Solar limb-darkening

Abstract

A series of Solar limb-darkening drift measurements in 13 wavelengths between 341mµ and 598mµ selected by second-order intference filters, and obtained using a 10-stage photomultiplier as detector, is described. The emphasis was on the last few per cent of the radius of the Solar disc, as suggested by the theory of Bohm. A slit was used, curved to match the preceding Solar limb. The image was focussed to within .1% of the focal length. The limb-darkening curves were displayed on the screen of an oscilloscope, thus freeing them from inertia-distortion, and recorded photographically. They were corrected for the effects of seeing-scattering using David and Elste's theory in a modified form. The results are tabulated, and the heliographic latitudes involved in each set of data given. The method is demonstrated to be completely satisfactory for wavelengths above 380mµ, but the results are weakened in the region of the Balmer discontinuity by convolutions of the true curves with the wide (20mµ) profiles of the interference filters. The author stresses the importance of accurately focussing any optical system used in limb-darkening measurements, and derives a criterion for tolerable defect-of-focus in terms on information theory, thus linking this with the contribution of Bohm. A review of the literature relating to limb-darkening experiments is given. The author suggests as the main reasons for the discrepancies between the data of different investigators: (1) the lack of attention to accurately focussing the image used, and (2) the absence of an internationally accepted definition of the limb itself.