Large Survey Telescope Research Articles

Identification of Blue Horizontal Branch Stars with Multimodal Fusion

Blue Horizontal Branch stars (BHBs) are ideal tracers to probe the global structure of the milky Way (MW), and the increased size of the BHB star sample could be helpful to accurately calculate the MW’s enclosed mass and kinematics. Large survey telescopes have produced an increasing number of astronomical images and spectra. However, traditional methods of identifying BHBs are limited in dealing with the large scale of astronomical data. A fast and efficient way of identifying BHBs can provide a more significant sample for further analysis and research. Therefore, in order to fully use the various data observed and further improve the identification accuracy of BHBs, we have innovatively proposed and implemented a Bi-level attention mechanism-based Transformer multimodal fusion model, called Bi-level Attention in the Transformer with Multimodality (BATMM). The model consists of a spectrum encoder, an image encoder, and a Transformer multimodal fusion module. The Transformer enables the effective fusion of data from two modalities, namely image and spectrum, by using the proposed Bi-level attention mechanism, including cross-attention and self-attention. As a result, the information from the different modalities complements each other, thus improving the accuracy of the identification of BHBs. The experimental results show that the F1 score of the proposed BATMM is 94.78%, which is 21.77% and 2.76% higher than the image and spectral unimodality, respectively. It is therefore demonstrated that higher identification accuracy of BHBs can be achieved by means of using data from multiple modalities and employing an efficient data fusion strategy.

Photonics Large-Survey Telescope Internal Motion Metrology System

Large survey telescopes are vital for mapping dark energy and dark matter in the deep universe. This study presents a fiber-linked internal motion metrology system that aligns the mirrors and large lenses in the telescopes to enhance alignment accuracy by improving the image quality at a lower weight, volume, power, and cost. The internal motion system comprises a photonic laser beam projector capable of projecting multiple Gaussian beams onto the detector of the telescope. The specific spatial frequency aberration component is determined by combining Gaussian beam location and the geometry model of the telescope. Furthermore, integrating the proposed system with the curvature-sensing wavefront system enables more precise alignment and camera sensing. In the experimental tests, the location precision was within 10 μm, and the rotation precision improved to 5 arcsecs, fulfilling the alignment and motion monitoring requirements of large survey telescopes. The results of this study can be used as a reference to improve the performance of closed-loop bandwidth systems and active camera optics.

In-Situ Metrology of Large Segmented Detector Based on Modified Optical Truss

To precisely map cosmic structures and capture time-domain events, future large survey telescopes will be equipped with segmented detectors that greatly expand their field of view. Segment flatness and assembly accuracy will be essential for them to make accurate observations. Instead of physically measuring distances between adjacent mirror surfaces, our modified optical truss enables the angle (slope) between them to be calculated. Furthermore, we calculated the relative tilt and piston between the segmented detectors.The modified optical truss can measure the flatness of a mosaicked detector before and after it is assembled into a telescope. We reduced the volume, weight, and power of our device compared with earlier versions. Its angular accuracy is now better than 0.1 arcsecs, and, at a maximum scope of 500 µm, the linear accuracy of the new truss is better than 25 µm. In fact, accuracy and testing ranges are comparable to instruments found in optics labs.

Hot white dwarfs and pre-white dwarfs discovered with SALT

ABSTRACT The Southern African Large Telescope survey of helium-rich hot subdwarfs aims to explore evolutionary pathways among groups of highly evolved stars. The selection criteria mean that several hot white dwarfs and related objects have also been included. This paper reports the discovery and analysis of eight new very hot white dwarf and pre-white dwarf stars with effective temperatures exceeding 100 000 K. They include two PG1159 stars, one DO white dwarf, three O(He), and two O(H) stars. One of the O(H) stars is the central star of a newly discovered planetary nebula, and the other is the hottest ‘naked’ O(H) star. Both of the PG1159 stars are GW Vir variables, one being the hottest GW Vir star measured and a crucial test for pulsation stability models. The DO white dwarf is also the hottest in its class.

