectrometer. II. Preliminary Results of Solar Spectrum near the CO 4.66 μm and Mg i 12.32 μm

An infrared solar spectrum observed by ground-based telescopes is seriously affected by the background radiation both from the telescope and sky, relative to the visible wavelengths. Its accuracy is also influenced by the spectral resolution of the Fourier transform spectrometer. In the paper, we developed a CO2 gas cell and installed it in the sample compartment to calibrate the spectral resolution of the Bruker IFS-125HR at infrared wavelengths. The measured spectral resolution is 0.00342 ± 0.00086 cm−1 and 0.0059 ± 0.00024 cm−1 at the wavenumbers of 798 cm−1 and 2136 cm−1, respectively. We also updated a fully reflective sunlight feeding system to observe the solar spectrum near CO 4.66 μm and Mg i 12.32 μm. By quickly pointing the sunlight feeding system about 1 degree away from the solar disk center, we are able to measure the background radiation from the telescope and the sky at Huairou Solar Observing Station. After removing the background radiation, our observed solar spectrum at CO 4.66 μm is consistent with that from the National Solar Observatory. The Mg i 12.32 μm working line selected by the Accurate Infrared Magnetic Field Measurements of the Sun (AIMS) project is also identified. Our method is helpful not only for the spectral resolution calibration and background radiation correction of AIMS but also for other infrared astronomical telescopes.

Optique adaptative : correction des effets de la turbulence atmosphérique sur les images astronomiques

Today, adaptive optics (AO) is installed on all very large astronomical telescopes. It allows to overcome the limitation in angular resolution imposed on large ground-based telescopes by the atmospheric turbulence perturbations. This review paper presents the state of the art of the field for astronomical applications. It gives the principles of AO and describes its main components which are the deformable mirrors, the wavefront sensors and the control algorithms. The paper also presents the main recent achievements and ongoing projects. First, for the direct imaging of extra-solar planets by extreme AO and coronography, taking as example the SPHERE instrument. Then to solve the problem of the very limited sky coverage by using laser guide star and to extend the AO correction beyond the isoplanatic patch. Thus, the various laser-assisted tomographic AO concepts are presented with examples of each realization as for the laser tomography AO, the multi-conjugate AO and the multi-object AO. Finally, the challenges of the new generation of giant telescopes are discussed at the end of the paper with a particular focus on the European Extremely Large Telescope (ELT) project.

Multiband optical variability on diverse time-scales of the TeV blazar TXS 0506 + 056, the first cosmic neutrino source

ABSTRACT We report the first extensive optical flux and spectral variability study of the TeV blazar TXS 0506 + 056 on intranight to long-term time-scales using BVRI data collected over 220 nights between 2017 January 21 to 2022 April 9 using eight optical ground-based telescopes. In our search for intraday variability (IDV), we have employed two statistical analysis techniques, the nested ANOVA test and the power enhanced F-test. We found the source was variable in 8 nights out of 35 in the R-band and in 2 of 14 in the V-band yielding duty cycles (DC) of 22.8 per cent and 14.3 per cent, respectively. Clear colour variation in V − R was seen in only 1 out of 14 observing nights, but no IDV was found in the more limited B, I, and B − I data. During our monitoring period the source showed a 1.18 mag variation in the R-band and similar variations are clearly seen at all optical wavelengths. We extracted the optical (BVRI) SEDs of the blazar for 44 nights when observations were carried out in all four of those wavebands. The mean spectral index (α) was determined to be 0.897 ± 0.171.

Multiparameter study for a new ground-based telescope in Egypt

ABSTRACT A multiparameter analysis was conducted to evaluate the impact of meteorological parameters, night sky brightness and seismic hazards on proposed sites for the new optical/infrared Egyptian astronomical telescope. The European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) data set is used to obtain the following meteorological parameters: total cloud coverage fraction, precipitable water vapour, relative humidity, wind speed and direction, and air temperature. To estimate the aerosol optical depth, we used the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2). Light pollution over the candidate sites was measured using the Visible Infrared Imaging Radiometer Suite (VIIRS) day/night band. In order to assess the seismic hazards for the candidate sites, the seismic input in terms of maximum acceleration and response spectra was computed using a physics-based ground motion approach to assess the seismic hazards and, consequently, the designation of a seismic-resistant structure for the proposed sites. Of the seven nominated sites, two sites are found to have the best measurements and might be considered as future sites for the new Egyptian astronomical telescope. The first site is located in the south of the Sinai peninsula, while the second site is located in the Red Sea mountains region.

