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I would welcome a on going discussion on SHS and the modern take off Spectroheliograph (SHG) that uses a web cam to collect the slit images and add them together to make a 2D image. Both a SHS and SHG have the advantage that one can get to band width of 0.4A or lower and can observe the Sun in any wavelength you wish.
I have compact SHS about 90% finished. One can make one for around $400 or less depending on how good you are at collecting parts.
Here is a quick review of how a SHS works. There are three basic parts 1) a telescope to form the image of a the Sun. A long focus 60mm to 70mm refractor is ideal. So 60x910mm refractor and 2x barlow would work well. I'm using 70mm x 910mm refractor with 2x barlow I got off on Ebay $35. 2) a long focus spectroscope. There are many different styles of spectroscope. I'm using a Littrow type which is made from a 50mm achromat with 1018mm focal length and 1200 lpi grating blazed at 500nm. The white light image of the Sun is focused on the entrance slit of the spectroscope. The spectroscope allows one to view the spectrum of the section of the Sun that is over the entrance slit. A second slit placed over the exit of the spectroscope narrows down the image you can view and defines the bandwidth of the light. The narrower the slit, the tighter the bandwidth. You center up the H-alpha in the exit slit by turning the diffraction grating and narrow the slit to the bandwidth you want. All one would see at this point is dark H-alpha line and as you moved the Sun over the entrance slit, any place on the surface that there was a H-alpha emmission feature, there would be a bright spot. If you looked at the limb, you can widen the exit slit and see prominences. 3) To see the whole disk or part of the disk in 2D you need the third part of the SHS, the image sythesizer. What this is a way of scanning the disk of the Sun over the entrance slit of the spectroscope quickly and taking the slit image produced at the exit slit and stacking them next to each other. It sort of works like a scanning in CRT.
You can have two rotating prism, moving slits, or oscillating mirrors. They all do the same thing. They move the image of the Sun quickly over the entrance slit and then scan the images of the monochrome line images in the eyepiece. It sort of works like a movie projector. So when the top of the Sun is projected onto the entrance slit, there is slit image in monochrome light projected at the top of image in the eyepiece. Then the next image a little farther down, etc, etc. Because of the persistance of vision one sees a 2D image.

- Dave

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