Thermal Infrared Sky Background for a High-Arctic Mountain Observatory

TL;DR

This study reports on the nighttime thermal infrared sky brightness at a High-Arctic observatory, demonstrating exceptionally dark skies suitable for infrared astronomy, based on three winters of continuous spectral measurements and meteorological data.

Contribution

It provides the first detailed spectral brightness measurements in the thermal infrared for a High-Arctic site, characterizing its stability and suitability for astronomical observations.

Findings

Sky brightness is extremely low across the thermal infrared spectrum.

The site has stable, dark skies comparable to the South Pole.

Infrared background levels are significantly lower than temperate sites.

Abstract

Nighttime zenith sky spectral brightness in the 3.3 to 20 micron wavelength region is reported for an observatory site nearby Eureka, on Ellesmere Island in the Canadian High Arctic. Measurements derive from an automated Fourier-transform spectrograph which operated continuously there over three consecutive winters. During that time the median through the most transparent portion of the Q window was 460 Jy/square-arcsec, falling below 32 Jy/square-arcsec in N band, and to sub-Jansky levels by M and shortwards; reaching only 36 mJy/square-arcsec within L. Nearly six decades of twice-daily balloonsonde launches from Eureka, together with contemporaneous meteorological data plus a simple model allows characterization of background stability and extrapolation into K band. This suggests the study location has dark skies across the whole thermal infrared spectrum, typically sub-200 micro-Jy/square-arcsec at 2.4 microns. That background is comparable to South Pole, and more than an order of magnitude less than estimates for the best temperate astronomical sites, all at much higher elevation. Considerations relevant to future facilities, including for polar transient surveys, are discussed.