Characterization of extrasolar giant planets with machine learning

ABSTRACT More than 5000 extrasolar planets have already been detected. JWST and near-term ground-based telescopes like the Extremely Large Telescope (ELT), Giant Magellan Telescope (GMT), Thirty Meter Telescope (TMT), and upcoming telescopes such as the Nancy Grace Roman Space Telescope, Xuntian, and Ariel are designed to characterize the atmosphere of directly imaged Jovian planets. Here, we used five diverse machine learning algorithms to investigate how well broad-band filter photometric fluxes could initially characterize giant exoplanets. We use an established grid of 8813 reflected light model spectra of different metallicities, planet–star distances, and cloud properties to assess the performance of several machine learning algorithms on both noiseless and noisy data to provide classification and regression results as a function of signal to noise of the data. In all cases, the algorithms were tested on noisy validation data. The results show that the use of machine learning to characterize giant planets from reflected broad-band filter photometry provides a promising tool for initial characterization, with over 65 per cent accuracy in characterizing metallicity for signal-to-noise ratios (S/N) ≳ 30, over 80 per cent for cloud coverage for S/N ≳ 30. This approach will allow initial characterization for large surveys of giant exoplanets and prioritization for spectroscopy observations of a subset of these worlds.

The high optical brightness of the BlueWalker 3 satellite

Large constellations of bright artificial satellites in low Earth orbit pose significant challenges to ground-based astronomy1. Current orbiting constellation satellites have brightnesses between apparent magnitudes 4 and 6, whereas in the near-infrared Ks band, they can reach magnitude 2 (ref. 2). Satellite operators, astronomers and other users of the night sky are working on brightness mitigation strategies3,4. Radio emissions induce further potential risk to ground-based radio telescopes that also need to be evaluated. Here we report the outcome of an international optical observation campaign of a prototype constellation satellite, AST SpaceMobile’s BlueWalker 3. BlueWalker 3 features a 64.3 m2 phased-array antenna as well as a launch vehicle adaptor (LVA)5. The peak brightness of the satellite reached an apparent magnitude of 0.4. This made the new satellite one of the brightest objects in the night sky. Additionally, the LVA reached an apparent V-band magnitude of 5.5, four times brighter than the current International Astronomical Union recommendation of magnitude 7 (refs. 3,6); it jettisoned on 10 November 2022 (Universal Time), and its orbital ephemeris was not publicly released until 4 days later. The expected build-out of constellations with hundreds of thousands of new bright objects1 will make active satellite tracking and avoidance strategies a necessity for ground-based telescopes.

Research on piston error sensing for segmented mirrors under atmospheric turbulence.

Large aperture ground-based segmented telescopes typically use electrical edge sensors to detect co-phase errors. However, complex observing environments can lead to zero-point drift of the edge sensors, making it challenging to maintain the long-term co-phase of the segmented primary mirror using only edge sensors. Therefore, employing optical piston error detection methods for short-term calibration of edge sensors can address the issue of zero-point drift in the sensors. However, atmospheric turbulence can affect calibration accuracy based on the observational target. To achieve high-precision calibration of electrical edge sensors, this study investigates the impact of atmospheric turbulence on optical piston error detection. Based on simulated results, it is found that the actual measured piston error in the presence of atmospheric turbulence is the difference between the average phases of the two segments. Subsequently, optical piston error detection experiments were conducted in a segmented mirror system under simulated turbulent conditions with varying turbulence intensities. Experimental studies have shown that the detection accuracy of the optical method is almost the same as without turbulence when using a detection aperture size that is 0.82 times the atmospheric coherence length and an exposure time of at least 40 ms. The root mean square of the cross-calibration is better than 3nm. These experimental results indicate that under conditions of good atmospheric seeing, the optical piston error detection method can meet the short-term calibration requirements of edge sensors by setting reasonable detection area size and exposure time. It may even be possible to directly use optical detection methods to replace edge sensors for real-time detection of piston errors.

Optical sparse telescope arrays and scintillation noise

ABSTRACT Fresnel propagation of starlight after it passes through high altitude turbulence in the Earth’s atmosphere results in random fluctuations of the intensity at ground level, known as scintillation. This effect adds random noise to photometric measurements with ground-based optical telescopes. Spatial correlation of the intensity fluctuations means that the fractional photometric noise due to scintillation may be substantially smaller for a sparse array of small aperture telescopes than for a single large aperture of the same total area. Assuming that the photometric noise for each telescope is independent, averaging the light curves measured by N telescopes reduces the noise by a factor of $\sqrt{N}$. For example, for bright stars, the signal-to-noise ratio of a 2.54 m telescope can be achieved for an array of thirty 20 cm telescopes if the scintillation noise measured for each telescope is uncorrelated. In this paper, we present results from simulation and from observations at the Isaac Newton Telescope. These explore the impact that several parameters have on the measured correlation of the scintillation noise between neighbouring telescopes. We show that there is significant correlation between neighbouring telescopes with separations parallel to the wind direction of the dominant high altitude turbulent layer. We find that the telescopes in an array should be separated by at least twice their aperture diameter so that there is negligible correlation of the photometric noise. We discuss additional benefits of using sparse telescope arrays, including reduced cost and increased field of view.

