Measurement of atmospheric MTF in a littoral environment

Measurement of atmospheric modulation transfer function (MTF) derived from the point spread function is an alternative to the use of scintillometry in characterizing the effects of turbulence as well as optical scattering. This experiment involved measurement of the system point spread function at 630 nm wavelength, over a 1.8 km path between the shoreline and Roman Rock lighthouse, which is offshore in False Bay near Simonstown, South Africa. Four telescopes at heights between 4 m and 14 m above the sea surface were mounted at the shoreline looking at two point sources mounted at heights of 6 m and 13 m above the sea surface on the lighthouse. The telescopes were equipped with digital charge-coupled device (CCD) cameras and the system MTF of the combination was measured. While the point spread function is also sensitive to atmospheric scattering, scintillation is driven largely by turbulence. Measurements were made under a wide variety of environmental conditions and the differences between image quality metrics and scintillation metrics were examined. Local weather parameters, visibility and sea temperature were also monitored during the campaign and the impact of these parameters on image quality was assessed. Divergent trends in scintillation and image quality were encountered.