Summary of Near-IR results:
The lower temperature of the sky and telescope
gives at least a factor of 20 reduction in sky
background beyond 2.5 mm.
The “cosmological window” from 2.27 to 2.45 mm
is a factor of 100 darker, because of a natural gap
in the air-glow.
The air-glow drops by a factor of 3 when the Sun
is more than 10° below horizon.