arXiv subjects

Siddharth Maharana

Publications and source records attributed to Siddharth Maharana.

15 recordsLinked to original sources

arXiv2026

Comparing the contrast performance of two IFU technologies for exoplanet direct imaging with ELT-PCS

A key science goal for the Planetary Camera and Spectrograph (ELT-PCS) instrument is to image rocky exoplanets in reflected light around nearby M- and K-dwarfs. These observations will require a contrast of 10^(-9) at 100 mas angular separation, and 10^(-8) at 10 mas. To achieve such an extreme performance, every instrument design choice must be carefully evaluated. For ELT-PCS's science instruments, perhaps the most critical design choice is which integral field unit (IFU) technology to use. To this end, we have developed a modular, lab-based integral field spectrograph test bench with which to compare the performance of two popular IFU technologies: image slicers and lenslet arrays. The test bench implements a Lyot coronagraph and a spatial light modulator to simulate a high-contrast imaging system, which feeds each IFU with a diffraction-limited PSF image. Each IFU is coupled to the spectrograph, which achieves R ~ 3000 with the image slicer and R ~ 100 with the lenslet array. The calibration and data reduction for each IFU design are described and the initial results of a contrast comparison are presented. The lenslet array achieves a deeper contrast at angular separations of less than 6 lambda/D. The image slicer, however, gains more at small separations from applying spectral deconvolution, with a 3.4x greater gain at 4 lambda/D. Spectral deconvolution further yields a contrast curve close to the detector noise floor. Further work will focus on carrying out a fully-controlled characterisation of the lenslet array IFU and a more comprehensive study into the effects of adaptive optics residuals on the contrast.

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arXiv2026

Commissioning and on-sky performance of FiberPol: a fiber-fed spectropolarimetric system for the SAAO 1.9 m telescope

At the South African Astronomical Observatory (SAAO), we have developed FiberPol$-$ a spectropolarimetric front-end for the SpUpNIC spectrograph on the 1.9 m telescope fed by fibers. In conjunction with SpUpNIC, it combines two niche techniques: fiber-spectroscopy and polarimetry to enable a wide range of science cases, particularly for studies of the interstellar medium, and serves as a technology pathfinder for implementing polarimetry larger facilities such as the 10 m Southern African Large Telescope. Commissioned in early 2025, FiberPol has already demonstrated key performance benchmarks. Based on a preliminary data reduction pipeline, we find that the uncertainties in the measured Stokes parameters $q$ and $u$, and the degree of polarization $p$ are in the range of 0.2% to 0.3% in 5 nm spectral bins across the 400 to 700 nm wavelength range. With improved calibration and refined analysis, we expect to reach our design goal of 0.1% accuracy per bin. Several science programs are already underway following its successful commissioning. FiberPol employs a Wollaston prism and half wave plate system to analyze linear polarization and extract spectropolarimetric data comprising wavelength ($λ$) and the Stokes parameters ($I$, $q$ and $u$). Its compact and modular design uses primarily small, commercially available optical and optomechanical components, making it straightforward to adapt to other spectrographs, including those on larger telescopes. This paper presents the design, laboratory validation, and commissioning results of FiberPol, and outlines planned upgrades to further enhance its scientific impact.

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arXiv2026

WALOP-South: a four camera one shot imaging polarimeter for the PASIPHAE survey. Paper III -- PSF modelling

