I've always found interesting reading about the projects some of you were working on : building a pier, a shed or an "observatory"... So I am going to share here the assembly of a "spectro-heliograph" (more on that here : http://www.astrosurf.com/solex/sol-ex-presentation-en.html )

This instrument, placed at the focal point of a refractor (using either a neutral filter at the entry of the refractor, or a Herschel prism at the end, that let ~5 % of the flux going on), allows you to "build" images of the sun at any wavelength your camera can record (H alpha, CaK, Helium etc.). You can also measure the rotation of the sun through Doppler effect (and produce images that shows it), and even measure magnetic field and produce magnetograms of the sun. All of that for the fraction of the cost of dedicated H alpha or CaK filters (but of course, the resolution is different....)

As you can see from the picture, this instrument has a "V" shape :

- entry side (connection to refractor / Herschel prism) you have a tiny slit : 10 microns large and 4 mm height, where the image of the sun is formed. Therefore, only a band of the image of the sun goes through the instrument. You then have to swipe the slit over the sun (slew your mount) to record the full disk, and later reconstruct the full image using this record.

- after the slit, you find a doublet, placed so that the slit is exactly at his focal point, in order to get all the rays to be parallel to the optical axis.

- this beam is then hitting an holographic gratings (2400 lines/mm) to diffract the beam and reflect it toward another doublet :

- the last doublet is the lens that allows the beam to converge toward the sensor of your camera (planetary camera, monochrome).

When all is set and collimated, what you get, at an instant T, is an image where horizontally you have the spatial dimension of the slit, and vertically the spectrum of the slit. Therefore, for a given pixel, its intensity is proportional to the energy received at a given point of the slit for a given wavelength of light (say H alpha).

To rebuild the full image of the sun in a given wavelength, you "just have to" stitch the relevant lines of the images you recorded while sweeping the sun. A free software was written by the person who originated this DIY project.

A kit of the optical pieces is sold by a company called Shelyak Instruments, but it seems you can find other suppliers as well, and the blueprints of the instruments are publicly availables on the link given above (the files for 3D printing).

Now I "just have to" start assembling all the pieces together and see how it works out....

I am going to create a user group of Sol'Ex (a few people here already use one) : for sun lovers, that's a fun project !