Polarization-dependent beam shifts upon metallic reflection in high-contrast imagers and telescopes

Context. To directly image rocky exoplanets in reflected (polarized) light, future space- and ground-based high-contrast imagers and telescopes aim to reach extreme contrasts at close separations from the star. However, the achievable contrast will be limited by reflection-induced polarization aberrations. While polarization aberrations can be modeled with numerical codes, these computations provide little insight into the full range of effects, their origin and characteristics, and possible ways to mitigate them. Aims. We aim to understand polarization aberrations produced by reflection off flat metallic mirrors at the fundamental level. Methods. We used polarization ray tracing to numerically compute polarization aberrations and interpret the results in terms of the polarization-dependent spatial and angular Goos-Hänchen and Imbert-Federov shifts of the beam of light as described with closed-form mathematical expressions in the physics literature. Results. We find that all four beam shifts are fully reproduced by polarization ray tracing. We study the origin and characteristics of the shifts as well as the dependence of their size and direction on the beam intensity profile, incident polarization state, angle of incidence, mirror material, and wavelength. Of the four beam shifts, only the spatial Goos-Hänchen and Imbert-Federov shifts are relevant for high-contrast imagers and telescopes because these shifts are visible in the focal plane and create a polarization structure in the point-spread function that reduces the performance of coronagraphs and the polarimetric speckle suppression close to the star. Conclusions. Our study provides a fundamental understanding of the polarization aberrations resulting from reflection off flat metallic mirrors in terms of beam shifts and lays out the analytical and numerical tools to describe these shifts. The beam shifts in an optical system can be mitigated by keeping the f-numbers large and angles of incidence small. Most importantly, mirror coatings should not be optimized for maximum reflectivity, but should be designed to have a retardance close to 180°. The insights from our study can be applied to improve the performance of SPHERE-ZIMPOL at the VLT and future telescopes and instruments such as the Roman Space Telescope, the Habitable Worlds Observatory, GMagAO-X at the GMT, PSI at the TMT, and PCS (or EPICS) at the ELT.

Non-modulated pyramid wavefront sensor

Context.The diffusion of adaptive optics systems in astronomical instrumentation for large ground-based telescopes is rapidly increasing and the pyramid wavefront sensor is replacing the Shack–Hartmann as the standard solution for single conjugate adaptive optics systems. The pyramid wavefront sensor is typically used with a tip-tilt modulation to increase the linearity range of the sensor, but the non-modulated case is interesting because it maximizes the sensor sensitivity. The latter case is generally avoided for the reduced linearity range that prevents robust operation in the presence of atmospheric turbulence.Aims.We aim to solve part of the issues of the non-modulated pyramid wavefront sensor by reducing the model error in the interaction matrix. We linearize the sensor response in the working conditions without extending the sensor linearity range.Methods.We developed a new calibration approach to model the response of pyramid wave front sensor in partial correction, whereby the working conditions in the presence of residual turbulence are considered.Results.We use in simulations to show how the new calibration approach allows for the pyramid wave front sensor without modulation to be used to sense and correct atmospheric turbulence and we discuss when this case is preferable over the modulated case.

Design and Experimental Analysis of an Optical Fiber Coupling System for a Ground-based Telescope with Adaptive Optics

Astronomy photonics has opened up a new era for the application of astronomical optical instruments. The fiber coupling system serves as the crucial link between the telescope and photonic devices. This paper explores a beam shaping method that utilizes a coupled lens to enhance the efficiency of coupling light into an optical single mode fiber. Compared to directly coupling the telescope beam into the fiber, this approach offers improved coupling efficiency and greater adjustment tolerance. The laboratory-based optical fiber coupling system described in this study comprises an imaging component with an F/50 ratio and a fiber coupling component. Theoretical analysis indicates that optimal coupling efficiency is achieved when the diameter of the focusing spot, limited by diffraction, matches the fiber core size. Any axial error, position error, or tip-tilt error between the lens and the fiber will reduce the coupling efficiency. Experimental results confirm that the coupling system achieves an efficiency of approximately 70%, which is close to the theoretical limit of 78%. These findings underscore the effectiveness of the fiber coupling method.

