Features of an infrared system (IWVM) for monitoring atmospheric water vapour content

• Operates near the peak of the Planck function for atmospheric temperatures
• Wavelength range carefully selected to include only transitions from water vapour
• Wavelength range includes mixture of strong and weak lines and therefore samples different heights, in effect yielding an average over whole atmosphere
• Theoretical atmospheric model supported by FTS measurements from Mauna Kea (Naylor et. al. PASP 96, 167 (1984)). Inputs are base pressure and temperature.
• IWVM sensitivity (expressed in terms of $\mu$m pwv) improves as the pwv decreases (Fig 1). For example, if the IWVM can detect variations of $\pm$1$\mu$m pwv in a given integration time when the atmosphere contains 1mm pwv, it will detect variations of $\pm$0.7$\mu$m and $\pm$1.85$\mu$m when the atmosphere contains 0.5 and 3mm pwv respectively
• Photoconductive detectors are simple devices offering high speed, sensitivity and stability
• Diffraction limit at these wavelengths allows use of smaller optics
• Design makes calibration intrinsically clean since optical train is identical in all observing modes