Atmospheric Calibration for Rubin-LSST – CARL

The Rubin-LSST (Large Survey of Space and Time) project is a deep survey of the sky with an 8.40 diameter telescope equipped with a 3.2 billion pixel camera. Starting in 2024, it will map the southern sky for 10 years, with 800 photometric measurements per object, obtained through 6 wide visible bandwidths. This survey will not only allow to measure precisely cosmological parameters, but also to revolutionize the studies of the variable sky.
In order to measure the transparency of the atmosphere in real time a spectrograph, equipped with a holographic optical element developed at IJCLab and LPNHE and realized in France, is installed on an auxiliary telescope in operation since early 2021. The process of atmospheric compensation that we want to develop consists in measuring the transmission of the atmosphere with this spectrograph, then to use it to restore estimates of fluxes relative to "standard" atmospheric conditions. This operation will allow the best use of the data from the LSST main telescope. The objective is to reach a photometric accuracy of the order of 0.1%, even under non-stable atmospheric conditions, which would be a remarkable breakthrough: first, the nights qualified as non-photometric would be more usable; second, the improvement of the photometric accuracy would have an effect equivalent to an increase of the collecting area of the main telescope.
Benefits are expected in all fields of astronomy, and in particular in cosmology thanks to the very significant improvement of the photometric follow-up of the 100,000 supernovae expected in LSST.
The French team has been leading this very high visibility project since its beginning, and the LSST management has delegated to us the responsibility to build on the effort made since 2017 to complete the implementation of the spectrograph and to finalize the photometric compensation procedures related to the atmosphere. To this end, our work program is first to complete the installation, calibration, and performance verification of the auxiliary telescope spectrograph in all possible atmospheric situations, in terms of humidity and aerosol absorption. We also estimate that at least 3 years of sustained effort are still needed to perfect and make reliable the spectra reduction and real-time atmospheric parameter extraction programs, to develop the photometric compensation procedures, to optimize the observing strategy of the spectrograph in synchronism with the main telescope, and to measure and understand the impact of the atmosphere on an annual cycle. Consequently, we first request from the ANR the financing of a post-doctoral fellow for 3 years, in anticipation of the end of a current contract, to allow us to maintain our efforts in these developments.
Secondly, to best measure the transparency of the atmosphere, it is essential to have an excellent knowledge of the complete transmission of the instrument consisting of the auxiliary telescope and its camera, as a function of wavelength. This transmission combines the reflectivity of the mirrors, the transmission of the optics and the hologram, and the quantum efficiency of the detector. We therefore need to complete our spectrograph with a device for calibrating the auxiliary telescope, the "travelling CBP" for Collimated Beam Projector, a device derived from a system already used for the StarDICE project, and of which a version has been built for the main LSST telescope. The version to be developed to regularly measure the transmission of the auxiliary telescope requires the acquisition of an intense laser-driven light source coupled to a monochromator, for which we are requesting funding from the ANR.