Wave propagation effect on differential image motion monitor measurements

Abstract

ABSTRACT The differential image motion monitor (DIMM) is widely used to measure the atmospheric optical turbulence (OT) integral over the line of sight. However, it is known that because of the wave propagation effect, the DIMM’s sensitivity falls with the distance to the turbulent layer and the variance of differential image motion is only approximately proportional to the OT integral. In this paper, we study this problem using the formalism of so-called weighting functions (WFs). We develop a method for calculating these WFs, including the case of the wide spectral band of detected radiation. The effect of underestimating the OT integral for different DIMM devices involved in previous site-testing campaigns is estimated using real measurements of OT vertical profiles. For instruments with small apertures (4–6 cm), the underestimation is ≈10 per cent, whereas for larger devices with apertures of 8–10 cm, the underestimation is less than 5 per cent, on average. This leads to a negative bias in seeing estimation of 7 and 3 per cent, respectively.