A portable S-DIMM developed for preliminary measurement of seeing for Solar Site Survey: Theory and one case study

Imaging quality is highly important for astronomical observations. The imaging quality of a telescope depends not only on the capabilities of the telescope itself but also on its location. When distant stars pass the atmosphere, due to the presence of atmospheric turbulence, the imaging quality becomes degraded. The atmospheric coherence length is the most commonly used parameter to evaluate the influence of the local atmospheric turbulence on telescopes. It is used to describe the degree of atmospheric attenuation of image quality. For the preliminary estimation of atmospheric daytime seeing from a lot of candidate sites, we have developed a portable S-DIMM (PS-DIMM),which is mainly composed of a light-duty tracking mount, a 9 cm aperture lens barrel and a Canon 60D SLR camera. With the PS-DIMM the daytime seeing of a site can be well estimated by measuring the relative displacement variance of the edge positions at both ends of the Sun’s diameter. The principle of PS-DIMM is introduced in this paper. The simultaneous seeing measurements by it and the traditional S-DIMM were carried out at the Fuxian Lake Solar Station, showing a very well correlated between them but with an offset of 2.6 cm. In contrast to the traditional S-DIMM, PS-DIMM offers the advantages of reduced weight and enhanced portability. The utilization of PS-DIMM has played an important role in the early stages of the site survey for the next-generation solar telescopes.