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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
Loc: Oort Cloud
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Re: Prism Diagonal
08/29/08 08:53 AM
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Hi Kenneth 'n' all.
An achromat might benefit a bit from a prism diagonal.
I believe Jim Burnett tried one in his long focus 4" achro and liked the result.
An achro (even an f/15 one) has quite a bit of c.a., compared to a good ED doublet (even a rather fast one). So, I doubt a prism will make a huge difference in the view... but for the price of trying one out, I'd say it's worth experimenting.
For a 100mm f/6 achro, well... it has much more c.a. and the subtle assistance a prism provides would get largely overwhelmed by the objective's spurious color.
I know how well a prism works in my F/5.95 110mm scope, so other ED's of smaller aperture in the F/6 range shouldn't present a problem using one.
IIRC, prisms really started to fall out of favor when folks with SCT's began using f/5 telecompressors to make them into "RFT" systems. Then, along came TeleVue with their F/5 (or thereabouts) Petzval refractors in the mid to late 1980's.
Using a prism with an F/5 probably isn't a good thing. Too much spurious color and spherical aberration gets added.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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