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Re: (IAAC) Cats's eye nebula



Lew
>Thanks for another set of intriguing observations, NM! I've been meaning to
>get my hands on a prism to try out on planetary nebulae. What are the specs
>on yours, and where did you obtain it?
I got the direct vision prism, a five prism version with 1 cm. square faces,
some thirty years ago from I cannot remember where but I suspect someone who
supplies educational opticaL equipment may sell them. Have you tried Edmund
Scientific? I have a catalog but having just retired (early)I don't know
where it is.
Diffraction gratings are no use for this as they will loose too much light.
Single prisms shouldn't be hard to find from Edmunds or from Government
surplus. 
You can get 60 degree or 45 degree ones. I've got a 45 degree one,  with
roughly one inch square faces, from a 7X50 monocular gunsight I very
stupidly demolished in my younger days. (It gave the sharpest star images I
have seen from any optical equipment but was very heavy.) 
The only difficulty with single prisms is getting your eye in the right
place but it only takes a bit of practice to find the right position for eye
and prism to allow for the angular deviation. The single prism is an
advantage in some cases as the lower dispersion gives brighrter images
useful at the red enmd of the spectrum.
At present I am just holding the prism up to the eyepiece but it should be
possible for someone a bit handy to make a holder to fit over the eyepiece
tube. Sadly I'm not very handy like that but if you come up with a simple to
make design please let me know.
Hope this is useful
Clear skies.
Nick
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|   nmartin@bonnyton.u-net.com    |
|"There are more things in heaven and earth ... than are dreamt of in your
philosophy"   (Hamlet, Act 1, Sc. 5)  |
Bonnyton House, Ayrshire,Scotland, UK. lat 55 24'56" N long 4 26' 00" W
Altitude 150 m.
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