Towards a physical understanding of the thermal background in large ground-based telescopes

Leonard Burtscher; Ioannis Politopoulos; Sergio Fernández-Acosta; Tibor Agocs; Mario van den Ancker; Roy van Boekel; Bernhard Brandl; Hans-Ulrich Käufl; Eric Pantin; Alex G. M. Pietrow; Ralf Siebenmorgen; Remko Stuik; Konrad R. W. Tristram; Willem-Jan de Wit

doi:10.1117/12.2576271

28 December 2020 Towards a physical understanding of the thermal background in large ground-based telescopes

Leonard Burtscher, Ioannis Politopoulos, Sergio Fernández-Acosta, Tibor Agocs, Mario van den Ancker, Roy van Boekel, Bernhard Brandl, Hans-Ulrich Käufl, Eric Pantin, Alex G. M. Pietrow, Ralf Siebenmorgen, Remko Stuik, Konrad R. W. Tristram, Willem-Jan de Wit

Author Affiliations +

Proceedings Volume 11447, Ground-based and Airborne Instrumentation for Astronomy VIII; 114477L (2020) https://doi.org/10.1117/12.2576271
Event: SPIE Astronomical Telescopes + Instrumentation, 2020, Online Only

Abstract

Ground-based thermal-infrared observations have a unique scientific potential, but are also extremely challenging due to the need to accurately subtract the high thermal background. Since the established techniques of chopping and nodding need to be modified for observations with the future mid-infrared ELT imager and spectrograph (METIS), we investigate the sources of thermal background subtraction residuals. Our aim is to either remove or at least minimise the need for nodding in order to increase the observing efficiency for METIS. To this end we need to improve our knowledge about the origin of chop residuals and devise observing methods to remove them most efficiently, i.e. with the slowest possible nodding frequency. Thanks to dedicated observations with VLT/VISIR and GranTeCan/CanariCam, we have successfully traced the origin of three kinds of chopping residuals to (1) the entrance window, (2) the spiders and (3) other warm emitters in the pupil, in particular the VLT M3 mirror cell in its parking position. We conclude that, in order to keep chopping residuals stable over a long time (and therefore allow for slower nodding cycles), the pupil illumination needs to be kept constant, i.e. (imaging) observations should be performed in pupil-stabilised, rather than field-stabilised mode, with image de-rotation in the post-processing pipeline. This is now foreseen as the default observing concept for all METIS imaging modes.

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Leonard Burtscher, Ioannis Politopoulos, Sergio Fernández-Acosta, Tibor Agocs, Mario van den Ancker, Roy van Boekel, Bernhard Brandl, Hans-Ulrich Käufl, Eric Pantin, Alex G. M. Pietrow, Ralf Siebenmorgen, Remko Stuik, Konrad R. W. Tristram, and Willem-Jan de Wit "Towards a physical understanding of the thermal background in large ground-based telescopes", Proc. SPIE 11447, Ground-based and Airborne Instrumentation for Astronomy VIII, 114477L (28 December 2020); https://doi.org/10.1117/12.2576271

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KEYWORDS

Mid-IR

Telescopes

Cameras

Cryogenics

Exoplanets

Imaging systems

Ray tracing

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