GRB optical and X-ray plateau properties classifier using unsupervised machine learning

ABSTRACT The division of gamma-ray bursts (GRBs) into different classes, other than the ‘short’ and ‘long’, has been an active field of research. We investigate whether GRBs can be classified based on a broader set of parameters, including prompt and plateau emission ones. Observational evidence suggests the existence of more GRB subclasses, but results so far are either conflicting or not statistically significant. The novelty here is producing a machine-learning-based classification of GRBs using their observed X-rays and optical properties. We used two data samples: the first, composed of 203 GRBs, is from the Neil Gehrels Swift Observatory (Swift/XRT), and the latter, composed of 134 GRBs, is from the ground-based Telescopes and Swift/UVOT. Both samples possess the plateau emission (a flat part of the light curve happening after the prompt emission, the main GRB event). We have applied the Gaussian mixture model (GMM) to explore multiple parameter spaces and subclass combinations to reveal if there is a match between the current observational subclasses and the statistical classification. With these samples and the algorithm, we spot a few microtrends in certain cases, but we cannot conclude that any clear trend exists in classifying GRBs. These microtrends could point towards a deeper understanding of the physical meaning of these classes (e.g. a different environment of the same progenitor or different progenitors). However, a larger sample and different algorithms could achieve such goals. Thus, this methodology can lead to deeper insights in the future.

Benefits of Intelligent Fuzzy Controllers in Comparison to Classical Methods for Adaptive Optics

Adaptive Optics (AO) systems have been developed throughout recent decades as a strategy to compensate for the effects of atmospheric turbulence, primarily caused by poor astronomical seeing. These systems reduce the wavefront distortions using deformable mirrors. Several AO simulation tools have been developed, such as the Object-Oriented, MATLAB, and Adaptive Optics Toolbox (OOMAO), to assist in the project of AO. However, the main AO simulators focus on AO models, not prioritizing the different control techniques. Moreover, the commonly applied control strategies in ground-based telescopes are based on Integral (I) or Proportional-Integral (PI) controllers. This work proposes the integration of OOMAO models to Simulink to support the development of advanced controllers and compares traditional controllers with intelligent systems based on fuzzy logic. The controllers were compared in three scenarios of different turbulence and atmosphere conditions. The simulations were performed using the characteristics/parameters of the Southern Astrophysical Research (SOAR) telescope and assessed with the Full Width at Half Maximum (FWHM), Half Light Radius (HLR), and Strehl ratio metrics to compare the performance of the controllers. The results demonstrate that adaptive optics can be satisfactorily simulated in OOMAO adapted to Simulink and thus further increase the number of control strategies available to OOMAO. The comparative results between the MATLAB script and the Simulink blocks designed showed a maximum relative error of 3% in the Strehl ratio and 1.59% in the FWHM measurement. In the assessment of the control algorithms, the fuzzy PI controller reported a 25% increase in the FWHM metrics in the critical scenario when compared with open-loop metrics. Furthermore, the fuzzy PI controller outperformed the results when compared with the I and PI controllers. The findings underscore the constraints of conventional control methods, whereas the implementation of fuzzy-based controllers showcases the promise of intelligent approaches in enhancing control performance under challenging atmospheric conditions.

Astrometry via close approach events: applications to main-belt asteroid (702) Alauda

ABSTRACT The release of Gaia catalogue is revolutionary to the astronomy of Solar system objects. After some effects such as atmospheric refraction and CCD geometric distortion have been taken into account, the astrometric precision for ground-based telescopes can reach the level of tens of milli-arcseconds (mas). If an object approaches a reference star in a small relative angular distance (less than 100 arcsec), which is called close approach event in this work, the relative positional precision between the object and reference star will be further improved since the systematic effects of atmospheric turbulence and local telescope optics can be reduced. To obtain the precise position of a main-belt asteroid in an close approach event, a second-order angular velocity model with time is supposed in the sky plane. By fitting the relationship between the relative angular distance and observed time, we can derive the time of maximum approximation and calculate the corresponding position of the asteroid. In practice, five nights’ CCD observations including 15 close approach events of main-belt asteroid (702) Alauda are taken for testing by the 1 m telescope at Yunnan Observatory, China. Compared with conventional solutions, our results show that the positional precision significantly improves, which reaches better than 4 mas, and 1 mas in the best case when referenced for the Jet Propulsion Laboratory (JPL) ephemeris in both right ascension and declination.