arXiv subjects

Devika Divakar

Publications and source records attributed to Devika Divakar.

6 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.

astro-ph.EP↗

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.

astro-ph.IM↗

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.

astro-ph.EP↗

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.

astro-ph.IM↗

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.

astro-ph.IM↗

arXiv2022

Design of SCALES: A 2-5 Micron Coronagraphic Integral Field Spectrograph for Keck Observatory

We present the design of SCALES (Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy) a new 2-5 micron coronagraphic integral field spectrograph under construction for Keck Observatory. SCALES enables low-resolution (R~50) spectroscopy, as well as medium-resolution (R~4,000) spectroscopy with the goal of discovering and characterizing cold exoplanets that are brightest in the thermal infrared. Additionally, SCALES has a 12x12" field-of-view imager that will be used for general adaptive optics science at Keck. We present SCALES's specifications, its science case, its overall design, and simulations of its expected performance. Additionally, we present progress on procuring, fabricating and testing long lead-time components.

astro-ph.IM↗