CONCERTO is playing drums!
CONCERTO is a brand-new instrument. Its installation started on April 6th 2021 and the ESO+OSO Large Program on the [CII] intensity mapping (800 hours granted) started on July, the 15th the same year. The [CII] survey started so soon after installation to be able to be completed before ESO's withdrawal from APEX at the end of 2022. At that time, the instrument was perfectly integrated in APEX infrastructure, characterized in photometry, and in perfect agreement with expectations (despite the very short time), but not in spectroscopy, the main observation mode of the [CII] survey. We soon discovered a strong systematic effect in the spectroscopic data, that we firmly identified in October 2021 as linked to the oscillations of the membrane of the polarizer (which is very large, 480mm×800mm). These oscillations are produced by all acoustic vibrations in the C-cabin, and in particular from the roof mirror motion which creates a wind flow and by the Menerga cooling system. These oscillations are causing a complex perturbation on the optical path difference (OPD) of the interferograms.
In December 2021, we installed a new laser to measure in real-time the OPD distortions, by directly measuring the main mode of vibrations on the central part of polarizer. These measurements can be used to correct the data at first order.
But more importantly, we developed in the lab in Fall 2021, and installed during the 2022 APEX shutdown, an active acoustic feedback system which is capable to counter the vibrations by sending proper sound waves. The new laser is used to calculate in real time the related acoustic counter-wave to be applied for minimizing the vibrations. This is working well, and the ultimate limitation of the correction is the relatively poor laser precision given the measured length. Details can be found in Fasano et al. (2022, SPIE).
