The Principle and Error Analysis of the DIMM with Three Sub—apertures

The differential image motion monitor (DIMM)with three sub—apertures has been constructed at Yunnan Observatory. The DIMM will be used for site testing of optical/infrared large telescope.#br#The theory of the DIMM is reviewed. Five types of erroes from the instrument noise, the statistial errors, the diameter of sub-apertures,sub-aperture orientations and focusing, for the wavelength variation in the DIMM measurements are discussed.