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Bojan Nikolic

Publications and source records attributed to Bojan Nikolic.

At least 19 recordsLinked to original sources
arXiv2026

New techniques for high-resolution imaging and high-precision wavefront sensing via masked-aperture interferometry

Achieving high-angular-resolution imaging on subarcsecond scales is fundamentally limited by wavefront aberrations imparted by the propagation medium and by optical elements along the light path. Accurate recovery of source structure at these fine angular scales therefore relies on precise, real-time sensing and correction of wavefront aberrations. Drawing inspiration from radio interferometry, we have developed different approaches using masked apertures that directly measure both the amplitude and phase of the distortions to the electromagnetic wavefront while simultaneously reconstructing the underlying source structure. First is radio interferometry style self-calibration which can recover the complex electric field aberrations across the aperture with subarcsecond phase accuracy, equivalent to nanometer-level precision in optical pathlength, and simultaneously reconstruct the source structure with milliarcsecond (sub-micron) accuracy. Second is closure invariant-based source reconstruction which allows to bypass self-calibration and errors therein entirely while achieving comparable fidelity in the recovered source structure. These methods have been validated on the ALBA synchrotron beamline. Together, these methods provide a reliable framework for nanometre-scale high-precision wavefront sensing and subarcsecond-scale high angular resolution imaging, enabling new possibilities for masked-aperture interferometry. Potential applications span laboratory and synchrotron facilities to astronomy, including beam diagnostics in the Large Hadron Collider and masked-aperture interferometry on space telescopes such as the James Webb Space Telescope.

physics.optics↗
arXiv2026

Establishing a relationship between the cosmological 21 cm power spectrum and interferometric closure phases

Measurements of the cosmic 21 cm signal need to achieve a high dynamic range to isolate it from bright foreground emissions. Calibration inaccuracies can compromise the spectral fidelity of the smooth foreground continuum, thereby limiting the dynamic range and potentially precluding the detection of the cosmic line signal. In light of this challenge, recent work has proposed using the calibration-independent closure phase to search for the spectral fluctuations of the cosmic 21 cm signal. However, so far there has been only a heuristic understanding of how closure phases map to the cosmological 21 cm power spectrum. This work aims to establish a more accurate mathematical relationship between closure phases and the cosmological power spectrum of the background line signal. Building on previous work, we treat the cosmic signal component as a perturbation to the closure phase and use a delay spectrum approach to estimate its power. We establish the relationship between this estimate and the cosmological power spectrum using standard Fourier transform techniques and validate it using simulated HERA observations. We find that, statistically, the power spectrum estimate from closure phases is approximately equal to the cosmological power spectrum convolved with a foreground-dependent window function, provided that the signal-to-foreground ratio is small. Compared with standard approaches, the foreground dependence of the window function results in an increased amount of mode-mixing and a more pronounced proliferation of foreground power along the line-of-sight dimension of the cylindrical power spectrum. These effects can be mitigated by flagging instances where the window function is broad. Crucial to gaining the necessary sensitivity, this mapping will allow us to average the measurements of closure triads of different shapes based on their imprint in cylindrical Fourier space.

astro-ph.CO↗
arXiv2025

First Results from HERA Phase II

We report the first upper limits on the power spectrum of 21-cm fluctuations during the Epoch of Reionization and Cosmic Dawn from Phase II of the Hydrogen Epoch of Reionization Array (HERA) experiment. HERA Phase II constitutes several significant improvements in the signal chain compared to Phase I, most notably resulting in expanded frequency bandwidth, from 50-250 MHz. In these first upper limits, we investigate a small two-week subset of the available Phase II observations, with a focus on identifying new systematic characteristics of the instrument, and establishing an analysis pipeline to account for them. We report 2$\sigma$ upper limits in eight spectral bands, spanning $5.6 \leq z \leq 24.4$ that are consistent with thermal noise at the $2\sigma$ level for $k \gtrsim 0.6-0.9 h{\rm Mpc}^{-1}$ (band dependent). Our tightest limit during Cosmic Dawn ($z>12$) is $1.13\times 10^6 {\rm mK}^2$ at ($k=0.55 h{\rm Mpc}^{-1}, z=16.78$), and during the EoR ($5.5<z<12$) it is $1.78\times 10^3 {\rm mK}^2$ at ($k=0.70 h{\rm Mpc}^{-1}, z=7.05$). We find that mutual coupling has become our dominant systematic, leaking foreground power that strongly contaminates the low-$k$ modes, resulting in the loss of modes from $k=0.35-0.55$ compared to Phase I data.

