Atmospheric Dispersion Measurement at Hanle Site

TL;DR

Atmospheric dispersion affects ground-based spectroscopy by wavelength-dependent throughput losses and radial-velocity systematics. The authors perform the first on-site dispersion measurements at the Hanle site (400–700 nm) using the HFOSC on the Himalayan Chandra Telescope and compare results to the Cassini atmospheric refraction model to separate instrumental dispersion. They find a measured dispersion of $0.6082''$ at a zenith angle of $50.61^\circ$ versus a model value of $0.7429''$, a discrepancy of $0.1346''$, highlighting the need for atmosphere-driven dispersion corrections and site-specific validation to inform the design of dispersion correctors for the upcoming NLOT. The work provides a practical baseline for high-precision spectroscopy at Hanle and points to incorporating real-time meteorological data and, potentially, closed-loop dispersion correction in future instrumentation.

Abstract

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.

Figures (5)

• Figure 1: Airmass and parallactic angle trends during the observation.
• Figure 2: Typical 2D image on the HFOSC detector and the extracted spectrum of the target.
• Figure 3: Example traces demonstrating random motion on the HFOSC detector during the observation.
• Figure 4: Traces in the common frame and pixel scale along the dispersion direction.
• Figure 5: Comparison of measured and modelled atmospheric dispersion at different zenith angles.