arXiv subjects

Manjunath Bestha

Publications and source records attributed to Manjunath Bestha.

10 recordsLinked to original sources

arXiv2026

An Observational Study of Systematics in Differential Transmission Spectroscopy Using HFOSC on the Himalayan Chandra Telescope

Ground-based low-resolution transmission spectroscopy requires photometric precision of a few hundred parts per million, making it sensitive to instrumental and atmospheric systematics. This work studies the systematic effects affecting differential transmission spectroscopy using the Hanle Faint Object Spectrograph Camera (HFOSC) on the 2-m Himalayan Chandra Telescope (HCT). The study was motivated by an additional flux drop observed in the white-light curve of HAT-P-1 b. HAT-P-1 b is an ideal target for differential spectrophotometry because it has a visual binary companion with similar brightness at a suitable separation, allowing the companion star to be used as a reference. To investigate the origin of this feature, we analyzed several observational parameters, including FWHM variations, spectral trace motion, centroid drift, and spectral shifts. We also observed WASP-33 b in slitless mode to test whether differential slit losses could explain the observed systematic. In addition, observations of WASP-12 b were used to derive a broadband optical transmission spectrum using common-mode correction. The additional flux drop is unlikely to be caused only by differential slit losses, since similar differential centroid and spectral shifts are present in both slit and slitless observations. The results suggest that the observed systematic may be related to field-dependent distortions and pointing-dependent instrumental flexure, although its exact cause is still unknown. Overall, this work highlights the importance of understanding and reducing observational systematics in ground-based exoplanet transmission spectroscopy, especially for measurements that require photometric precision of a few hundred parts per million.

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arXiv2025

A new broadband atmospheric dispersion corrector for HROS-TMT

Atmospheric dispersion causes light from celestial objects with different wavelengths to refract at varying angles as it passes through Earth's atmosphere. This effect results in an elongated image at the focal plane of a telescope and diminishes fiber coupling efficiency into spectrographs. We propose an optical design that incorporates a Rotational Atmospheric Dispersion Corrector (RADC) to address the broadband dispersion encountered in the multi-object mode of the High-Resolution Optical Spectrograph (HROS) on the Thirty Meter Telescope (TMT). The RADC corrects the dispersion across the entire wavelength range (0.31-1 micron), using Amici prisms optimized for over 90% transmission efficiency and minimal angular deviation of the beam from the optical axis after dispersion correction. For enhanced accuracy, particularly in the blue region, we have, for the first time, implemented the Filippenko (1982) model in Zemax via a custom Dynamic-Link Library (DLL) file.

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arXiv2025

Addressing wavelength-correlated systematics in exoplanet transmission spectroscopy: a 2D Gaussian Process approach

Ground-based transmission spectroscopy is often dominated by systematics, which obstructs our ability to leverage the advantages of larger aperture sizes compared to space-based observations. These systematics could be time-correlated, uniform across all spectroscopic light curves, or wavelength-correlated, which could significantly affect the characterization of exoplanet atmospheres. Gaussian Processes were introduced in transmission spectroscopy by Gibson et al. (2012) to model correlated systematics in a non-parametric way. The technique uses auxiliary information about the observation and independently fits each spectroscopic light curve to provide robust atmospheric retrievals. However, this method assumes that the uncertainties in the transmission spectrum are uncorrelated in wavelength, which can cause discrepancies and degrade the precision of atmospheric retrievals. To address this limitation, we explore a 2D GP framework formulated by Fortune et al. (2024) to simultaneously model time- and wavelength-correlated systematics. We present its application to ground-based observations of TOI-4153b obtained using the 2-m Himalayan Chandra Telescope (HCT). As we move towards detecting smaller and cooler planets, developing new methods to address complex systematics becomes increasingly essential.

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arXiv2025

High-Resolution Echelle Spectroscopy for Solar System Planets: A Planet-as-Point-Source Analogy

Transmission spectroscopy has proven to be an effective technique for characterizing exoplanet atmospheres. However, transmission spectroscopy requires planetary transits, which occur for only a small fraction of planetary systems due to geometric alignment constraints; hence, characterizing exoplanets through their reflected spectrum of host stars will be helpful for a large number of exoplanets. The upcoming extremely large telescopes (ELTs) will be able to study the reflected spectra of exoplanets. Here, we present a preliminary optical design and a detailed throughput analysis of the instrumentation that interfaces the 2.34 m Vainu Bappu Telescope prime focus to an existing high-resolution echelle spectrograph with disk-integrated light from solar system objects. One of the primary objectives is to obtain high-resolution, high signal-to-noise reflected spectra from the solar system objects.

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arXiv2025

A multi-object approach for studying exoplanet atmospheres using high-resolution spectrographs

Atmospheric characterization of exoplanets has traditionally relied on Low-Resolution Transmission Spectroscopy (LRTS), obtained from both space- and ground-based facilities, as well as on High-Resolution Transmission Spectroscopy (HRTS). Although HRTS can resolve individual spectral lines, it is subject to normalization degeneracies that limit the accurate retrieval of key atmospheric parameters such as pressure, abundance, and cloud opacity. A promising strategy to mitigate this issue is to combine ground-based HRTS with space-based LRTS. However, this approach depends on two separate datasets, thereby requiring two independent observations. In this study, we explore the feasibility of Multi-Object High-Resolution Transmission Spectroscopy (Mo-HRTS) as a means to constrain atmospheric parameters in retrievals using a single dataset. Through simulations based on existing spectrograph specifications for a well-studied target, we demonstrate that low-resolution broadband transmission spectra can be extracted from Mo-HRTS data.

