Ground-based Optical Telescopes Research Articles

A Possibly Short GRB 180418A and Phenomenology of Reverse-Shock Emission in the Optical Afterglows of Gamma-Ray Bursts

We present early-time ground-based optical follow-up observations of GRB 180418A, which was discovered by both Swift/BAT and Fermi/GBM. Its broadband afterglow was well monitored by Swift/XRT and ground-based optical telescopes. The optical light curve of GRB 180418A can be modeled by forward shock (FS) plus reverse shock (RS). We fit the light curves with standard external shock models and derive the physical properties of the outflow. It is found that the ratio R B ≡ ε B,r /ε B,f is 11.22, indicating a moderate degree of magnetization in the RS region. The reported duration of GRB 180418A, T 90, lies in the intermediate region between short and long gamma-ray bursts (GRBs). We further discuss the classification of GRB 180418A, and calculate ε = E γ,iso,52/E p,z,2 values of 0.026 and 0.018 (assuming the redshift z is 1.0 and 1.5, respectively), which is closer to short GRBs (SGRBs) in the ε-T 90,z plane. If GRB 180418A is an SGRB, it is the only reported SGRB thus far with RS emission in optical light curves. In order to compare the properties of GRB 180418A, we collected three SGRBs that may have RS emission (GRBs 060313, 090426, and 210207B) and also 22 long GRBs (LGRBs) with RS emission. We find that the parameters of LGRBs are in a wider range than those of SGRBs. Also, SGRBs appear to have very small R B values, but the results are generally similar to those of LGRBs. The fitting parameters of GRB 180418A are generally consistent with those of the other three SGRBs, implying that GRB 180418A may belong to the category of SGRBs.

PASAT: pathfinder in solar adaptive telescope

In the forefront of quantitative solar physics research using large-aperture ground-based solar optical telescopes, high-contrast observation along with high-accuracy polarimetric measurement in the solar active region are required. In this paper, we propose a novel high-contrast imaging telescope construction with a 60 cm medium aperture, namely, the PAthfinder in Solar Adaptive Telescope (PASAT), in which a deformable secondary mirror is used as the adaptive optical correction device and a symmetrical optical path design is employed, leading to the least Muller matrix polarization instruments. The telescope can provide a high-resolution magnetic field with high accuracy for the solar active regions, as well as high-contrast images with a superior signal-to-noise ratio and photometric accuracy of the solar photosphere and chromosphere. These data will be directly used for a better understanding of the evolution and release of magnetic energy, which will help in improving space weather forecasting. Meanwhile, PASAT will accumulate the relevant techniques for constructing similar, larger solar telescopes in the future.

Challenges in Identifying Artificial Objects in the Near-Earth Object Population: Spectral Characterization of 2020 SO

Since the dawn of the Space Age, hundreds of payloads have been launched into heliocentric space. As near-Earth object (NEO) surveys search deeper for small asteroids, more artificial objects in heliocentric orbits are being discovered. We now face a challenge to identify the true nature of these objects and avoid contaminating the NEO catalog. Here, we present the methods used to characterize one such object. 2020 SO was discovered by the Pan-STARRS1 survey on 2020 September 17. Originally classified as a NEO, the object’s artificial nature became evident due to its low velocity relative to Earth and solar radiation pressure affecting its orbit about the Sun. Based on a backward propagation of its orbit, 2020 SO is thought to be a Centaur rocket body (R/B) from the launch of the Surveyor 2 mission to the Moon. We characterized 2020 SO using a range of ground-based optical and near-infrared telescopes to constrain its true nature. We find that its reflectance spectrum is consistent with that of other Centaur R/B launched during a similar time frame, and we identify 1.4, 1.7, and 2.3 μm absorption bands consistent with polyvinyl fluoride used on the aft bulkhead radiation shield exterior of Centaur-D R/B at the time.

