The three-Subaperture Seeing Monitor at Yunnan Observatory

In this paper, the Three-subaperture Differential Image Motion Monitor is described. This instrument has been used in the site-testing of Gaomeigu Station at Yunnan. As a paper on the designing of an instrument, it goes as something of mixture, so we divided it into several parts: 1. The evolvement of the seeing-testing inst rument 2. The designing of the Three-subaperture DIMM at Yunnan Observatory. 3. The calibratio n and formally using of the Three-subaperture DIMM.