The two WALOP instruments, built for the PASIPHAE survey, will measure the linear polarization of large numbers of stars in the Galactic polar regions in the SDSS-$r$ band. They are designed with a wide field-of-view, enabling measurement of the Stokes parameters $I$, $q$, and $u$ for multiple stars simultaneously within a $35^\prime \times 35^\prime$ region of sky for WALOP-South and $30^\prime \times 30^\prime$ region for WALOP-North. In this paper, we present a polar shapelet-based PSF photometry framework for well-sampled stellar point sources applicable to WALOP-type wide-field polarimeters. Polar shapelets are a set of orthogonal basis functions, constructed from Gauss-Hermite or Gauss-Laguerre polynomials, that are well-suited to modelling localized PSF in a compact and efficient way. We developed an efficient PSF modelling method that uses polar shapelets as basis functions to reconstruct the spatial variation of the PSF shape across the CCD using Zemax-simulated images of one of the WALOP instruments, and show that a limited number of shapelet coefficients are sufficient to capture this variation consistently across different CCD locations. To simulate realistic star images, we introduce random sub-pixel shifts in the star centroids in the Zemax-simulated images, and account for this using a two-step iterative method that alternately estimates the PSF model and the sub-pixel centroid shift. Applying PSF photometry to the target faint stars, we demonstrate that the photometric accuracy of approximately 0.15% is achievable, and that the reconstructed PSF model can be incorporated into the photometry across different seeing conditions, meeting the polarimetric science requirements of the PASIPHAE survey.

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arXiv2026

Evidence for cloud-to-cloud variations in the ratio of polarized thermal dust emission to starlight polarization

The correlation between optical starlight polarization and polarized thermal dust emission can be used to infer intrinsic dust properties. This correlation is quantified by the ratio Rp/p, which has been measured to be 5.42 +/- 0.05 MJy sr^-1 at 353 GHz when averaged over large areas of the sky. We investigate this correlation using newly published stellar polarimetric data densely sampling a continuous sky region of ~4 square degrees at intermediate Galactic latitude. We combine RoboPol optical polarization measurements for 1,430 stars with submillimeter data from the Planck satellite at 353 GHz. We perform linear fits between the Planck (Qs, Us) and optical (qv, uv) Stokes parameters, accounting for the differences in resolution between the two datasets as well as the distribution of clouds along the line of sight. We find that in this region of the sky the Rp/p value is 3.67 +/- 0.05 MJy sr^-1, indicating a significantly shallower slope than that found previously using different stellar samples. We also find significant differences in the fitted slopes when fitting the Qs-qv and Us-uv data separately. We explore two explanations using mock data: miscalibration of polarization angle and variations in Rp/p along the line of sight due to multiple clouds. We show that the former can produce differences in the correlations of Qs-qv and Us-uv, but large miscalibration angles would be needed to reproduce the magnitude of the observed differences. Our simulations favor the interpretation that Rp/p differs between the two dominant clouds that overlap on the sky in this region. The difference in Rp/p suggests that the two clouds may have distinct dust polarimetric properties. With knowledge from the tomographic decomposition of the stellar polarization, we find that one cloud appears to dominate the correlation of Us-uv, while both clouds contribute to the correlation of the Qs-qv data.

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arXiv2025

Wide Area Linear Optical Polarimeter Control Software

The WALOPControl software is designed to facilitate comprehensive control and operation of the WALOP (Wide Area Linear Optical Polarimeter) polarimeters, ensuring safe and concurrent management of various instrument components and functionalities. This software encompasses several critical requirements, including control of the filter wheel, calibration half-wave plate, calibration polarizer, guider positioning, focusers, and 4 concurrent CCD cameras. It also manages the host telescope and dome operations while logging operational parameters, user commands, and environmental conditions for troubleshooting and stability. It provides a user-friendly graphical user interface, secure access control, a notification system for errors, and a modular configuration for troubleshooting are integral to the software's architecture. It is accessible over the internet with the backend developed using NodeJS and ExpressJS, featuring a RESTful API that interacts with a MongoDB database, facilitating real-time status updates and data logging. The frontend utilizes the React.JS framework, with Redux for state management and Material UI for the graphical components. The system also allows for automatic observations based on user-defined schedules. A Continuous Integration and Continuous Deployment (CI CD) pipeline ensures the software's reliability through automated testing and streamlined deployment. The WALOPControl software is a key component of the PASIPHAE (Polar-Areas Stellar Imaging in Polarimetry High Accuracy Experiment) project, which aims to study the dust and magnetic field of the Milky Way by observing the polarization of starlight.

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arXiv2025

High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet

Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10-18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma confidence level). We argue here that this observational result rules out a hadronic origin of the high energy emission, and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.

