DIMM - Differential Image Motion Monitor

The DIMM method consists of measuring wavefront slope differences over two small pupils some distance apart. Because it is a differential method, the technique is inherently insensitive to tracking errors. It had already been used as early as 1960 to provide qualitative seeing estimates. The theory of differential measurements is now well understood. It shows that the differential motion exceeds the absolute motion as soon as the distance between the two apertures equals a few times their diameter. Hence a compact instrument can be built and still provide good sensitivity. One needs to select the starlight having passed through two circular apertures in the entrance pupil plane (Hartmann holes) so as to obtain dual star images whose relative motion in the image plane represent local wavefront tilts. This is shown in figure 3.2. The theory is reviewed below, ESO and Hartmann seeing monitors are described.

Figure 3.2: Visualisation Of The DIMM Method