Abstract
The frequency analysis of image motion (IM) at the solar limb was carried out in the frequency range from 0.5 to 50 Hz using a photoelectric equipment. For a telescopic aperture of 35 cm and a bandwidth of 0.65 Hz a typical frequency spectrum under average observing conditions shows a decrease of amplitude from 2 arcsec at 0.5 Hz to 0.4 arcsec at 5 Hz, 0.03 arsec at 50 Hz (and < 0.01 arcsec at 500 Hz). Visually estimated values of image steadiness seem to be in better agreement with the r.m.s. value of image motion (scattering parameter σ) than with the amplitude at a certain frequency (Figures 5a, b). The influence of IM on the quality of photographic pictures or on spectra of solar fine structures is calculated as a function of exposure time. Table II gives the IM scattering parameters σ (0.01 arcsec to 4 arcsec) calculated for exposure times from 0.001 to 0.5 sec — valid for a time average. The modulation transfer functions (MTF, one-dimensional) derived from the IM scattering parameters are presented in Figure 7 together with the MTF for a diffraction-limited telescope of 35 cm aperture at λ 6000 A. Exposure times of less than approximately 0.01 sec (certain within a factor of 2) render the influence of IM negligible compared to the MTF of the objective used for this investigation.
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