The instrumental profile of a Lyot-filter with 0.5 A, pass-band for Ha has been determined from photographic spectra obtained with a large grating spectrograph and a continuous light source.
After line adjustment of the filter, the transmission profile in parallel light agrees closely with the theoretical profile as given by crystal optics. The peak transmission has been found to bc 12% of the incident unpolarized radiation.
Convolving the measured instrumental profile with the average solar spectral distribution near H~, it can be shown that the parasitic light (transmission outside the main pass-band) contributes I 0 % to the total light flux with the filter centered on the line. At wavelength shifts of 0.5 and 0.8 A, the parasitic light contribution is 25 % and 40~ respectively.
The deformation of the line-profile of various solar phenomena (faculae, filaments, flares) by the instrumental profile is discussed quantitativcly. * Mitteilungen aus dem Fraunhofcr lnstitut Nr. 82. Solar Physics 5 (1968) 462-470; ~5 D. Reidel Publishing (.bmpany, Dordrecht-Holland * h(2)= intensity distribution of chromosph, phenomenon (ELLISON, 1952). 12 (2)-intensity distribution of quiet chromosphere. ** limits of integration are Ha --5 H and Ha + 5 H.