Optical Transient Source AT2021lfa: A Possible “Dirty Fireball”

AT2021lfa, also known as ZTF21aayokph, was detected by the Zwicky Transient Facility on 2021 May 4, at 05:34:48 UTC. Follow-up observations were conducted using a range of ground-based optical telescopes, as well as Swift/XRT and VLA instruments. AT2021lfa is classified as an “orphan afterglow” candidate due to its rapid flux decline and its reddened color (g − r = 0.17 ± 0.14 mag). For an optical transient source without prompt gamma-ray detection, one key point is to determine its burst time. Here we measure the burst time through fitting the initial bump feature of AT2021lfa and obtain its burst time as 2021 May 3, at 22:09:50 UTC. Using afterglowpy, we model the multi-band afterglow of AT2021lfa and find that the standard model cannot reproduce the late radio observations well. Considering that the microphysical parameters ϵ e , ϵ B (the energy fraction given to electrons and magnetic field), and ξ N (the fraction of accelerated electrons) may vary with time, we then model the afterglow of AT2021lfa taking into account the temporal evolution of the physical parameters ϵ e , ϵ B , and ξ N and find in this case the multi-wavelength observations can be reproduced well. The initial Lorentz factor of AT2021lfa can be estimated from the peak time of the early afterglow, which yields a value of about 18, suggesting that AT2021lfa should be classified as a “dirty fireball.” From the upper limit for the prompt emission energy of AT2021lfa, we obtain that the radiation efficiency is less than 0.02%, which is much smaller than that of ordinary gamma-ray bursts (GRBs). It is also interesting that the fitted values of jet angle and viewing angle are very large, θ c ∼ 0.66 rad, θ v ∼ 0.53 rad, which may lead to the low Lorentz factor and radiation efficiency. When compared with GRB afterglow samples, it is evident that the onset bump timescale of AT2021lfa satisfies the empirical relationships observed in GRB samples. Additionally, the luminosity of AT2021lfa falls within the range of observations for GRB samples; however, approximately 1 day after the burst, its luminosity exceeds that of the majority of GRB samples.

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.

Photonics Scanning Pentaprism System for the Integrated Inspection of Large-Aperture Telescopes.

To improve their spatial resolution and detection capabilities, future ground-based optical telescopes will have a size of 30 m, and the aperture of space telescopes will be increased to 10 m. Such large optical systems necessitate the development of large integrated testing equipment. In this study, spectrum and system alignment measurements and wavefront quality checking were performed using the sub-aperture detection method and a fiber-connected Photonics Scanning Pentaprism (PSP). First, the system was aligned using an optical truss, ensuring that the optical axis was properly positioned. Second, using a sub-aperture light beam though the entrance pupil, light spots were formed on the focal plane and transmitted to the spectrometer via fibers to obtain the corresponding spectral components. Then, by taking measurements at different system positions, a full-aperture spectrum response could be reached. Lastly, by photon-integrated interference on the focal plane, intensity interference fringes could be projected at the entrance pupil of the system. And the wavefront quality of the system could be verified by observing the fringe deformation. The measurement accuracy of the optical axis of the system is better than 2 mrad. The spectral measurement accuracy was better than 5%, and the wavefront measurement accuracy surpassed 0.1 wavelengths (1 wavelength = 633 nm). This study effectively enhanced the detection and in situ calibration capabilities of large telescope systems, ensuring that the performance requirements can be met in the design of future telescopes.

A simulation framework for telescope array and its application in distributed reinforcement learning-based scheduling of telescope arrays

Time-domain astronomy necessitates continuous observation of celestial objects across the entire sky, with specific observation depth and cadence requirements. Telescope arrays, comprised of numerous wide-field optical telescopes, have emerged as a novel observational instrument for time-domain astronomy. However, the observation capabilities of ground-based optical telescope arrays are often constrained by various dynamic factors such as clouds, satellites, and sky background. To meet the observation requirements, active scheduling of telescopes within a telescope array is essential, leveraging telemetry data from the environment. However, due to the complexity and cost of telescope arrays, it is impractical to directly design or test algorithms using a physical telescope array. In this paper, we propose a framework for simulating telescope arrays that incorporates the majority of real observation effects. Building upon this framework, we further introduce a scheduler based on a distributed reinforcement learning framework to optimize the observation strategy of telescopes within the array. The scheduler is trained and evaluated using the simulator. Results demonstrate that the distributed control framework-based scheduler enhances the observation efficiency of the telescope array.

