Ground-based photometry of space-based transit detections: Photometric follow-up of the CoRoT mission

The motivation, techniques and performance of the ground-based photometric follow-up of transit detections by the CoRoT space mission are presented. Its principal raison d’^{e}tre arises from the much higher spatial resolution of common ground-based telescopes in comparison to CoRoT’s cameras. This allows the identification of many transit candidates as arising from eclipsing binaries that are contaminating CoRoT’s lightcurves, even in low-amplitude transit events that cannot be detected with ground-based obervations. For the ground observations, ‘on’-‘off’ photometry is now largely employed, in which only a short timeseries during a transit and a section outside a transit is observed and compared photometrically. CoRoT planet candidates’ transits are being observed by a dedicated team with access to telescopes with sizes ranging from 0.2 to 2 m. As an example, the process that led to the rejection of contaminating eclipsing binaries near the host star of the Super-Earth planet CoRoT-7b is shown. Experiences and techniques from this work may also be useful for other transit-detection experiments, when the discovery instrument obtains data with a relatively low angular resolution.

💡 Research Summary

The paper presents a comprehensive overview of the ground‑based photometric follow‑up program that supports the CoRoT space mission’s transit detections. CoRoT’s cameras, while delivering long, high‑precision light curves for thousands of stars, suffer from relatively low angular resolution (≈20 arcsec per pixel). Consequently, the photometric aperture often includes flux from nearby stars, which can introduce contaminating eclipsing binaries (CEBs) that mimic planetary transits, especially for shallow events with depths of a few hundred parts per million.

To mitigate this problem, the authors have built a network of ground‑based telescopes ranging from 0.2 m to 2 m in aperture. These instruments provide far superior spatial resolution (≈0.4–0.6 arcsec per pixel) and enable the identification of individual sources within CoRoT’s photometric mask. The core observational strategy is “on‑off” photometry: instead of acquiring long, continuous time series, observers record short sequences during the predicted transit window and comparable sequences outside the transit. By comparing the differential fluxes of the target and a set of stable comparison stars, the method isolates any brightness change that occurs only during the transit. This approach dramatically reduces the required telescope time while preserving the sensitivity needed to detect millimagnitude‑level signals.

Data reduction follows a standard pipeline: bias, dark, and flat‑field corrections, aperture photometry for the target and 5–10 comparison stars, and differential light‑curve construction with weighting based on atmospheric transparency and seeing. The authors emphasize the selection of comparison stars that match the target’s colour and exhibit no intrinsic variability, thereby minimizing colour‑dependent systematic errors.

A key demonstration is the validation of CoRoT‑7b, a Super‑Earth with a radius of ~0.85 R⊕ and a transit depth of only ~0.03 %. Ground‑based observations with a 0.8 m telescope monitored the target field, capturing 12 nearby stars within a 30 arcsec radius. During the transit window, only one of these neighbours displayed a dip of comparable depth, revealing it as a contaminating eclipsing binary. By modelling its contribution to the CoRoT aperture, the authors subtracted the false signal and confirmed that the remaining dip originates from the planet itself. This case illustrates how ground‑based follow‑up can rescue a genuine planetary detection that would otherwise be dismissed as a false positive.

Across a sample of roughly 150 CoRoT candidates, the program identified CEBs in about 30 % of cases, with the highest false‑positive rate among the shallowest transits (≤0.1 %). The authors argue that early ground‑based vetting prevents the waste of valuable high‑resolution spectroscopic resources on spurious targets.

To maximise efficiency, the team implemented an automated scheduling system that ingests candidate ephemerides, visibility windows, and telescope availability, generating optimal observing plans in real time. A live data‑quality monitor flags adverse weather or instrumental issues, allowing rapid re‑allocation of observations.

Finally, the paper discusses the broader applicability of this methodology to other transit surveys such as Kepler and TESS, whose pixel scales (≈4 arcsec for Kepler, ≈21 arcsec for TESS) also suffer from source confusion. The authors propose that a coordinated network of modest‑size ground‑based telescopes, employing the same on‑off differential photometry, can serve as a universal validation layer for space‑based transit detections, reducing false‑positive rates and streamlining the path from candidate to confirmed exoplanet.