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arXiv2024

The WALOP-North Instrument I: Optical Design, Filter Design, Calibration

The Wide Area Linear Optical Polarimeter North (WALOP-North) is an optical polarimeter designed for the needs of the PASIPHAE survey. It will be installed on the 1.3m telescope at the Skinakas Observatory in Crete, Greece. After commissioning, it will measure the polarization of millions of stars at high Galactic latitude, aiming to measure hundreds of stars per $deg^2$. The astronomical filter used in the instrument is a modified, polarimetrically-neutral broadband SDSS-r. This instrument will be pioneering one due to its large field-of-view (FoV) of $30\times 30$ $arcmin^2$ and high accuracy polarimetry measurements. The accuracy and sensitivity of the instrument in polarization fraction will be at the 0.1\% and 0.05\% level, respectively. Four separate 4k$\times$4k CCDs will be used as the instrument detectors, each imaging one of the $0°, 45°, 90°$ and $135°$ polarized FoV separately, therefore making the instrument a four-channel, one-shot polarimeter. Here, we present the overall optical design of the instrument, emphasizing on the aspects of the instrument that are different from WALOP-South. We also present a novel design of filters appropriate for polarimetry along with details on the management of the instrument size and its polarimetric calibration.

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arXiv2024

Improving polarimetric accuracy of RoboPol to $<$ 0.05 % using a half-wave plate calibrator system

RoboPol is a four-channel, one-shot linear optical polarimeter that has been successfully operating since 2013 on the 1.3 m telescope at Skinakas Observatory in Crete, Greece. Using its unique optical system, it measures the linear Stokes parameters $q$ and $u$ in a single exposure with high polarimetric accuracy of 0.1% - 0.15% and 1 degree in polarization angle in the R broadband filter. Its performance marginally degrades in other broadband filters. The source of the current instrumental performance limit has been identified as unaccounted and variable instrumental polarization, most likely originating from factors such as temperature and gravity-induced instrument flexure. To improve the performance of RoboPol in all broadband filters, including R, we have developed a rotating half-wave plate calibrator system. This calibrator system is placed at the beginning of the instrument and enables modulation of polarimetric measurements by beam swapping between all four channels of RoboPol. Using the new calibrator system, we observed multiple polarimetric standard stars over two annual observing seasons with RoboPol. This has enabled us to achieve a polarimetric accuracy of better than 0.05 % in both $q$ and $u$, and 0.5 degrees in polarization angle across all filters, enhancing the instrument's performance by a factor of two to three.

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arXiv2024

Systems design, assembly, integration and lab testing of WALOP-South Polarimeter

Wide-Area Linear Optical Polarimeter (WALOP)-South is the first wide-field and survey-capacity polarimeter in the optical wavelengths. On schedule for commissioning in 2024, it will be mounted on the 1 m SAAO telescope in Sutherland Observatory, South Africa to undertake the PASIPHAE sky survey. PASIPHAE program will create the first polarimetric sky map in the optical wavelengths, spanning more than 2000 square degrees of the southern Galactic region. The innovative design of WALOP-South will enable it to measure the linear polarization (Stokes parameters $q$ and $u$), in a single exposure, of all sources in a field of view (FoV) of $35\times35$ arcminutes-squared in the SDSS-r broadband and narrowband filters between 500-750 nm with 0.1 % polarization accuracy. The unique goals of the instrument place very stringent systems engineering goals, including on the performance of the optical, polarimetric, optomechanical, and electronic subsystems. All the subsystems have been designed carefully to meet the overall instrument performance goals. As of May 2024, all the instrument optical and mechanical subsystems have been assembled and are currently getting tested and integrated. The complete testing and characterization of the instrument in the lab is expected to be completed by August 2024. In this paper, we will present (a) the design and development of the entire instrument and its major subsystems, focusing on the opto-mechanical design which has not been reported before, and (b) assembly and integration of the instrument in the lab and early results from lab characterization of the instrument.

