Vertical path atmospheric MTF measurements

Measurements of the atmospheric modulation transfer function (MTF) have been made at three sites simultaneously along a vertical path using stars as point light sources. A relatively simple photographic technique was employed to compile “climatological” summaries of the atmospheric MTF characterized by the transverse coherence length r0 over a seven-month period during the day and night. The results show a factor of 2 diurnal change in r0 with relative peaks during the morning and evening neutral zones. The average r0 was found to be 90 ± 40 mm at night and 45 ± 18 mm during the day with a relatively constant standard deviation of 43%. The night-time averages appear to be among the highest recorded to date. The measurement technique is presented along with the average data trends. The differences among site locations are investigated.