Observing exoplanets from Antarctica in two colours: set-up and operation of ASTEP+

Description

On December 2021, a new camera box for two-colour simultaneous visible photometry was successfully installed on the ASTEP telescope at the Concordia station in Antarctica. The new focal box offers increased capabilities for the ASTEP+ project. The opto-mechanical design of the camera was described in a previous paper.1 Here, we focus on the laboratory tests of each of the two cameras, the low-temperature behaviour of the focal box in a thermal chamber, the on-site installation and alignment of the new focal box on the telescope, the measurement of the turbulence in the tube and the operation of the telescope equipped with the new focal box. We also describe the data acquisition and the telescope guiding procedure and provide a first assessment of the performances reached during the first part of the 2022 observation campaign. Observations of the WASP19 field, already observed previously with ASTEP, demonstrates an improvement of the SNR by a factor 1.7, coherent with an increased number of photon by a factor of 3. The throughput of the two cameras is assessed both by calculation of the characteristics of the optics and quantum efficiency of the cameras, and by direct observations on the sky. We find that the ASTEP+ two-colour transmission curves (with a dichroic separating the fluxes at 690nm) are similar to those of GAIA in the blue and red channels, but with a lower transmission in the ASTEP+ red channel leading to a 1.5 magnitude higher B-R value compared to the GAIA B-R value. With this new setting, the ASTEP+ telescope will ensure the follow-up and the characterization of a large number of exoplanetary transits in the coming years in view of the future space missions JWST and Ariel.

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