Satellite Star Tracker Breadboard with Space Debris Detection Capability for LEO

This paper evaluates the possibility of having a star tracker device running space debris algorithms. A simple star tracker breadboard was developed to evaluate the possibility of having a device running both stellar identification and space debris algorithms. The breadboard was built with commercial off-the-shelf components, representing the current star tracker resolution and field of view. A star tracker device and space debris algorithms were implemented and tested, respectively: Tetra and ASTRiDE. The device concept was tested by taking pictures of the night sky with satellite streaks. Seeking to overcome such limitations, a dual-purpose star tracker with stars detection and optical debris detection capability is proposed. Star trackers are usually used in satellites for attitude determination and therefore have a vast potential to be a major tool for space debris detection. The rapid increase of space debris poses a risk to space activities, so it is vital to detect it. Ground-based radar and optical telescope techniques used for debris detection are limited by a size threshold, detecting only a tiny amount of the total, reason why evaluating the possibility of detecting them in space is of major importance.

Conceptual design of the optical system of the 6.5m wide field multiplexed survey telescope with excellent image quality

Astrophysics and cosmology in the coming decades urgently need a large field-of-view (FOV), highly multiplexed spectroscopic survey telescope satisfying challenging image quality and stability requirements. The 6.5 m MUltiplexed Survey Telescope (MUST) proposed by Tsinghua University will be constructed on the Saishiteng Mountain of Northwest China to improve the spectroscopic survey capability of ground-based optical telescopes. In this paper, we demonstrate the conceptual design of the optical system of MUST. MUST will adopt a 6.5 m primary mirror, a 2.45 m secondary mirror, and a multiple-element widefield corrector (WFC) to ensure excellent image quality with an 80% encircled energy size of image spots less than ~ 0.6 arcsec in diameter for the entire 3° FOV and the whole 50° zenith angle range. Thanks to its compact 6.5 m Ritchey-Chretien system and 20,000 optical fibers on its Cassegrain focus, MUST will carry out state-of-the-art wide-field spectroscopic surveys with efficiency ~ 19 times higher than the Dark Energy Spectroscopic Instrument (DESI) using a measure proposed by Ellis et al. Upon completion around 2029, MUST will be one of the world's most advanced wide-field spectroscopic survey telescopes and a new essential reference for the future development of wide-field survey telescopes. It will enable significant advances in many fields in astrophysics and cosmology.

Discovery of Calm Astronomical Sites Over the Antarctic Continent

A calm astronomical site means a site where astronomical observation would be less likely to be interfered with by optical turbulence. Previous turbulence measurements at a few sites in Antarctica have demonstrated very calm atmospheric conditions here. So far, to realize a wide range of measurements of the turbulence conditions above the Antarctic plateau will be a great hardship. Thus, in this study, the numerical weather model outputs provided by the Antarctic Mesoscale Prediction System (AMPS) have been used. Based on the AMPS outputs, the boundary layer height and the atmospheric Richardson number were obtained, from which the turbulence conditions above the Antarctic plateau have been evaluated. Finally, a statistical conclusion evaluating the total atmospheric turbulence above the whole Antarctic continent for an entire year is first reported. We find some sites (or regions) have a calmer atmosphere than Dome A; this is of great instructional significance for planning the next generation of ground-based optical astronomical telescopes.

SHIMM: a versatile seeing monitor for astronomy

ABSTRACTCharacterization of atmospheric optical turbulence is crucial for the design and operation of modern ground-based optical telescopes. In particular, the effective application of adaptive optics correction on large and extremely large telescopes relies on a detailed knowledge of the prevailing atmospheric conditions, including the vertical profile of the optical turbulence strength and the atmospheric coherence time-scale. The Differential Image Motion Monitor (DIMM) has been employed as a facility seeing monitor at many astronomical observing sites across the world for several decades, providing a reliable estimate of the seeing angle. Here, we present the Shack–Hartmann Image Motion Monitor (SHIMM), which is a development of the DIMM instrument, in that it exploits differential image motion measurements of bright target stars. However, the SHIMM employs a Shack–Hartmann wavefront sensor in place of the two-hole aperture mask utilized by the DIMM. This allows the SHIMM to provide an estimate of the seeing, unbiased by shot noise or scintillation effects. The SHIMM also produces a low-resolution (three-layer) measure of the vertical turbulence profile, as well as an estimate of the coherence time-scale. The SHIMM is designed as a low-cost, portable instrument. It is comprised of off-the-shelf components so that it is easy to duplicate and well suited for comparisons of atmospheric conditions within and between different observing sites. Here, the SHIMM design and methodology for estimating key atmospheric parameters will be presented, as well as initial field test results with comparisons to the Stereo-SCIntillation Detection And Ranging instrument.

