Bolometric Night Sky Temperature and Subcooling of Telescope Structures. (arXiv:2010.01978v2 [astro-ph.IM] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Holzlohner_R/0/1/0/all/0/1">Ronald Holzlöhner</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kimeswenger_S/0/1/0/all/0/1">Stefan Kimeswenger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kausch_W/0/1/0/all/0/1">Wolfgang Kausch</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Noll_S/0/1/0/all/0/1">Stefan Noll</a>
Context. The term sky temperature is used in the literature in different
contexts which often leads to confusion. In this work, we study $T_text{sky}$,
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
$1-100,mu$m. We then analyze the thermal physics of radiative cooling with
special focus on telescopes and discuss mitigation strategies.
Aims. The quantity $T_text{sky}$ 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 $T_text{sky}$ 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 (2600 m) and Cerro Armazones (3060 m) in the
Atacama desert, $T_text{sky}$ towards the zenith mostly lies $20-50$ Kelvin
below the ambient temperature near the ground, depending strongly 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.
Context. The term sky temperature is used in the literature in different
contexts which often leads to confusion. In this work, we study $T_text{sky}$,
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
$1-100,mu$m. We then analyze the thermal physics of radiative cooling with
special focus on telescopes and discuss mitigation strategies.
Aims. The quantity $T_text{sky}$ 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 $T_text{sky}$ 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 (2600 m) and Cerro Armazones (3060 m) in the
Atacama desert, $T_text{sky}$ towards the zenith mostly lies $20-50$ Kelvin
below the ambient temperature near the ground, depending strongly 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.
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