DIMM with large aperture separation and simultaneous seeing measurements at Mauna Kea

We developed two DIMMs (Differential Image Motion Monitor) for simultaneous seeing measurements at multiple sites. Simultaneous seeing measurements enable us to distinguish temporal variation and site to site variation of seeing, and clarify which site has better seeing. We set the aperture separation of the DIMM at 50 cm for accurate measurements under good (~0.3 arcsec) seeing conditions for Mauna Kea. Our system can also be tuned for moderate to bad seeing conditions by changing the effective focal length of the optics. The frame of our DIMM device is made of CFRP in order to avoid deformation with temperature change and to reduce its weight. We will present the details of our DIMM system and some results of simultaneous seeing measurements at Mauna Kea, Hawaii.