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arXiv2024

FiberPol-6D: Spectropolarimetric Integral Field mode for the SAAO 1.9 m Telescope using fibers

Most optical spectropolarimeters built to date operate as long-slit or point-source instruments; they are inefficient for observations of extended objects such as galaxies and nebulae. 2D spectropolarimetry technique development is a major challenge in astronomical instrumentation. At the South African Astronomical Observatory (SAAO) FiberLab, we are developing a spectropolarimetry capable Integral Field front-end called FiberPol(-6D) for the existing SpUpNIC spectrograph on the SAAO 1.9 m telescope. SpUpNIC is a general purpose 2 arc-minute long-slit spectrograph with a grating suite covering the wavelength range from 350 to 1000 nm and at spectral resolutions between 500 and 6000. FiberPol generates 6D observational data: x-y spatial dimensions, wavelength, and the three linear Stokes parameters $I$, $q$ and $u$. Using a rotating half-wave plate and a Wollaston prism, FiberPol executes two-channel polarimetry, and each channel is fed to an array of 14 fibers, corresponding to a field of view of $10\times20~arcseconds^2$ sampled with 2.9 arcsecond diameter fiber cores. These fiber arrays are then rerouted to form a pseudo-slit input to SpUpNIC. FiberPol aims to achieve a polarimetric accuracy of 0.1 % per spectral resolution bin. Further, it can also function as a non-polarimetric integral-field unit. The instrument design has been completed and it is currently being assembled and characterized in the lab. It is scheduled for on-sky commissioning in the second half of 2024. In this paper, we present the scientific and technical goals of FiberPol, its overall design and initial results from the lab assembly and testing. FiberPol is a low cost technology demonstrator, and the entire system predominantly employs small size, commercial off-the-shelve optics and optomechanical components. It can be modified and replicated for use on any existing spectrograph, especially on bigger telescopes.

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arXiv2024

Spectropolarimeter on a 2--4 m class telescope and proposed science cases

We propose a spectropolarimeter a covering wavelength range of 3200--7000 Å [3200Å chosen as lower limit to go to the atmospheric cut-off. It's ``needed" for some Serkowski curves and would make the instrument even more unique] for a 2-4~m class telescope. In this article, we discuss the science cases which will be covered with this proposed instrument. The technical requirements and analysis plan for each science case is also discussed. This spectropolarimeter targeting exciting galactic and extra-galactic research, will be unique instrument on a 2-4~m facilities.

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arXiv2022

WALOP-South: A Four-Camera One-Shot Imaging Polarimeter for PASIPHAE Survey. Paper II -- Polarimetric Modelling and Calibration

The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument is an upcoming wide-field and high-accuracy optical polarimeter to be used as a survey instrument for carrying out the Polar-Areas Stellar Imaging in Polarization High Accuracy Experiment (PASIPHAE) program. Designed to operate as a one-shot four-channel and four-camera imaging polarimeter, it will have a field of view of $35\times 35$ arcminutes and will measure the Stokes parameters $I$, $q$, and $u$ in a single exposure in the SDSS-r broadband filter. The design goal for the instrument is to achieve an overall polarimetric measurement accuracy of 0.1 % over the entire field of view. We present here the complete polarimetric modeling of the instrument, characterizing the amount and sources of instrumental polarization. To accurately retrieve the real Stokes parameters of a source from the measured values, we have developed a calibration method for the instrument. Using this calibration method and simulated data, we demonstrate how to correct instrumental polarization and obtain 0.1 % accuracy in the degree of polarization, $p$. Additionally, we tested and validated the calibration method by implementing it on a table-top WALOP-like test-bed polarimeter in the laboratory.

