C-DIMM : an autonomous, outdoor and fixed seeing monitor for astronomy, atmospheric studies and free space optical communications

Abstract

Many applications such as ground to satellite optical communications or astronomy require precise knowledge of cloud cover, turbulence and absorption. In the case of telecoms, this data is critical for the initial ground station sites survey; during ground station operation to inform link availability and bandwidth; and finally, to predict atmospheric conditions over different ground stations for network planning. Historically the turbulence by night time has been measured by astronomers with research class solutions installed on observatories sites. Many implementations exist using either the moon or the stars as reference target. One of them is the Differential Image Motion Monitor (DIMM) from M. Sarazin and F. Roddier with the first implementations back in the 80’s for the ESO. All these turbulence monitors have in common the integration of a small telescope in the 20 to 40cm aperture range with various aperture masks on an automatic tracking mount within a protective dome. This form factor and cost is not in line with the requirement of a more industrial utilization as expected by telecom operators or for atmospheric studies. Since 2018, Miratlas has been using a simpler implementation of the image motion monitor (NSM) with a fixed outdoor system using a single aperture aiming at Polaris. Nevertheless this single aperture system requires a very stable fixture which is not always available and doesn’t apply in southern hemisphere. Therefore, Miratlas has developed a small outdoor implementation of a legacy DIMM named the C(ompact)-DIMM. It uses two different optical assemblies and two identical synchronized cameras to fulfil the same features. The C-DIMM is small enough to be installed anywhere, is not sensitive to vibration and therefore can be installed either on a fixed mount aiming at Polaris, or on a small outdoor tracking mount to operate on any sufficiently bright star and therefore under any latitude.