Characterisation of the Dicke-Switch Chopper System and AGPM Coronagraph for the NEAR Experiment at the ESO-VLT

Abstract

As part of the New Earths in the Alpha Cen Region (NEAR) experiment, VISIR, an imager and spectrometer for the mid-infrared at the Very Large Telescope (VLT) of the European Southern Observatory, will be soon upgraded to improve its performance in terms of high contrast and sensitivity for the detection of Earth-like planets in the Alpha Centauri star system, the closest to the Sun. Part of the VISIR upgrade consists in a novel chopper system based on the Dicke-switch concept to suppress the excess low frequency noise (ELFN) suffered by the detector array. The ELFN is a form of temporally correlated noise and can be mitigated by modulating the incident light at high enough frequency. The other main challenge of the NEAR experiment is the need of high contrast to image the circumstellar environment, and it will be achieved with the implementation of an annular groove phase mask (AGPM) coronagraph for the suppression of the starlight. The main goals of the thesis project were the validation of the Dicke-switch chopper systems to assess its ability to mitigate background level and ELFN, and the characterisation of the performance of the AGPM coronagraph in terms of null depth and contrast level. The first part of the laboratory characterisation of the Dicke-switch chopper system focused on the verification of the radiometric flux level achievable by its internal blackbody unit and by the so called artificial sky, a cold blackbody needed to reproduce the background level typical at the VLT, where the combined sky and telescope thermal radiation is the same as the one of a greybody at 280K and 10% emissivity. After the completion of the radiometric tests and mitigation of parasitic light effects caused by reflection of internal and external thermal radiation, the tests to validate the ability of the Dicke-switch chopper system to remove background level and suppress the ELFN started. The obtained results confirm that the Dicke-switch chopper system mitigate the ELFN as done also with pseudo-chopping. This was verified also for large offsets between the flux level of internal blackbody unit and artificial sky. Similarly, the characterisation of the AGPM coronagraph started with the preparation of the laboratory setup, in particular design of the optical setup, alignment of all the optical components and measurement of the optical quality of the system to limit residual aberrations and vignetting. The results of the coronagraphic tests revealed that the two candidate AGPMs for the NEAR campaign, both optimised for wavelengths between 10 and 12.5 μm, are not able to provide the required contrast level and null depth. On the contrary, an older AGPM, spare part of the one currently mounted in VISIR, and thus optimised for wavelengths between 11 and 13.1 μm, provided better coronagraphic performance, and even though it is still not compliant with the requirements, at present it represents the baseline for the NEAR experiment.