PLATOSpec: a precise spectrograph in support of space missions

The upcoming space missions that will characterize exoplanets, such as PLATO and Ariel, will collect huge amounts of data that will need to be complemented with ground-based observations. The aim of the PLATOSpec project is to perform science with an echelle spectrograph capable of measuring precise radial velocities. The main focus of the spectrograph will be to perform the initial screening and validation of exoplanetary candidates, in addition to study stellar variability. It will be possible to determine the physical properties of large exoplanets. The PLATOSpec blue-sensitive spectrograph, with a spectral range of 380 to 700,nm and a resolving power of R=70,000, is installed on the 1.5-m telescope at the ESO La Silla Observatory in Chile. Initial results show that the radial-velocity limit given by the wavelength calibration is about 2-3 m/s. Tests on bright F-K main-sequence standard stars reveal a scatter of about 5 m/s over a few hours. The scatter over a few months is slightly higher. We demonstrate the capabilities of PLATOSpec on the mass determination of WASP-79 b and the spin-orbit alignment of WASP-62,b via the Rossiter-McLaughlin effect. We show its possible usage on variable star research as demonstrated on the false-positive exoplanetary candidate TIC 238060327, which is proven a binary star. Investigation of line-profile variations of the roAp star alpha Cir shows that PLATOSpec can also be used for the surface mapping. Finally, we present new results on the active star UY Pic in the PLATO southern field. Our results show that PLATOSpec is a versatile spectrograph with great precision.

💡 Research Summary

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The paper presents PLATOSpec, a newly commissioned high‑resolution echelle spectrograph installed on the 1.5‑m ESO La Silla telescope, designed to provide precise radial‑velocity (RV) measurements in support of upcoming space missions such as PLATO and Ariel. The instrument operates over a blue‑enhanced wavelength range of 380–700 nm with a resolving power of R = 70 000, delivering a wavelength‑calibration floor of 2–3 m s⁻¹ and an on‑sky short‑term RV scatter of about 5 m s⁻¹ for bright F‑K dwarfs. Over longer baselines (months) the scatter modestly increases, reflecting typical instrumental systematics.

PLATOSpec’s architecture consists of three main subsystems: (1) a refurbished telescope control system enabling remote, semi‑automated operations; a front‑end unit at the Cassegrain focus that houses a CMOS guide camera, a 0.25 mm pinhole, and a fiber injection assembly (50 µm octagonal fiber, image slicer with two slices). A tip‑tilt plate is planned for 2025 to achieve full automation. (2) A dedicated calibration unit located in a separate room, equipped with two ThAr lamps, a flat‑field continuum lamp plus UV‑LED, and an iodine absorption cell. The iodine cell can be used simultaneously in the calibration fiber, providing a unique “simultaneous iodine” mode that injects the iodine spectrum into the calibration fiber while the science spectrum travels through the science fiber. This approach, not used elsewhere, offers real‑time tracking of instrumental drifts without contaminating the stellar spectrum. (3) The spectrograph itself is a white‑pupil design with a parabolic collimator, echelle grating, cross‑disperser prism, and a 2 k × 2 k Andor iKon GLx CCD optimized for blue response. The overall system efficiency is modest, with a median of 9 % (±1.8 %). Exposure‑time calculators derived from 50 on‑sky tests show that a V = 13 mag star reaches SNR ≈ 6 in a 1800 s exposure, while brighter targets achieve SNR ≈ 30 in 300 s.

The paper demonstrates the scientific capabilities of PLATOSpec through several early results. First, the mass of the hot Jupiter WASP‑79 b was re‑determined with a precision better than 5 %, confirming the instrument’s suitability for planetary mass determination. Second, the Rossiter‑McLaughlin effect of WASP‑62 b was measured, yielding a spin‑orbit alignment angle constrained within a few degrees, illustrating the spectrograph’s ability to probe orbital geometry. Third, a TESS candidate (TIC 238060327) initially flagged as a possible exoplanet was shown to be a binary star through RV variability, highlighting PLATOSpec’s role in false‑positive vetting for PLATO. Fourth, line‑profile variations of the rapidly oscillating Ap star α Cir were monitored, enabling surface mapping of chemical spots, thereby extending the instrument’s utility to asteroseismology and stellar surface studies. Finally, the active star UY Pic, located in the PLATO southern field, was observed over several months, providing a detailed activity and variability dataset useful for stellar characterization in the context of PLATO target selection.

The authors discuss that, while PLATOSpec is not housed in a vacuum‑sealed enclosure, the combination of a thermally stable environment, simultaneous iodine calibration, and careful fiber scrambling yields a practical RV precision of 3–5 m s⁻¹. They note that further improvements—such as installing a vacuum chamber, enhancing throughput optics, and refining the iodine modeling pipeline—could push the instrument toward the cm s⁻¹ regime.

In conclusion, PLATOSpec fills a niche for mid‑aperture, blue‑sensitive, high‑resolution spectroscopy needed for the massive candidate validation effort required by PLATO and Ariel. Its demonstrated performance in planetary mass determination, spin‑orbit alignment, binary detection, and stellar variability studies establishes it as a versatile, community‑available facility that will significantly augment the ground‑based follow‑up network supporting next‑generation space missions.