Atmospheric optical turbulence profiling by stellar scintillometers: an instrument evaluation experiment.

We operated two identical atmospheric optical turbulence profiling instruments over a period from 15 Mar. to 20 Apr. 1985 at the Air Force Maui Optical Station on Mount Haleakala. The daily average results of looking along common lines of sight through identical atmospheres show systematic differences at lower altitudes and near agreement at higher altitudes. We suspect that spatially nonuniform PMT photocathode responses and incompletely subtracted background noise could significantly affect the turbulence values measured at lower altitudes and contribute to the low altitude systematic differences. The average results of looking along two different lines of sight through different regions of the atmosphere show additional systematic differences at higher altitudes, which are not due to instrument bias. We believe these to be due to genuine horizontal structure in the distribution of atmospheric optical turbulence above the 10-km level.