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arXiv2025

Atmospheric Dispersion Measurement at Hanle Site

Atmospheric dispersion introduces wavelength-dependent effects that significantly impact ground-based observations, particularly in slit- and fibre-fed spectroscopic studies. These effects reduce the signal entering the spectrograph and introduce systematic errors in radial velocity measurements. To address this challenge, atmospheric dispersion correctors are utilised. However, many existing designs of these correctors, which are based on theoretical models, often lack practical validation and consistency. The forthcoming National Large Optical Telescope (NLOT) will be installed at Hanle, a site known for its favourable astronomical sky conditions. Thus, the design of an effective dispersion corrector for the instruments on the NLOT, specifically one that compensates for the measured dispersion, is crucial. For the first time, we have directly measured atmospheric dispersion at the Hanle site using the Himalayan Faint Object Spectrograph mounted on the Himalayan Chandra Telescope. In this study, we present our methodology, the dispersion measurements obtained within the 400 to 700 nm wavelength range, and a comparison with modelled dispersion values.

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arXiv2025

An Initial Assessment of the Hanle Echelle Spectrograph for Exoplanet Atmosphere Studies

Transmission spectroscopy is an effective technique for probing exoplanetary atmospheres. While most observations have relied on space facilities such as HST and JWST, ground-based high-resolution transmission spectroscopy (HRTS) has also provided valuable insights by resolving individual atomic features. In this work, we present an initial performance assessment and feasibility test of the Hanle Echelle Spectrograph (HESP) on the 2 m Himalayan Chandra Telescope (HCT) for HRTS. As a benchmark, we observed the hot Jupiter HD 209458b during a single transit at a resolution of R = 30,000. We developed a Python-based, semi-automated data reduction and analysis pipeline that includes corrections for telluric contamination and stellar radial velocity shifts. The final achieved signal-to-noise ratio and spectral stability allow us to probe for features at the 0.1% level. This work establishes a methodology and demonstrates the operational capability of the HESP-HCT for obtaining high-resolution transmission spectra.

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arXiv2025

Doppler Shifted Transient Sodium Detection by KECK/HIRES

We carried out the first high-resolution transit observations of the exoplanet WASP-49 Ab with Keck/HIRES. Upon custom wavelength calibration we achieve a Doppler RV precision of $<$ 60 ${\rm m\,s}^{-1}$. This is an improvement in RV stability of roughly 240 ${\rm m\,s}^{-1}$ with respect to the instrument standard. We report an average sodium flux residual of $\Delta \mathcal{F}_{NaD}/ \mathcal{F}_{\star} (\lambda) \sim$ 3.2 $\pm$ 0.4 $\%$ (8.0 $\sigma$) comparable to previous studies. Interestingly, an average Doppler shift of -6.2 $\pm$ 0.5 ${\rm km\,s}^{-1}$ (12.4 $\sigma$) is identified offset from the exoplanet rest frame. The velocity residuals \textit{in time} trace a blueshift (v$_{\Gamma, ingress} \sim$ -10.3 $\pm$ 1.9 ${\rm km\,s}^{-1}$) to redshift (v$_{\Gamma, egress} \sim$ +4.1 $\pm$ 1.5 ${\rm km\,s}^{-1}$) suggesting the origin of the observed sodium is unlikely from the atmosphere of the planet. The average Na light curves indicate a depth of $\Delta \mathcal{F}_{NaD} /\mathcal{F}_{\star} (t) \sim$ 0.47 $\pm$ 0.04 % (11.7 $\sigma$) enduring $\lesssim$ 90 minutes with a half-max duration of $\sim$ 40.1 minutes. Frequent high-resolution spectroscopic observations will be able to characterize the periodicity of the observed Doppler shifts. Considering the origin of the transient sodium gas is of unknown geometry, a co-orbiting natural satellite may be a likely source.

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arXiv2024

A multi-functional fiber positioning system for Extremely Large Telescopes

We present a conceptual design for a fiber positioning system for multi-object high-resolution spectroscopy, designed to be compatible with the upcoming large telescopes with a wide field of view. The design incorporates multiple Atmospheric Dispersion Correctors (ADCs) and tip-tilt mirrors that receive non-telecentric input from individual targets and direct it to the ADCs. Here, we introduce a mechanical design for the fiber positioner that accommodates the optics and operates in a curved focal plane with a Radius of Curvature (R) of 3m. This mechanical design provides four degrees of freedom to access the focal volume, enhancing targeting efficiency. The proposed design and an efficient target allocation algorithm ensure a targeting efficiency of approximately 80-100% for a primary observation session. We also present a methodology for target assignment, positioning, and quantification based on sequential and Monte Carlo (MC) algorithms. This method has been tested on realistic fields with varying target densities to validate its performance.

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arXiv2023

Atmospheric dispersion corrector for a multi-object spectroscopic mode of HROS-TMT

Highly multiplexed spectroscopic surveys have changed the astronomy landscape in recent years. However, these surveys are limited to low and medium spectral resolution. High spectral resolution spectroscopy is often photon starved and will benefit from a large telescope aperture. Multiplexed high-resolution surveys require a wide field of view and a large aperture for a suitable large number of bright targets. This requirement introduces several practical difficulties, especially for large telescopes, such as the future ELTs. Some of the challenges are the need for a wide field atmospheric dispersion corrector and to deal with the curved non-telecentric focal plane. Here, we present a concept of Multi-Object Spectroscopy (MOS) mode for TMT High-Resolution Optical Spectrograph (HROS), we have designed an atmospheric dispersion corrector for individual objects that fit inside a fiber positioner. We present the ZEMAX design and the performance of the atmospheric dispersion corrector for all elevations accessible by TMT.

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