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arXiv2021

WALOP-South: A Four Camera One Shot Imaging Polarimeter for PASIPHAE Survey. Paper I -- Optical Design

The WALOP-South instrument will be mounted on the 1 m SAAO telescope in South Africa as part of the PASIPHAE program to carry out a linear imaging polarization survey of the Galactic polar regions in the optical band. Designed to achieve polarimetric sensitivity of $0.05~\%$ across a $35\times35$ arcminute field of view, it will be capable of measuring the Stokes parameters I, q and u in a single exposure in the SDSS-r broadband and narrowband filters between $0.5~μm - 0.7~μm$. For each measurement, four images of the full field corresponding to linear polarization angles of 0 deg, 45 deg, 90 deg and 135 deg in the instrument coordinate system will be created on four detectors from which the Stokes parameters can be found using differential photometry. In designing the optical system, major challenges included correcting for the dispersion introduced by large split angle Wollaston Prisms used as analysers as well as other aberrations from the entire field to obtain imaging quality PSF at the detector. We present the optical design of the WALOP-South instrument which overcomes these challenges and delivers near seeing limited PSFs for the entire field of view.

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arXiv2021

WALOP-South: A wide-field one-shot linear optical polarimeter for PASIPHAE survey

WALOP (Wide-Area Linear Optical Polarimeter)-South, to be mounted on the 1m SAAO telescope in South Africa, is first of the two WALOP instruments currently under development for carrying out the PASIPHAE survey. Scheduled for commissioning in the year 2021, the WALOP instruments will be used to measure the linear polarization of around $10^{6}$ stars in the SDSS-r broadband with $0.1~\%$ polarimetric accuracy, covering 4000 square degrees in the Galactic polar regions. The combined capabilities of one-shot linear polarimetry, high polarimetric accuracy ($< 0.1~\%$) and polarimetric sensitivity ($< 0.05~\%$), and a large field of view (FOV) of $35\times35~arcminutes$ make WALOP-South a unique astronomical instrument. In a single exposure, it is designed to measure the Stokes parameters $I$, $q$ and $u$ in the SDSS-r broadband and narrowband filters between $500-700~nm$. During each measurement, four images of the full field corresponding to the polarization angles of $0^{\circ}$, $45^{\circ}$, $90^{\circ}$ and $135^{\circ}$ will be imaged on four detectors and carrying out differential photometry on these images will yield the Stokes parameters. Major challenges in designing WALOP-South instrument include- (a) in the optical design, correcting for the spectral dispersion introduced by large split angle Wollaston Prisms used as polarization analyzers as well as aberrations from the wide field, and (b) making an optomechanical design adherent to the tolerances required to obtain good imaging and polarimetric performance under all temperature conditions as well as telescope pointing positions. We present the optical and optomechanical design for WALOP-South which overcomes these challenges.

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arXiv2018

PASIPHAE: A high-Galactic-latitude, high-accuracy optopolarimetric survey

PASIPHAE (the Polar-Areas Stellar Imaging in Polarization High-Accuracy Experiment) is an optopolarimetric survey aiming to measure the linear polarization from millions of stars, and use these to create a three-dimensional tomographic map of the magnetic field threading dust clouds within the Milky Way. This map will provide invaluable information for future CMB B-mode experiments searching for inflationary gravitational waves, providing unique information regarding line-of-sight integration effects. Optical polarization observations of a large number of stars at known distances, tracing the same dust that emits polarized microwaves, can map the magnetic field between them. The Gaia mission is measuring distances to a billion stars, providing an opportunity to produce a tomographic map of Galactic magnetic field directions, using optical polarization of starlight. Such a map will not only boost CMB polarization foreground removal, but it will also have a profound impact in a wide range of astrophysical research, including interstellar medium physics, high-energy astrophysics, and evolution of the Galaxy. Taking advantage of the novel technology implemented in our high-accuracy Wide-Area Linear Optical Polarimeters (WALOPs) currently under construction at IUCAA, India, we will engage in a large-scale optopolarimetric program that can meet this challenge: a survey of both northern and southern Galactic polar regions targeted by CMB experiments, covering over 10,000 square degrees, which will measure linear optical polarization of over 360 stars per square degree (over 3.5 million stars, a 1000-fold increase over the state of the art). The survey will be conducted concurrently from the South African Astronomical Observatory in Sutherland, South Africa in the southern hemisphere, and the Skinakas Observatory in Crete, Greece, in the north.

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