KenMerlin66 wrote:Bob,
I re-read your CN posting but still have problems with your 'double edge' = continuum leakage.
The spicule edge region is visible on band and off band....at 0.3 FWHM even with the "tails" you are still working within the wings of the Ha....
OK
My real issue is when you use a spectroheliograph (SHG) to generate a solar spectrum and then extract a strip from the absorption line the bandwidth of which will depend on the dispersion of the spectrograph and the PSF of the instrument. I'd like to think, that at least in theory, the extraction say of a 0.3A wide strip (based on a dispersion of >0.05 A/pixel and a measured resolution (using a neon reference line) of <0.1A) would NOT give the same "leakage" results of the etalons etc. but an almost "clean" 0.3A image with NO side band interference.
I have had images submitted for the new book on the digital SHG which don't show a double limb (but plenty of spicules!!).
Any comment?
Bob's explanation is quite complete, but one detail is missing. Ironically, the image that started the whole double limb controversy in 1963 came from a spectroheliograph, the one at the Mount Wilson 60 foot tower telescope. This instrument was considered to have extremely low stray light, but the test used to determine this was flawed in an important way. The Mount Wilson astronomers used a mercury arc and looked for Rowland ghosts and other features. The mercury arc was primarily a line emitter, and the level of stray light from the lines was too low to see. On the other hand, the solar continuum provided plenty of off band light, and what was considered a high quality grating in 1915 doesn't compare with what was available even a short time later. Thus, the scattered continuum light formed a ghost image on top of the Ha image. The double limb seen in etalon filters and Lyot filters have mixed sources (sidebands and wing effects) but the effect is the same. Modern gratings are much better than the early ones, and much cheaper. With a modern grating and care in keeping the post-slit optics clean a spectroheliograph today will provide the spectrally purest images available without going to a six figure investment.