arXiv subjects

Albert Wai Kit Lau

Publications and source records attributed to Albert Wai Kit Lau.

12 recordsLinked to original sources

arXiv2026

Eclipses by Artificial Satellites to Measure the Angular Sizes of Stars

Direct measurements of the angular sizes of stars (other than our Sun) are inherently limited by telescopes' optical diffraction limit. The spatial diffraction limit can be overcome by using time-domain information when stars are `eclipsed' by anything moving in the foreground: artificial satellites or the Moon (known as `lunar occultation imaging'). Here we analyse the angular resolution achievable by high-speed photon counting devices, when stars are eclipsed for several microseconds by one of the thousands of satellites now in low or mid-Earth orbit. We also build a full Bayesian statistical treatment for analysing simulated observations. We show that eclipses by the moon and satellites deliver $<2$ milliarcsecond resolution for $\sim$$10^4$ stars every year, with the diameters of $\sim$$10^1 - 10^2$ stars constrained to better than $0.5$ milliarcseconds. The application of satellites requires an ultra-fast, photon-resolving detector, capable of continuous readout with inter-frame delay controlled within $\lesssim 1$ $μ$s. Such a fast rate can easily be handled by the SPINA instrument, which has demonstrated $8$ ns inter-frame delay, and potentially by the Cherenkov Telescope Array.

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arXiv2026

Low-cost, ultra-wideband, differential low-noise amplifier for interferometric radio telescopes

We present a low-cost, ultra-wideband, dual-polarized differential low-noise amplifier that eliminates the need for a pre-amplification balun. Built entirely from commercial surface-mount components, the design costs approximately 20 US dollars per unit. The amplifier has been characterized through laboratory gain and noise measurements together with feed-coupled hot-cold testing at the Dominion Radio Astrophysical Observatory. A noise figure close to 0.3 dB is demonstrated across a 10:1 bandwidth when matched to a constant 130 $Ω$ source, while feed-coupled measurements yield system noise temperatures as low as 25 K when matched to a Vivaldi feed operating over a 300-1500 MHz bandwidth.

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arXiv2026

Remembering George Smoot

George Smoot (1945-2025) was a major figure in observational cosmology, particularly through his role in the precision measurement of the CMB dipole and the discovery of the primordial CMB temperature anisotropy as part of the COBE team. This collection of 33 essays from collaborators, colleagues, students, and friends offers varied insights into the work, career, and life of George Smoot.

astro-ph.CO↗

arXiv2026

IQUEYE at Gemini South: instrument, science commission, and first results

The Italian quantum eye (IQUEYE) is a fast photon counter based on the single photon avalanche diode detectors and capable of preserving a ~0.5 ns/h accuracy photon time of arrival. IQUEYE was originally developed for intensity interferometry experiments, but now its scientific scope has been extended towards ultra fast astronomy, including optical pulsars, millisecond pulsars and the enigmatic fast radio bursts. IQUEYE's capabilities are mainly restricted by the number of photons detected, a quantity that scales with the collector size of an optical telescope. Through the visitor instrument program at Gemini South (Cerro Pachón, Chile) we brought IQUEYE to the 8.1-m dish, reaching an order magnitude sensitivity increased from previous operations. At Gemini South we installed IQUEYE to observe giant pulse emitters, millisecond pulsars, and transitional millisecond pulsars for over 40 hours in the span of a week.

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arXiv2025

False Alarm Rate based Statistical Detection Limit for Astronomical Photon Detectors

In ultra-fast astronomical observations featuring fast transients on sub-$μ$s time scales, the conventional Signal-to-Noise Ratio (SNR) threshold, often fixed at $5σ$, becomes inadequate as observational window timescales shorten, leading to unsustainably high False Alarm Rates (FAR). We provide a basic statistical framework that captures the essential noise generation processes relevant to the analysis of time series data from photon-counting detectors. In particular, we establish a protocol of defining detection limits in astronomical photon-counting experiments, such that a FAR-based criterion is preferred over the traditional SNR-based threshold scheme. We developed statistical models that account for noise sources such as dark counts, sky background, and crosstalk, and establish a probabilistic detection criterion applicable to high-speed detectors. The model is testified against the on-site data obtained in the Single-Photon Imager for Nanosecond Astrophysics (SPINA) experiment and consistency is confirmed. We compare the performance of several detector technologies, including photon-counting CMOS/CCDs, SPADs, SiPMs, and PMTs, in detecting faint astronomical signals. These findings offer insights into optimizing detector choice for future ultra-fast astronomical instruments and suggest pathways for improving detection fidelity under rapid observational conditions.

