[Wavytone]

I made something like this in the 70's, but I used a 4" f/10 mirror tilted off-axis to reflect the image off two glass wedges in an arrangement similar to a 3 mirror schiefspielger (though in my case the two wedges were flat mirrors, not curved). The two wedges were originally prisms extracted from some surplus optical junk I scored in a disposal store.

FWIW a f/10 mirror is effectively spherical at apertures up to and including 25cm, you won't be able to tell the difference.

It takes preferably 4 reflections off plain (uncoated) glass to reduce the brightness. The light from 3 uncoated reflections will still be painfully bright and probably damaging to your eyes.

Regarding the mirrors, to get maximum contrast the glass each mirror is made of needs to be fairly clear and the back surface should be polished clear- put a black matte surface behind it to absorb the light.

The reason for mentioning this is that many cheap 4" mirrors are made from glass which is less than clear, or have a fairly rough ground back that will create a fairly bright background scatter, so your view of the sun will be against a rather bright background of scattered light. An alternative is to grind and polish your own from a thick piece of plate glass and a mirror grinding kit - a 4" f/10 can be done in under 20 hours hard labour.

Then there is the problem of the star diagonal - most are cut from a plane-parallel piece of glass., though the backside won't be optically flat. If you strip the aluminium off the front surface you will get an annoying second reflection from the back surface, which will degrade your image quite badly.

A better solution for the diagonal mirror would be to use the hypotenuse (the long side) of a 45-45-90 degree prism, this way the unwanted 90% will go through the back of the prism or be bounced around through odd angles - you won't get a second image at the focal plane. For a prism, you could cannibalise an old star diagonal or a small pair of binoculars. Some IIS members may even have a spare old one lying around. Another prism would work nicely in place of the Herschel wedge too.


Have fun, but be careful.