Large survey camera collimation and alignment at the observation site by curvature sensing

Collimation and alignment of large survey cameras is an essential link for large survey telescope performance. A simple, stable, and fast collimation strategy is urgently needed after the correction lens assemblies are translated part-by-part at the observation site. This study proposes a method for the collimation of the correction lens assembly of a large survey telescope at the observation site. First, we investigated the method theoretically. The method was then demonstrated based on simulations in terms of reliability, while the wavefront of large transmission systems was characterized effectively. Finally, we optimized the alignment station to optimize the performance of the correction lens assembly. This study can enhance large survey telescopic developments and the design of similar large-transmission systems.

Alignment of DECam-like large survey telescope for real-time active optics and error analysis

To obtain relatively high imaging quality in the whole field of view and simultaneously create petabytes of data with a high signal-to-noise ratio, the alignment requirements for the dark energy camera (DECam)-like wide field active optics survey telescope are strict. Ensuring the real-time alignment of the system is the most important target for the future DECam-like large survey telescope due to the requirement for ellipticity of stellar image across the full field of view is much higher than the traditional telescopes. In such case, the alignment procedure needs to be designed more carefully to consider the large dynamical range and tight residual optical aberration at the same time The error of coarse alignment, influence of camera seeing, error of wavefront sensing, and interaction between the active optics and primary focus assembly and the active optics nonlinear factors, are analyzed by focusing on the DECam-like wide-field active optics survey telescope. Finally, the error of a real time active optics system is analyzed using the normalized point source sensitivity (PSSn). The PSSn of an ideal telescope is 0.9698 and the PSSn of a telescope with an alignment error is 0.8359. This work will be a good guide for the design of future large survey telescopes.

Two bright z > 6 quasars from VST ATLAS and a new method of optical plus mid-infrared colour selection

Abstract We present the discovery of two z > 6 quasars, selected as i-band dropouts in the Very Large Telescope Survey Telescope ATLAS survey. Our first quasar has redshift, z = 6.31 ± 0.03, z-band magnitude, zAB = 19.63 ± 0.08 and rest frame 1450 Å absolute magnitude, M1450 = −27.8 ± 0.2, making it the joint second most luminous quasar known at z > 6. The second quasar has z = 6.02 ± 0.03, zAB = 19.54 ± 0.08 and M1450 = −27.0 ± 0.1. We also recover a z = 5.86 quasar discovered by Venemans et al., in preparation. To select our quasars, we use a new 3D colour space, combining the atlas optical colours with mid-infrared data from the Wide-field Infrared Survey Explorer. We use iAB − zAB colour to exclude main-sequence stars, galaxies and lower redshift quasars, W1 − W2 to exclude L dwarfs and zAB − W2 to exclude T dwarfs. A restrictive set of colour cuts returns only our three high redshift quasars and no contaminants, albeit with a sample completeness of ∼50 per cent. We discuss how our 3D colour space can be used to reject the majority of contaminants from samples of bright 5.7 < z < 6.3 quasars, replacing follow-up near-infrared photometry, whilst retaining high completeness.

Photometric AGN reverberation mapping – an efficient tool for BLR sizes, black hole masses, and host-subtracted AGN luminosities

