Measurements of mirror seeing in the laboratory and at the telescope

Abstract

Abstract We present a new technique for determining mirror seeing: this method is unobtrusive and could be employed on a working telescope. We first describe a series of experiments in the laboratory to determine how the correlation of thermal fluctuations between two points above a warm surface varies with separation. These fluctuations have been measured directly using an array of fast thermistors. By considering the attendant variations in the refractive index of the air, we show how this relation, the structure function, may be used to estimate the Strehl ratio and the point spread function. We have also measured the structure function using a shearing interferometer, and compared the results. The combined data indicate a structure function which is either much flatter than the conventional Kolmogorov 5/3 power law, or indicates that the outer scale of turbulence is much less than the size of the mirror. In both cases this implies a considerable reduction in the importance of mirror seeing. The thermistors were then deployed on the William Herschel Telescope. As in the laboratory, we found that the environment of the telescope had a marked effect. However, while we were at the telescope the temperature of the primary mirror was close to ambient, or slightly lower, which resulted in negligible mirror seeing.