Stray light determination in small sunspots

For the correct analysis of spectral line profiles arising from the spot, the fraction α of photospheric stray light has to be determined either by an approximate formula derived by Maltby, or by the method introduced by Zwaan and Staveland. The second method needs a determination of several blurring and scattering parameters. In the case of the observations made by a new horizontal solar telescope spectrograph in Ondrejov, the amount of scattering is negligible. The blurring parameters can be determined from an observed photometric profile of the solar limb. Such a profile is the convolution of a real limb profile and an appropriate spread function, i.e. weighted sum of two or more Gaussian profiles with various parameters bsub(i). According to the preliminary measurements of 12 sunspot spectra, the values of α, calculated by the above method, are on an average lower by a factor 0.85 as compared to those obtained from Maltby's approximate formula. (author)