Nonuniform Correction of Ground-Based Optical Telescope Image Based on Conditional Generative Adversarial Network.

Ground-based telescopes are often affected by vignetting, stray light and detector nonuniformity when acquiring space images. This paper presents a space image nonuniform correction method using the conditional generative adversarial network (CGAN). Firstly, we create a dataset for training by introducing the physical vignetting model and by designing the simulation polynomial to realize the nonuniform background. Secondly, we develop a robust conditional generative adversarial network (CGAN) for learning the nonuniform background, in which we improve the network structure of the generator. The experimental results include a simulated dataset and authentic space images. The proposed method can effectively remove the nonuniform background of space images, achieve the Mean Square Error (MSE) of 4.56 in the simulation dataset, and improve the target's signal-to-noise ratio (SNR) by 43.87% in the real image correction.

First Study of Optical Turbulence Over an Astronomical Site in the Central Himalayas Using ST Radar Observations

Optical turbulence in the atmosphere is a key factor which governs the sharpness of images produced from ground-based optical telescopes. The intensity of optical turbulence can be conveniently assessed by the estimation of the refractive index structure constant ([Formula: see text]). Here, observations from a stratosphere troposphere radar (206.5[Formula: see text]MHz) that is co-located with the optical telescope facilities in the central Himalayas, are used for the estimation of [Formula: see text]. The integrated profile of [Formula: see text] and winds over about 2–20[Formula: see text]km have been used to estimate seasonal and diurnal variation of “seeing”, wavefront coherence time, isoplanatic angle and scintillation rate. The mean [Formula: see text] over the site varies from 10[Formula: see text] to 10[Formula: see text][Formula: see text]m[Formula: see text] with the largest values in the monsoon and post-monsoon season. The best “seeing” conditions are observed in the winter and post-monsoon season with median “seeing” varying from 0.39[Formula: see text] to 0.81[Formula: see text]. The largest contribution to the diurnal variation of “seeing” comes from turbulences at 2–5[Formula: see text]km height region. A weak correlation of diurnal variation of “seeing” has been observed with surface temperature and relative humidity (RH). Among astroclimatic parameters, the largest isoplanatic angle is associated with winter season with median value of 2.57[Formula: see text] and wavefront coherence time of ∼4.2[Formula: see text]ms. The least scintillation in intensity is observed in the winter season with a rate of ∼10% and most in monsoon season with ∼45% rate. This study is expected to have a positive impact on the optimization of the operation of adaptive optical techniques, observing time and scheduling of scientific programmes for optical telescope facilities in this region of the central Himalayas.

Correction performance uniformity on ground-layer adaptive optics

ABSTRACTGround-layer adaptive optics (GLAO) is generally used to improve the seeing for large-aperture ground-based optical telescopes across a wide field of view with uniform imaging quality. In this paper, a new criterion, which is defined as the weighted sum of the mean and mean-square-error values of the full width at half-maximum of the field-dependent point spread function of the image over the whole field of view, is proposed to evaluate GLAO performance uniformity. An analytic model of the GLAO-corrected point spread function based on spatial filtering methods is used to investigate the uniformity and image quality over the whole field of view for different guide-star layouts, telescope apertures, numbers of actuators for the deformable mirror, and turbulence profiles. Simulations are performed on the GLAO configurations for the 1-m New Vacuum Solar Telescope at Fuxian Solar Observatory. The results show that, compared with that of the classical evaluation, the correction uniformity improves by 63 per cent, with the resolution sacrificing only 5 per cent for a measured turbulence profile at Fuxian Solar Observatory. Furthermore, this criterion is applied to various requirements, and some critical system parameters are suggested for the wide field-of-view GLAO.

