Calibration Problems in Solar Speckle Interferometry

摘要

Techniques were developed which allow to estimate the Speckle transfer function (STF) from the observations directly. The Welter and Worden (1978; see also Bruck and Sodin, 1980) technique relies on the perception that the STF may be approximated by the square of the short-term transfer function (Fried, 1966) plus a term proportional to the diffraction-limited transfer function. This technique is not quite satisfactory when seeing is reasonably good, because intermediate spatial frequency regions, which may contribute a significant portion to the power spectrum of solar structure, are not very well corrected. In another approach, the Fried (1966) parameter r sub o is inferred from the data and theoretical STF models such as Korff's (1974) are used to correct Fourier amplitudes. In the technique proposed by Aime et al. (1980), the ratio of the average power spectra from two sets of specklegrams is analyzed in order to obtain r sub o for both sets. This method may pose a problem when a sufficient number of specklegrams cannot be taken within the typical change time of the fine structure. Also, information at high spatial frequencies, where the signal-to-noise ratio may be low, is analyzed. In order to eliminate these problems, I proposed to analyze the ratio of the power spectrum of the average picture and the average power spectrum instead (von der Luhe, 1984). This approach allows to estimate r sub o as well as the signal-to-noise ratio (SNR) of the speckle signal at higher frequencies. In this contribution, I briefly describe the technique and demonstrate its performance.