A Design of a DIMM System for Long-Term Seeing Measurement

The Fried (or seeing) parameter r₀ is very important in modern site surveys, and a DIMM (Differential-Image Motion Monitor) is a general-purpose equipment to measure r₀. We introduce a design of a remote-control DIMM system in detail, including target-star pointing, automatic guiding, automatic measurement, data storage, and etc. Employing a DIMM system to monitor long-term seeing will be helpful to find the intrinsic site seeing for a checked or candidate site, and to evaluate the imaging quality. We use a 12″ telescope—Meade LX200GPS for measuring the atmospheric seeing. With conventional methods we would have to manually adjust the telescope pointing for tracking the target during the measurement, as the tracking accuracy of the small telescope is not excellent. However, we often need to observe a bright star for several hours for r₀ and to keep the target in the field of view of the CCD. We adopt a more effective method, i.e. to install a guide camera for the telescope. For our remote DIMM system we choose the LPI as the guide camera (instead of DSI or others) and the Guider Master as our guiding software, which leads to a very good performance. Through the Remote Desktop Protocol or VNC, the system acquires a remote observation mode, which brings about many advantages for long-term seeing measurement. In addition, the system is capable of real time processing of images, which are recorded at a rate of 250 frames per 10 seconds with the exposure time of 8 miliseconds per frame. In this way we can measure nearly six sets of r₀ values per minute. In the night of April 9, 2011 we measured the seeing at the Phoenix Mountain, Kunming using our DIMM system, and obtained the r₀ curve from 20∶50 to 2∶30. We also obtained the FWHM values of stellar images (for targets not identical to those for the DIMM) of the 1m telescope of the Yunnan Observatory. We find a good correlation between the seeing values measured by the DIMM and the FWHM values measured by the 1m telescope. This indicates that our DIMM system is a successful design for long-term seeing measurement.