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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 11:26 AM
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Hi Dylan.
I think you'll see the best image with the Mak, when you don't use any diagonal. A friend of mine tried 1.25" dielectric and prism diagonals with his 6" SCT recently. He then looked through the scope without a diagonal. Target was Jupiter (thankfully quite low in the sky-- no neck straining required). The view "straight through" was contrastier. Of course, taking the diagonal out of the optical system will result in a somewhat shorter effective focal length if the scope uses a moving primary mirror to focus. The shorter e.f.l will lower the magnification a bit, which naturally results in a brighter, contrastier appearing image.
His opinion about the prism vs. mirror in the SCT:
The mirror (dielectric 99% reflectivity) looked brighter but the prism appeared a little bit contrastier. Best view was "straight thru".
FWIW, Questar Mak-Cassegrain telescopes have always used a built-in star diagonal prism, so that company seems to think they're worthwhile.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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