Seeing measurements using the solar limb - I. Comparison of evaluation methods for the Differential Image Motion Monitor

Abstract

Differential Image Motion Monitors (DIMMs) are used not only for stellar observations but also for solar observations with the limb as the target to evaluate the seeing. In stellar observations, the differential image motions along and perpendicular to the DIMM baseline are consistently employed to evaluate Fried’s parameter. However this is not the case for solar-limb observations, in which we can measure the image motion only perpendicular to the limb. Therefore the validity of the methods so far proposed to calculate Fried’s parameter from a DIMM using the solar limb is still open to examination. We have focused on this problem and carried out DIMM observations of the solar limb using a Shack–Hartmann wavefront sensor of the Domeless Solar Telescope at Hida Observatory. Pairs of apertures in the Shack–Hartmann sensor act as multiple DIMMs that have different baselines in distance and orientation. We calculated Fried’s parameters based on three evaluating methods (those of Sarazin & Roddier, Sasiela and Conan et al.) and found that the method of Sarazin & Roddier gives a consistent value of Fried’s parameter over the set of multiple DIMMs.