Differential image motion in the short exposure regime

TL;DR

This paper investigates the impact of wind on differential image motion measurements in short exposure regimes, proposing a correction method using image correlation to improve atmospheric seeing estimates.

Contribution

It introduces a new correction technique based on image motion correlation to account for wind effects in short exposure measurements.

Findings

Correlation method estimates mean wind speed and coherence time.

The correction improves seeing measurements accuracy.

Theoretical results agree with observational data from Mount Shatdjatmaz.

Abstract

Whole atmosphere seeing \beta_0 is the most important parameter in site testing measurements. Estimation of the seeing from a variance of differential image motion is always biased by a non-zero DIMM exposure, which results in a wind smoothing. In the paper, the wind effects are studied within short exposure approximation, i.e. when the wind shifts turbulence during exposure by distance lesser than device aperture. The method of correction for this effect on the base of image motion correlation between adjacent frames is proposed. It is shown that the correlation can be used for estimation of the mean wind speed V_2 and atmospheric coherence time \tau_0. Total power of longitudinal and transverse image motion is suggested for elimination of dependence on the wind direction. Obtained theoretical results were tested on the data obtained on Mount Shatdjatmaz in 2007--2010 with MASS/DIMM device and good agreement was found.