Image Subtraction in Fourier Space

Image subtraction is essential for transient detection in time-domain astronomy. The point-spread function (PSF), photometric scaling, and sky background generally vary with time and across the field of view for imaging data taken with ground-based optical telescopes. Image subtraction algorithms need to match these variations for the detection of flux variability. An algorithm that can be fully parallelized is highly desirable for future time-domain surveys. Here we introduce the saccadic fast Fourier transform (SFFT) algorithm we developed for image differencing. SFFT uses a δ-function basis for kernel decomposition, and the image subtraction is performed in Fourier space. This brings about a remarkable improvement in computational performance of about an order of magnitude compared to other published image subtraction codes. SFFT can accommodate the spatial variations in wide-field imaging data, including PSF, photometric scaling, and sky background. However, the flexibility of the δ-function basis may also make it more prone to overfitting. The algorithm has been tested extensively on real astronomical data taken by a variety of telescopes. Moreover, the SFFT code allows for the spatial variations of the PSF and sky background to be fitted by spline functions.

Flip-flop modulation method used with a pyramid wavefront sensor to correct piston segmentation on ELTs

The next generation of ground-based optical telescopes, such as the European Southern Observatory’s Extremely Large Telescope (the ELT), will have large support structures (spiders) for the secondary mirror. These spiders have the effect of segmenting the pupil. Without careful control of the wavefront, segment piston (petal modes) errors can develop. We present a flip-flop modulated/unmodulated method for the pyramid wavefront sensor (PWFS) enabling the PWFS to sense petal piston modes. This flip-flop modulation method uses a single PWFS operating in two states: a modulated state and an unmodulated state. An independent controller is used in each state; the modulated state controls the atmospheric turbulence and the unmodulated state only controls petal piston modes. In simulation, we show the flip-flop method working with the wavefront sensor in both K- and R-bands, providing an improvement of 9.9% and 13%, respectively, over a standard modulated PWFS.

Warning of asteroids approaching Earth from the sunward direction using two Earth-leading heliocentric orbiting telescopes

Ground-based optical telescopes suffer from a blind zone surrounding the Sun. In this article, two telescopes were proposed to be deployed on an Earth‑leading heliocentric orbit approximately 10 million kilometers ahead of the Earth to warn of asteroids approaching Earth from the sunward direction. Considering the initial orbit determination, a discovery model for asteroids was established. Based on Granvik's impactor and near-Earth asteroid (NEA) populations, the warning efficiency of the Earth‑leading heliocentric orbiting telescopes for asteroids approaching Earth from the sunward direction was simulated. The simulation results show that under the designed search strategy, with a limiting apparent magnitude of 24, the warning efficiency of two Earth‑leading heliocentric orbiting telescopes for impactors and NEAs approaching Earth from the sunward direction reaches 99.0% and 94.3% (with 0.4% and 0.8% uncertainty, respectively) within 6 years, which is 95.8% and 57.5% (with 0.7% and 1.7% uncertainty, respectively) for one telescope deployed on an Earth‑leading heliocentric orbit and 22.1% (with 1.5% uncertainty) for two telescopes deployed on a Sun-Earth L1 orbit. Earth‑leading heliocentric orbiting telescopes thus provide a possible option to cover the blind zone of ground-based optical telescopes.

Three dimensional isolation system for large ground-based optical telescope

Three dimensional isolation system for large ground-based optical telescope

Wind disturbance simulation and optical performance prediction for large ground-based optical telescopes based on two-dimensional stochastic fields

Wind disturbance simulation and optical performance prediction for large ground-based optical telescopes based on two-dimensional stochastic fields

Automatic Scheduling Tool for Balloon-Borne Planetary Optical Remote Sensing

The balloon-borne Planetary Atmosphere Spectroscopic Telescope (PAST), China’s first planetary optical remote-sensing project, will be launched for testing and conducting scientific flights during 2021 and 2022. Images of the planetary atmosphere and plasma in ultraviolet and visible wavelengths will be used to investigate the diversity of the planetary space environment in the solar system and their different drivers. Because simultaneous observation of multiple target planets in the solar system is possible, effective observation scheduling is critical to acquire high scientific merit spectroscopic imaging data. Herein, we demonstrate an automatic scheduling tool (AST) to aid the planning of observation schedules. The AST is primarily based on a planetary ephemeris and is realized on the basis of the geometrical information and optical requirements of the telescope. The temporal variations of the planetary reference frames can also be obtained to assist in the positioning and data processing of the telescope. As a part of the Chinese deep-space exploration plan, several ground-based planetary optical telescopes will be constructed in China in the future. With the use of the proposed AST, such telescopes can achieve maximum efficiency.