A critical part of SOFIA's operations is the Water Vapor Monitor of which Tom Roellig of NASA Ames Research Center is the Prinicipal Investigator.
The Water Vapor Monitor measures the water vapor content of the atmosphere integrated along the line-of-sight at a 40 degree elevation angle by making submm radiometric measurements of the center and wings of the 183.3 GHz rotational line of water. These measurements are then converted to the integrated water vapor along the telescope line-of-sight.
The monitor hardware consists of three physically distinct sub-systems: 1) a 12 lbs. radiometer head mounted in a pressure vessel, 2) an IF Converter Box Assembly, and 3) an operating computer, which are mounted in a cabinet just under the radiometer head. The window plug contains a 5" size 0.15 inch thick stretched acrylic window suitable for optical observations.
The Water Vapor Monitor is powered on 15 VDC at 1.5 amps maximum, but taps power from a supply of 110V/60Hz. A flow of dry air is collected from the air ducts that keeps the windows free of frost.
This is a FAA approved setup that can be copied to accommodate other instruments.