Re: sun's spectrum

[Gary Karshner <karshner@STMARYTX.EDU>]

Paul,
you wrote:
> If we don't see emission lines in the Sun's spectrum then what does 
> an H-alpha filter do? There has to be some sort of emission in order 
> for the band pass filter to pass it through to your telescope.

        An absorption line is does contain some photons, most are scattered
out. A narrow band H-alpha filter passes just those photons from the center
of the absorption band. It allows one to see changes in the excitation of
the solar surface (plages, and flares) as well as clouds of hydrogen
floating above the solar surface (filaments) that further absorb the
radiation. If the filter is narrow enough it can be "tuned" by changing its
temperature to see hydrogen that is rising from the surface that is slightly
Doppler shifted to the blue, for example. 
        There are also wide band filters, but they are not useful for
observing the sun except in eclipse. They are useful for seeing hydrogen in
nebulae and other gas clouds in space, where ionized hydrogen is recombining
and emitting photons in the process.
        I hope this helps

Gary


  
Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU