Table 1: The optical filtering scheme employed by B03. In order to take advantage of the low backgrounds available at float altitudes, much care must be taken to reduce the background originating from within the cryostat. While the metal mesh filters, which consist of bonded layers of polyethylene, exhibit in-band emissivities at the percent level, the PTFE antireflection coating is several times more emissive. It is crucial that these filters remain well heat-sunk and protected from infrared emission from the warmer stages.
Optical Filters
Temp. 145 GHz 245 GHz 345 GHz
77 K $\sim $$100~\mu$m IR blocker
$\vdots$ 540 GHz AR-coated LPF
2 K $\sim $$100~\mu$m IR blocker
$\vdots$ 450 GHz AR-coated LPF
$\vdots$ Removable NDF$^\dagger$
$\vdots$   Polarizing Grid
$\vdots$   420 GHz LPF
$\vdots$   $180~\mu$m BP/Yosh LPF$^\ddagger$
$\vdots$   295 GHz dichroic LPF 295 GHz dichroic HPF
0.3 K $180~\mu$m BP/Yosh LPF$^\ddagger$ 360 GHz LPF 410 GHz LPF
$\vdots$ 255 GHz LPF  
$\vdots$ 540 GHz LPF  
$\vdots$ 168 GHz LPF  
$^\dagger$ The neutral density filter can be mechanically rotated in and out of the beam while the system is cooled down.
$^\ddagger$ A black-polyethilene (BP) Yoshinaga filter is an absorptive filter consisting of a thallium salt deposited on a black polyethylene substrate, the thickness of which is tuned to minimize reflections (Yamada et al. 1962).


Source LaTeX | All tables | In the text