astro-ph.CO↗
arXiv2025

Balanced Domination in Convex Polytopes, Trees, and Grid Graphs

This paper addresses two open questions posed in [27] regarding the balanced domination number in graphs. We show that three new classes of graphs, those of convex polytopes A_n, D_n, and Rn'', are d-balanced. Further, we provide a characterization of d-balancedness for rooted trees with two levels of descendants and prove that each full binary tree is d-balanced. Several results for caterpillar graphs are established. Moreover, we determine and prove the exact balanced domination number for grid graphs. Finally, we conclude by providing several open problems of interest.

math.CO↗
arXiv2025

Simultaneous bosonic and fermionic T-dualization of the type II superstring theory -- Buscher approach and double space representation

In this article I consider type II superstring in the pure spinor formulation with constant background fields in the context of T-dualization. First I prove that bosonic and fermionic T-dualization commute using already known T-dual transformation laws for bosonic and fermionic T-dualization. Consequently, the T-dual transformation laws of the full T-dualization are obtained. At the end the full T-dualization is realized in double space and it is showed that Buscher procedure and double space approach are equivalent in this specific case.

hep-th↗
arXiv2025

A New Method for Aperture Masking Interferometric Imaging: Demonstration with the JWST

We present a new method for aperture masking interferometric (AMI) imaging at near-IR wavelengths using radio astronomical techniques. The method starts with derivation of interferometric visibilities from a Fourier transform of the interferograms. An iterative joint optimization process is then employed, using self-calibration of the interferometric element-based complex voltage gains (i.e. electric fields), and CLEAN deconvolution to obtain the source structure. We demonstrate the efficacy of the method using the NIRISS aperture masking interferometer on the James Webb Space Telescope (JWST) at 4.8~$\mu$m and 3.8~$\mu$m. Due to a number of effects (the large pixel size, charge migration, near-field optics), the method also requires an initial visibility-based amplitude normalization using observations of a well know point-source calibration star. We employ early science observations of the dusty binary Wolf-Rayet star WR137. Images with a dynamic range (peak/rms) of $\sim 240$ on the target, and $\sim 1000$ on the calibrator, are synthesized from a short integration. The self-calibration process determines the photon path-lengths through the optical system to each aperture using data on the target source itself, thereby representing an essentially 'real-time', precise wavefront error sensor. Four independent measures of the JWST mirror segment pistons (two wavelengths for two sources), agree to within 10~nm to 15~nm, comparable to the expected errors based on an analysis of closure phases on the calibrator star. Including a baseline-based phase correction improves the dynamic range of the final images by about 23\%.

astro-ph.IM↗
arXiv2025

Two-dimensional Light Beam Shape Characterization using Interferometric Closure Amplitudes

We introduce a novel technique using closure amplitudes, inspired by radio interferometry, to determine with high angular resolution the two-dimensional profile of a light beam using an interferogram from a non-redundantly masked aperture. Previous techniques have required multiple interferograms or accurate estimates of the non-uniform illuminations across the aperture. In contrast, our method using closure amplitudes avoids the need to estimate the aperture illuminations while determining the two-dimensional beam shape from a single interferogram. The invariance of closure amplitudes to even time-varying aperture illuminations makes it suitable to longer averaging intervals, with potential to reducing data rates and computational overheads. By using data from the ALBA synchrotron light source to validate the method and its results against existing methods, this paper represents the first real-world application of closure amplitudes to directly determine the light beam's profile using optical interferometry in the high angular resolution regime.

physics.optics↗
arXiv2024

New interferometric aperture masking technique for full transverse beam characterization using synchrotron radiation

Emittance measurements using synchrotron radiation are usually performed using x-rays to avoid diffraction limits. Interferometric techniques using visible light are also used to measure either the horizontal or the vertical beam projection. Several measurements rotating the interferometry axis are needed to obtain a full beam reconstruction. In this report we present a new interferometric multi-aperture masking technique and data analysis, inspired by astronomical methods, that are able to provide a full 2-D transverse beam reconstruction in a single acquisition. Results of beam characterization obtained at ALBA synchrotron light source will also been shown.

physics.acc-ph↗
arXiv2024

Two-dimensional Synchrotron Beam Characterisation from a Single Interferogram

Double-aperture Young interferometry is widely used in accelerators to provide a one-dimensional beam measurement. We improve this technique by combining and further developing techniques of non-redundant, two-dimensional, aperture masking and self-calibration from astronomy. Using visible synchrotron radiation, tests at the ALBA synchrotron show that this method provides an accurate two-dimensional beam transverse characterisation, even from a single 1 ms interferogram. The non-redundancy of the aperture mask in the technique enables it to be resistant to spatial phase fluctuations that might be introduced by vibration of optical components, or in the laboratory atmosphere.

