What is a DIMM?
The Differential Image Motion Monitor (DIMM) is a proven technique used by observatories worldwide to measure atmospheric seeing — the effect of turbulence in Earth’s atmosphere on astronomical image quality.
The DIMM method works by measuring wavefront slope differences over two small, spaced apertures known as Hartmann holes in the telescope’s entrance pupil. As starlight passes through each aperture, two distinct star images form in the focal plane. By analysing the relative motion between these two images, astronomers can determine the local wavefront tilts caused by atmospheric turbulence.
Since the DIMM is a differential measurement, it is not affected by telescope tracking errors, making it a reliable and widely used tool for characterising site seeing conditions.