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arXiv2025

CHIME/FRB Outriggers: Design Overview

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has emerged as the world's premier facility for studying fast radio bursts (FRBs) through its fast transient search backend CHIME/FRB\@. The CHIME/FRB Outriggers project will augment this high detection rate of 2--3 FRBs per day with the ability to precisely localize them using very long baseline interferometry (VLBI). Using three strategically located stations in North America and deploying recently developed synoptic VLBI observing techniques, the Outriggers will provide $\sim 50$~milliarcsecond localization precision for the majority of detected FRBs. This paper presents an overview of the design and implementation of the Outriggers, covering their geographic distribution, structural design, and observational capabilities. We detail the scientific objectives driving the project, including the characterization of FRB populations, host galaxy demographics, and the use of FRBs as cosmological probes. We also discuss the calibration strategies available to mitigate ionospheric and instrumental effects, ensuring high-precision localization. With two stations currently in science operations, and the third in commissioning, the CHIME/FRB Outriggers project is poised to become a cornerstone of the FRB field, offering unprecedented insights into this enigmatic cosmic phenomenon.

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arXiv2023

WTH! Wok the Hydrogen: Measurement of Galactic Neutral Hydrogen in Noisy Urban Environment Using Kitchenware

Astronomy observation is difficult in urban environments due to the background noise generated by human activities. Consequently, promoting astronomy in metropolitan areas is challenging. In this work, we propose a low-cost, educational experiment called Wok the Hydrogen (WTH) that offers opportunities for scientific observation in urban environments, specifically the observation of the $21$ cm ($f_{21} = 1420.4$ MHz) emission from neutral hydrogen in the Milky Way. We demonstrate how to construct a radio telescope using kitchenware, along with additional electronic equipment that can be easily purchased online. The total system cost is controlled within 150 dollars. We also outline the subsequent data analysis procedures for deriving the recession velocity of galactic hydrogen from the raw data. The system was tested on the campus of the Hong Kong University of Science and Technology, which is located approximately 2 km northeast of the nearest residential area with a population of 0.4 million and about 10 km east of the downtown area with a population of 2 million. We show that a precision of $Δv \approx \pm 20$ km s$^{-1}$ can be achieved for determining the recession velocity of neutral hydrogen with this relatively simple setup, and the precision can be further improved with longer exposure time.

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arXiv2023

Initial On-Sky Performance testing of the Single-Photon Imager for Nanosecond Astrophysics (SPINA) system

This work presents an initial on-sky performance measurement of the Single-Photon Imager for Nanosecond Astrophysics (SPINA) system, part of our Ultra-Fast Astronomy (UFA) program. We developed the SPINA system based on the position-sensitive silicon photomultiplier (PS-SiPM) detector to record both photoelectron (P.E.) temporal and spatial information. The initial on-sky testing of the SPINA system was successfully performed on UT 2022 Jul 10, on the 0.7-meter aperture Nazarbayev University Transient Telescope at the Assy-Turgen Astrophysical Observatory (NUTTelA-TAO). We measured stars with a wide range of brightness and a dark region of the sky without stars $< 18$ mag. We measured the SPINA system's spatial resolution to be $<232μm$ (full-width half-maximum, FWHM), limited by the unstable atmosphere. We measured the total background noise (detector dark counts and sky background) of 1914 counts per second (cps) within this resolution element. We also performed a crosstalk mapping of the detector, obtaining the crosstalk probability of $\sim0.18$ near the detector's center while reaching $\sim 50\%$ at the edges. We derived a $5σ$ sensitivity of $17.45$ Gaia-BP magnitude in a 1s exposure with no atmospheric extinction by comparing the received flux with Gaia-BP band data. For a $10ms$ window and a false alarm rate of once per 100 nights, we derived a transient sensitivity of 14.06 mag. For a $1μs$ or faster time scale, we are limited by crosstalk to a 15 P.E. detection threshold. In addition, we demonstrated that the SPINA system is capable of capturing changes in the stellar profile FWHM of $\pm1.8\%$ and $\pm5\%$ change in the stellar profile FWHM in $20ms$ and $2ms$ exposures, respectively, as well as capturing stellar light curves on the $ms$ and $μs$ scales.

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arXiv2022

A SiPM photon-counting readout system for Ultra-Fast Astronomy

Very little work has been done searching for astrophysical transient optical emission in the millisecond to nanosecond regime with significant sensitivity. We call this regime "Ultra-Fast Astronomy", or UFA. To investigate transients on as short time scales as possible, we developed our own customized readout system for a silicon photomultiplier (SiPM)-based UFA camera, intended for use on conventional astronomical telescopes. SiPMs, available in array packages for imaging a field, are capable of time-tagged single-photon detection in the visible wavelength range. Our readout system consists of 16 channels of 14-bit data logging. Each channel includes a 50-dB gain pre-amplifier, signal shaping circuits, an analogue front end, an analogue to digital converter, and a Xilinx UltraScale+ Field Programable Gate Array Multipurpose System on Chip (FPGA-MPSoC)board for data-logging. We show that our system successfully read out the data from SiPM at 16 ns intervals with a maximum power consumption of 300 mW per channel and capability to perform concurrent 16 channels readout.