physics.acc-ph↗
arXiv2024

Novelty Detection on Radio Astronomy Data using Signatures

We introduce SigNova, a new semi-supervised framework for detecting anomalies in streamed data. While our initial examples focus on detecting radio-frequency interference (RFI) in digitized signals within the field of radio astronomy, it is important to note that SigNova's applicability extends to any type of streamed data. The framework comprises three primary components. Firstly, we use the signature transform to extract a canonical collection of summary statistics from observational sequences. This allows us to represent variable-length visibility samples as finite-dimensional feature vectors. Secondly, each feature vector is assigned a novelty score, calculated as the Mahalanobis distance to its nearest neighbor in an RFI-free training set. By thresholding these scores we identify observation ranges that deviate from the expected behavior of RFI-free visibility samples without relying on stringent distributional assumptions. Thirdly, we integrate this anomaly detector with Pysegments, a segmentation algorithm, to localize consecutive observations contaminated with RFI, if any. This approach provides a compelling alternative to classical windowing techniques commonly used for RFI detection. Importantly, the complexity of our algorithm depends on the RFI pattern rather than on the size of the observation window. We demonstrate how SigNova improves the detection of various types of RFI (e.g., broadband and narrowband) in time-frequency visibility data. We validate our framework on the Murchison Widefield Array (MWA) telescope and simulated data and the Hydrogen Epoch of Reionization Array (HERA).

astro-ph.IM↗
arXiv2024

A demonstration of the effect of fringe-rate filtering in the Hydrogen Epoch of Reionization Array delay power spectrum pipeline

Radio interferometers targeting the 21cm brightness temperature fluctuations at high redshift are subject to systematic effects that operate over a range of different timescales. These can be isolated by designing appropriate Fourier filters that operate in fringe-rate (FR) space, the Fourier pair of local sidereal time (LST). Applications of FR filtering include separating effects that are correlated with the rotating sky vs. those relative to the ground, down-weighting emission in the primary beam sidelobes, and suppressing noise. FR filtering causes the noise contributions to the visibility data to become correlated in time however, making interpretation of subsequent averaging and error estimation steps more subtle. In this paper, we describe fringe rate filters that are implemented using discrete prolate spheroidal sequences, and designed for two different purposes -- beam sidelobe/horizon suppression (the `mainlobe' filter), and ground-locked systematics removal (the `notch' filter). We apply these to simulated data, and study how their properties affect visibilities and power spectra generated from the simulations. Included is an introduction to fringe-rate filtering and a demonstration of fringe-rate filters applied to simple situations to aid understanding.

astro-ph.CO↗
arXiv2024

Hydrogen Epoch of Reionization Array (HERA) Phase II Deployment and Commissioning

This paper presents the design and deployment of the Hydrogen Epoch of Reionization Array (HERA) phase II system. HERA is designed as a staged experiment targeting 21 cm emission measurements of the Epoch of Reionization. First results from the phase I array are published as of early 2022, and deployment of the phase II system is nearing completion. We describe the design of the phase II system and discuss progress on commissioning and future upgrades. As HERA is a designated Square Kilometer Array (SKA) pathfinder instrument, we also show a number of "case studies" that investigate systematics seen while commissioning the phase II system, which may be of use in the design and operation of future arrays. Common pathologies are likely to manifest in similar ways across instruments, and many of these sources of contamination can be mitigated once the source is identified.

astro-ph.IM↗
arXiv2023

matvis: A matrix-based visibility simulator for fast forward modelling of many-element 21 cm arrays

Detection of the faint 21 cm line emission from the Cosmic Dawn and Epoch of Reionisation will require not only exquisite control over instrumental calibration and systematics to achieve the necessary dynamic range of observations but also validation of analysis techniques to demonstrate their statistical properties and signal loss characteristics. A key ingredient in achieving this is the ability to perform high-fidelity simulations of the kinds of data that are produced by the large, many-element, radio interferometric arrays that have been purpose-built for these studies. The large scale of these arrays presents a computational challenge, as one must simulate a detailed sky and instrumental model across many hundreds of frequency channels, thousands of time samples, and tens of thousands of baselines for arrays with hundreds of antennas. In this paper, we present a fast matrix-based method for simulating radio interferometric measurements (visibilities) at the necessary scale. We achieve this through judicious use of primary beam interpolation, fast approximations for coordinate transforms, and a vectorised outer product to expand per-antenna quantities to per-baseline visibilities, coupled with standard parallelisation techniques. We validate the results of this method, implemented in the publicly-available matvis code, against a high-precision reference simulator, and explore its computational scaling on a variety of problems.

astro-ph.IM↗
arXiv2023

Bayesian estimation of cross-coupling and reflection systematics in 21cm array visibility data

