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

TL;DR

This work addresses the challenge of characterizing exoplanets without transits by using solar system planets as proxies for planet-as-point-source observations with a high-resolution spectrograph. It develops a practical interface to feed disk-integrated light from the Vainu Bappu Telescope into the HiRES spectrograph by reimaging the pupil to a ~200 µm diameter at an approximate $f/3$ to enable coupling into a 100 µm fiber, evaluated with Zemax and off-the-shelf microlenses. Throughput modeling yields $C(\lambda)$ and an $SNR$ per pixel that is sufficient for bright targets like Venus and Jupiter, while fainter outer planets would require longer exposures; mechanical integration with the autoguider remains a constraint. The study provides a concrete template for exploiting ELT-era capabilities by validating planet-as-point-source spectroscopy on a 2.34 m telescope and outlining paths for hardware integration and future enhancements. Overall, the approach offers a viable pathway to high-resolution reflected-light spectroscopy of Solar System analogs and exoplanets, with emphasis on maximizing throughput and minimizing optics modifications, while highlighting practical mechanical considerations for instrument integration. $SNR$ per pixel scales approximately as $SNR \approx \sqrt{N_{\mathrm{ph}}}$, linking photon counts to detectability across wavelengths.

Abstract

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.

Figures (6)

• Figure 1: Pictorial representation of the requirement for feeding a planet-as-a-point-source from VBT prime focus (a case study with Jupiter)
• Figure 2: Transmittance of the lens materials LASFN-35 (green) and LB2000 (blue)
• Figure 3: Optical layout of two microlens configurations explored for reimaging the VBT prime focus onto the fiber input. (Top) The reimaging optics design with the ball lens. (Bottom) The reimaging optics design with the aspheric lens.
• Figure 4: Footprint diagrams for two microlens configurations used to reimage the VBT prime focus onto the fiber input. The top panel shows the footprint for the ball lens design, and the bottom panel shows the footprint for the aspheric lens design. The aspheric lens configuration produces a more uniform pupil distribution at the fiber input compared to the ball lens
• Figure 5: (a) Calculated photon counts per pixel for each planet across the observed wavelength range. (b) Corresponding signal-to-noise ratio (SNR) per pixel for a 1 s exposure, derived from the total system response.