Photometric reverberation mapping employs a wide band pass to measure the AGN continuum variations and a suitable narrow band to trace the echo of an emission line in the broad line region (BLR). The narrow band catches both the emission line and the underlying continuum, and one needs to extract the pure emission line light curve. We performed a test on two local AGNs, PG0003+199 and Ark120, by observing well-sampled broad- (B, V) and narrow-band light curves with the robotic 15 cm telescope VYSOS-6 on Cerro Armazones, Chile. We find that, as long as the emission line contributes 50% to the band pass, the pure emission line light curve can be reconstructed from photometric monitoring data so that the time lag τ can be measured. For both objects the lags are consistent with spectroscopic reverberation results. We calculated virial black hole masses in agreement with literature values, by combining the BLR size RBLR (τ) from photometric monitoring with the velocity dispersion of a single contemporaneous spectrum. Applying the flux variation gradient method, we estimate the host galaxy contribution in the apertures used and the host-subtracted restframe 5100 Å luminosity LAGN. Our LAGN differs significantly from previous estimates, placing both sources ~50% closer to the RBLR − LAGN relation. This suggests that the scatter in the current RBLR − LAGN relation is largely caused by uncertainties in RBLR due to undersampled light curves and by uncertainties in the host-subtracted AGN luminosities inferred so far. If the scatter can be reduced, then two quasar samples matching in RBLR should also match in intrinsic LAGN, independent of redshift, thus offering the prospect of probing cosmological models. Photometric reverberation mapping opens the door to efficiently measuring hundreds of BLR sizes and host-subtracted AGN luminosities even with small telescopes, but also routinely with upcoming large survey telescopes like the LSST.

Active optics control of VST telescope secondary mirror

In telescopes based on active optics, defocus and coma are usually compensated for by secondary mirror movements. They are performed at the Very Large Telescope Survey Telescope (VST) with a hexapod--a parallel robot with six degrees of freedom positioning capability. We describe the application of the two-mirror telescope theory to the VST case and the solutions adopted for the hexapod control. We present the results of performance and reliability tests performed both in the laboratory and at the telescope.

Active optics primary mirror support system for the 26m VST telescope

The Very Large Telescope Survey Telescope (VST) is equipped with an active optics system in order to correct low-order aberrations. The 2.6 m primary mirror is supported both axially and laterally and is surrounded by several safety devices for earthquake protection. We describe the mirror support system and discuss the results of the qualification test campaign.

Correlated Noise and Gain in Unfilled and Epoxy‐Underfilled Hybridized HgCdTe Detectors

ABSTRACTSpatially correlated noise in focal plane arrays (FPAs) affects the photometric signal‐to‐noise ratio and can lead to overestimation of conversion gain (detector nodal capacitance). Capacitive coupling leads to charge sharing between neighboring pixels and correlates the measured pixel signal. This correlation can lead to image degradation, in particular for undersampled large survey telescopes. We present new comparative measurements of the interpixel coupling of 1.7 and 2.5 μm cutoff HgCdTe FPAs. Rockwell Scientific FPAs with epoxy underfill exhibit correlations that result in a 16%–20% systematic overestimation of the conversion gain. Results for a Raytheon Vision Systems 1.7 μm cutoff HgCdTe detector tested both before and after epoxy underfill show increased coupling in the presence of the epoxy, but the coupling is not spatially symmetric. We speculate that the difference in the observed correlation for each vendor is related to multiplexer geometry and pixel nodal capacitance.

A Strategy for Finding Near-Earth Objects with the SDSS Telescope

We present a detailed observational strategy for finding near-Earth objects (NEOs) with the Sloan Digital Sky Survey (SDSS) telescope. We investigate strategies in normal, unbinned mode, as well as binning the CCDs 2 × 2 or 3 × 3, which affects the sky coverage rate and the limiting apparent magnitude. We present results from 1 month, 3 year, and 10 year simulations of such surveys. For each cadence and binning mode, we evaluate the possibility of achieving the Spaceguard goal of detecting 90% of 1 km NEOs (absolute magnitude H ≤ 18 for an albedo of 0.1). We find that an unbinned survey is most effective at detecting H ≤ 20 NEOs in our sample. However, a 3 × 3 binned survey reaches the Spaceguard goal after only 7 years of operation. As the proposed large survey telescopes (Pan-STARRS, LSST) are at least 5–10 years from operation, an SDSS NEO survey could make a significant contribution to the detection and photometric characterization of the NEO population.