The Differential Image Motion Monitor with Three Sub-aperture at Yunnan Observatory

The Differential Image Motion Monitor (DIMM) with three sub-aperture was developed at Yunnan Observatory. The DIMM is attached to a 35cm telescope with f/15. The apertures are 23cm or 16.3cm apart and have a diameter of 10cm. Two 20" prism wedges are positioned over two of the three sub-apertures to separate two images from the other one by 10.4" at the focal plane. The monitor was first tasted the seeing near the dome of 1m telescope at Yunnan Observatory on Apr.8, 1994, and since then 1770 ro at 16 nights have been recorded. The average ro measured is 12cm, and has good correlation with diffraction ring observation. On May 24 and 25, 1994, Full Width at Half Maximum (FWHM) of stellar images was measured on 20s' exposure time in the direct CCD imaging mode with the 1m telescope. The mean FWHM of the images is 1.15", and the seeing measured by DIMM is 0.906".