DIMMWIT measurements of the spatial and temporal scale of atmospheric turbulence at COAST and MROI

Abstract

The DIMMWIT (Differential Image Motion Monitor, Which Is Transportable) is a portable DIMM that can measure the Fried parameter r₀ and the average wind speed of the turbulent layers. Analysing DIMM images to calculate r₀ is a standard procedure, but wind speeds have rarely been calculated from differential image motion before. Here, we describe how wind speeds can be derived from either differential image motion power spectra or differential image velocities. The DIMMWIT wind speeds are then compared with a wind speed derived from the coherence times, t₀, of interferometric fringes recorded simultaneously at COAST (Cambridge Optical Aperture Synthesis Telescope). Although t₀, and hence the wind speed, is routinely measured by the interferometer at the COAST site, the Fried parameter had not been studied. The results of seeing campaigns at COAST and MROI (Magdalena Ridge Observatory Interferometer) are presented, along with a comparison of DIMMWIT r₀ measurements with the FWHM of long exposure images recorded at the same time.