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arXiv2021

Constraining HeII Reionization Detection Uncertainties via Fast Radio Bursts

Context. The increased detection rate of Fast Radio Bursts (FRBs) makes it likely to get samples of sizes $\mathcal{O}(10^2)$ to $\mathcal{O}(10^3)$ in the near future. Because of their extragalactic origin can help us in understanding the epoch of helium reionization. Aims. We try to identify the epoch of Helium II (HeII) reionization, via the observations of early FRBs in range of $z=3$ to $4$. Methods. We build a model of FRB Dispersion Measure following the HeII reionization model, density fluctuation in large scale structure, host galaxy interstellar medium and local environment of FRB contribution. The model is fit to the ideal intergalactic medium (IGM) dispersion measure model to check the goodness of constraining the HeII reionization via FRB measurement statistics. Conclusion. We report our findings under two categories, accuracy in detection of HeII reionization via FRBs assuming no uncertainty in the redshift measurement and alternatively assuming a varied level of uncertainty in redshift measurement of the FRBs. We show that under the first case, a detection of $N\sim\mathcal{O} (10^2)$ FRBs give an uncertainty of $σ(z_{r, fit})\sim0.5$ from the fit model, and a detection of $N\sim\mathcal{O} (10^3)$ gives an uncertainty of $σ(z_{r, fit})\sim0.1$. While assuming a redshift uncertainty of level $5-20\%$, changes the $σ(z_{r, fit})\sim0.5$ to $0.6$ for $N\sim 100$ and $σ(z_{r, fit})\sim0.1$ to $0.15$ for $N \sim 1000$ case.

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arXiv2020

On-sky silicon photomultiplier detector performance measurements for millisecond to sub-microsecond optical source variability studies

In our Ultra-Fast Astronomy (UFA) program, we aim to improve measurements of variability of astronomical targets on millisecond and shorter time scales. In this work, we present initial on-sky measurements of the performance of silicon photomultiplier detectors (SiPMs) for UFA. We mounted two different SiPMs at the focal plane of the 0.7-meter aperture Nazarbayev University Transient Telescope at the Assy-Turgen Astrophysical Observatory (NUTTelA-TAO), with no filter in front of the detector. The $3mm\times3mm$ SiPM single-channel detectors have a field of view of $2.2716'\times2.2716'$. During the nights of 2019 October 28-29, we measured sky background, bright stars, and an artificial source with a 100Hz flashing frequency. We compared detected SiPM counts with Gaia satellite G-band flux values to show that our SiPMs have a linear response. With our two SiPMs (models S14520-3050VS and S14160-3050HS), we measured a dark current of $\sim$130 and $\sim$85 kilo counts per second (kcps), and a sky background of $\sim$201 and $\sim$203 kcps, respectively. We measured an intrinsic crosstalk of 10.34$\%$ and 10.52$\%$ and derived a 5$σ$ sensitivity of 13.9 and 14.0 Gaia G-band magnitude for 200ms exposures, for the two detectors respectively. For a 10 $μ$s window, and allowing a false alarm rate of once per 100 nights, we derived a sensitivity of 22 detected photons, or 6 Gaia G-band magnitudes. For nanosecond timescales, our detection is limited by crosstalk to 12 detected photons, which corresponds to a fluence of $\sim$155 photons per square meter.

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arXiv2019

Program objectives and specifications for the Ultra-Fast Astronomy observatory

We present program objectives and specifications for the first generation Ultra-Fast Astronomy (UFA) observatory which will explore a new astrophysical phase space by characterizing the variability of the optical (320 nm - 650 nm) sky in the millisecond to nanosecond timescales. One of the first objectives of the UFA observatory will be to search for optical counterparts to fast radio bursts (FRB) that can be used to identify the origins of FRB and probe the epoch of reionization and baryonic matter in the interstellar and intergalactic mediums. The UFA camera will consist of two single-photon resolution fast-response detector 16x16 arrays operated in coincidence mounted on the 0.7 meter Nazarbayev University Transient Telescope at the Assy-Turgen Astrophysical Observatory (NUTTelA-TAO) located near Almaty, Kazakhstan. We are currently developing two readout systems that can measure down to the microsecond and nanosecond timescales and characterizing two silicon photomultipliers (SiPM) and one photomultiplier tube (PMT) to compare the detectors for the UFA observatory and astrophysical observations in general.

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