  The Millimeter Sky Transparency Imager (MiSTI) is a small millimeter-wave
scanning telescope with a 25-cm diameter dish operating at 183 GHz. MiSTI is
installed at Atacama, Chile, and it measures emission from atmospheric water
vapor and its fluctuations to estimate atmospheric absorption in the millimeter
to submillimeter. MiSTI observes the water vapor distribution at a spatial
resolution of 0.5 deg, and it is sensitive enough to detect an excess path
length of <~ 0.05 mm for an integration time of 1 s. By comparing the MiSTI
measurements with those by a 220 GHz tipper, we validate that the 183 GHz
measurements of MiSTI are correct, down to the level of any residual systematic
errors in the 220 GHz measurements. Since 2008, MiSTI has provided real-time
(every 1 hr) monitoring of the all-sky opacity distribution and atmospheric
transmission curves in the (sub)millimeter through the internet, allowing to
know the (sub)millimeter sky conditions at Atacama.