Observations with radio arrays that target the 21-cm signal originating from the early Universe suffer from a variety of systematic effects. An important class of these are reflections and spurious couplings between antennas. We apply a Hamiltonian Monte Carlo sampler to the modelling and mitigation of these systematics in simulated Hydrogen Epoch of Reionisation Array (HERA) data. This method allows us to form statistical uncertainty estimates for both our models and the recovered visibilities, which is an important ingredient in establishing robust upper limits on the Epoch of Reionisation (EoR) power spectrum. In cases where the noise is large compared to the EoR signal, this approach can constrain the systematics well enough to mitigate them down to the noise level for both systematics studied. Incoherently averaging the recovered power spectra can further reduce the noise and improve recovery. Where the noise level is lower than the EoR, our modelling can mitigate the majority of the reflections and coupling with there being only a minor level of residual systematics. Our approach performs similarly to existing filtering/fitting techniques used in the HERA pipeline, but with the added benefit of rigorously propagating uncertainties. In all cases it does not significantly attenuate the underlying signal.

astro-ph.IM↗
arXiv2023

Direct Optimal Mapping Image Power Spectrum and its Window Functions

The key to detecting neutral hydrogen during the epoch of reionization (EoR) is to separate the cosmological signal from the dominating foreground radiation. We developed direct optimal mapping (DOM) to map interferometric visibilities; it contains only linear operations, with full knowledge of point spread functions from visibilities to images. Here, we demonstrate a fast Fourier transform-based image power spectrum and its window functions computed from the DOM images. We use noiseless simulation, based on the Hydrogen Epoch of Reionization Array Phase I configuration, to study the image power spectrum properties. The window functions show $<10^{-11}$ of the integrated power leaks from the foreground-dominated region into the EoR window; the 2D and 1D power spectra also verify the separation between the foregrounds and the EoR.

astro-ph.IM↗
arXiv2023

Characterization Of Inpaint Residuals In Interferometric Measurements of the Epoch Of Reionization

Radio Frequency Interference (RFI) is one of the systematic challenges preventing 21cm interferometric instruments from detecting the Epoch of Reionization. To mitigate the effects of RFI on data analysis pipelines, numerous inpaint techniques have been developed to restore RFI corrupted data. We examine the qualitative and quantitative errors introduced into the visibilities and power spectrum due to inpainting. We perform our analysis on simulated data as well as real data from the Hydrogen Epoch of Reionization Array (HERA) Phase 1 upper limits. We also introduce a convolutional neural network that capable of inpainting RFI corrupted data in interferometric instruments. We train our network on simulated data and show that our network is capable at inpainting real data without requiring to be retrained. We find that techniques that incorporate high wavenumbers in delay space in their modeling are best suited for inpainting over narrowband RFI. We also show that with our fiducial parameters Discrete Prolate Spheroidal Sequences (DPSS) and CLEAN provide the best performance for intermittent ``narrowband'' RFI while Gaussian Progress Regression (GPR) and Least Squares Spectral Analysis (LSSA) provide the best performance for larger RFI gaps. However we caution that these qualitative conclusions are sensitive to the chosen hyperparameters of each inpainting technique. We find these results to be consistent in both simulated and real visibilities. We show that all inpainting techniques reliably reproduce foreground dominated modes in the power spectrum. Since the inpainting techniques should not be capable of reproducing noise realizations, we find that the largest errors occur in the noise dominated delay modes. We show that in the future, as the noise level of the data comes down, CLEAN and DPSS are most capable of reproducing the fine frequency structure in the visibilities of HERA data.

astro-ph.IM↗
arXiv2023

Search for the Epoch of Reionisation with HERA: Upper Limits on the Closure Phase Delay Power Spectrum

Radio interferometers aiming to measure the power spectrum of the redshifted 21 cm line during the Epoch of Reionisation (EoR) need to achieve an unprecedented dynamic range to separate the weak signal from overwhelming foreground emissions. Calibration inaccuracies can compromise the sensitivity of these measurements to the effect that a detection of the EoR is precluded. An alternative to standard analysis techniques makes use of the closure phase, which allows one to bypass antenna-based direction-independent calibration. Similarly to standard approaches, we use a delay spectrum technique to search for the EoR signal. Using 94 nights of data observed with Phase I of the Hydrogen Epoch of Reionization Array (HERA), we place approximate constraints on the 21 cm power spectrum at $z=7.7$. We find at 95% confidence that the 21 cm EoR brightness temperature is $\le$(372)$^2$ "pseudo" mK$^2$ at 1.14 "pseudo" $h$ Mpc$^{-1}$, where the "pseudo" emphasises that these limits are to be interpreted as approximations to the actual distance scales and brightness temperatures. Using a fiducial EoR model, we demonstrate the feasibility of detecting the EoR with the full array. Compared to standard methods, the closure phase processing is relatively simple, thereby providing an important independent check on results derived using visibility intensities, or related.

astro-ph.CO↗