The role of telescopic stray light in limb-darkening scans obtained in April 1981 (and later)

Abstract

The suspicion of Elste (1990), that telescopic stray light together with imperfect collimation of telescope and spectrograph could be a possible explanation for the systematic differences and variations found by Neckel and Labs (1987) in many limb-darkening scans, proves to be unfounded for the following reasons: (1)The collimation was performed very precisely; (2) the telescope mirrors remained fixed in position and direction during most of the observing period; (3) stray light effects depending on hour angle were not detectable; (4) in the same collimation status, also many almost symmetric scans had been recorded; (5) the observed east-west differences in the solar intensities are partly even larger than the total amount of stray light (from telescope and sky!) observed as ‘sky’-background just outside the limb; (6) any east-west differences in the ‘sky’-background near the limb are just a few 0.01% of the disk center intensity; (7) the differences of the average intensities along eastern and western radius appear to be correlated with the east-west differences of the intensity's R.M.S.