Bolometric night sky temperature and subcooling of telescope structures

Abstract

Context. The term sky temperature is used in the literature to refer to different phenomena in different contexts which often leads to confusion. In this work, we study T sky , the effective bolometric sky temperature at which a hemispherical black body would radiate the same power onto a flat horizontal structure on the ground as the night sky, integrated over the entire thermal wavelength range of 1-100 µm. We then analyze the thermal physics of radiative cooling with special focus on telescopes and discuss mitigation strategies. Aims. The quantity T sky is useful to quantify the subcooling in telescopes which can deteriorate the image quality by introducing an optical path difference (OPD) and induce thermal stress and mechanical deflections on structures. Methods. We employ the Cerro Paranal Sky Model of the European Southern Observatory to derive a simple formula of T sky as a function of atmospheric parameters. The structural subcooling and the induced OPD are then expressed as a function of surface emissivity, sky view factor, local air speed, and structure dimensions. Results. At Cerro Paranal (2 600 m) and Cerro Armazones (3 060 m) in the Atacama desert, T sky towards the zenith mostly lies 25-50 Kelvin below the ambient temperature near the ground, depending to a great extent on the precipitable water vapor (PWV) column in the atmosphere. The temperature difference can decrease by several Kelvin for higher zenith distances. The subcooling OPD scales linearly to quadratically with the telescope diameter and is inversely proportional to the local air speed near the telescope structure.

Redundant apodization for direct imaging of exoplanets

“…The LWE is also expected on upcoming giant telescopes like the ELT (51 cm spider) and the Thirty Meter Telescope (TMT, 225 mm spider) (Holzlöhner et al 2021). In addition, other island phenomena are expected, in particular the petaling effect.…”

“…The ELT, with its 51 cm spiders, might be subject to these island effects. While the low-wind effect has mainly been studied on existing telescopes like VLT or Subaru, studies are also ongoing for the ELT (Holzlöhner et al 2021); in this paper we assume that LWE has the same properties as those observed on SPHERE (i.e., primarily consisting of differential pistons, tips, and tilts). On the other hand, petaling has mainly been studied in simulation and appears as a strong limitation for HARMONI and MI-CADO, which use visible pyramid wavefront sensors (Schwartz et al 2018;Bertrou-Cantou et al 2022).…”

“…However, instruments already in operation, such as VLT/SPHERE or Subaru telescope/SCExAO have been proved sensitive to a specific aberration called the low-wind effect (LWE), a phenomenon that happens for high-emissivity spider telescopes when the wind speed at the level of the telescope aperture is lower than a few m/s (Sauvage et al 2015;Milli et al 2018;Vievard et al 2019;Holzlöhner et al 2021), which becomes dominant under good observing conditions (low seeing). With LWE the phase is fragmented into differential piston, tip, and tilt on the pupil petals delimited by the spiders that cannot be corrected by traditional adaptive optics (AO) systems (N'Diaye et al 2018).…”

Redundant apodization for direct imaging of exoplanets 2: Application to island effects

“…However, instruments already in operation, such as VLT/SPHERE or Subaru telescope/SCExAO have revealed themselves sensitive to a specific aberration, called the low-wind effect (LWE), a phenomenon that happens for high emissivity spider telescopes when the wind speed at the level of the telescope aperture is lower than a few m/s (Sauvage et al 2015;Milli et al 2018;Vievard et al 2019;Holzlöhner et al 2021), which becomes dominant under good observing conditions (low seeing). With LWE, the phase is fragmented into differential piston, tip, and tilt on the pupil petals delimited by the spiders that cannot be corrected by traditional Adaptive Optics (AO) systems (N'Diaye et al 2018).…”

Flip-flop modulation method used with a pyramid wavefront sensor to correct piston segmentation on ELTs

“…The low wind effect is a phenomenon where the low-velocity wind blowing over the telescope structure is cooled to subambient temperatures, resulting in optical path differences across the pupil of the telescope. 12 Previous work on the topic of segment piston control has led to several approaches to solving the problem. Schwartz et al 11 shown a phase closure algorithm to ensure a continuous phase across pupil segments.…”