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[Login] Entrance · Main Index · Search · New user · Who's Online FAQ · Calendar Announcements and News >> Discussion of CN Articles, Reviews, and Reports Previous   Index   Next   Threaded   Pages: 1 | 2 | 3 | (show all) rustynailz911 super member Reged: 01/30/08 Posts: 177 Loc: Nova Scotia Re: Prism Diagonal [Re: Clive Gibbons]       #2626362 - 09/05/08 09:33 PM Edit   Reply   Quote Quick Reply   i have budget equipment and have a 45deg Meade diagonal with a prism. i had some serious ghosting as well i was thinking it was the MA eyepieces i removed the diagonal and went just eyepiece perfect focus and no ghosting so it was the diagonal.i just ordered a 90 deg mirror diagonal.im not saying that prisms are bad they just didnt work for me it might work awsome in another scope .plus i really disliked the 45 deg angle it is uncomfortable on my scope for viewing. Rusty Post Extras:       Clive Gibbons Mostly Harmless Reged: 05/26/05 Posts: 10200 Loc: Oort Cloud Re: Prism Diagonal [Re: rustynailz911]       #2630375 - 09/08/08 08:49 AM Edit   Reply   Quote Quick Reply   Just to reiterate... a 45-degree erecting prism diagonal is an entirely different animal, compared to a 90-degree star diagonal prism. If you had a poor experience with an erecting prism (either 45 degree or 90 degree type), I'd strongly recommend that you try the simpler, traditional star diagonal prism. Much less chance of introducing stray reflections and other optical errors with that type. -------------------- A few telescopes of dubious value. Understanding wife and three curious cats. "Semper ubi sub ubi" Post Extras:       Bradley B sage Reged: 05/14/07 Posts: 341 Loc: Sacramento, California Re: Prism Diagonal [Re: Clive Gibbons]       #2630547 - 09/08/08 10:57 AM Edit   Reply   Quote Quick Reply   That old Burgess 2" Diagonal was of the "correct image" variety, so no wonder there may be issues of collimation . . . however 8mm is extreme. -------------------- Vixen NA140SSf Pentax 105SD Meade 12" LightBridge Takahashi 7X50IL "Astronomer" Binoculars * * * * * * * * * It is error only, and not truth, that shrinks from inquiry. -Thomas Paine Post Extras:       Clive Gibbons Mostly Harmless Reged: 05/26/05 Posts: 10200 Loc: Oort Cloud Re: Prism Diagonal [Re: asaint]       #2669604 - 09/29/08 09:34 AM Edit   Reply   Quote Quick Reply   Further testing with ED doublets has shown that star diagonal prisms using BK7 glass typically have a greater benefit with regards to improving color correction in many ED doublet refractors. It's usually the less expensive star diagonal prisms which use BK7 glass. More expensive prisms, such as the Baader T-2 model (Zeiss) and the Takahashi 1.25" star diagonal, use a higher index glass (BAK-4, or analogous). This results in more complete "total internal reflection" when used in a very fast optical system (f/5 or faster). However, since most refractors aren't of such a short f/ratio, it's not necessary to use a high-index prism. BK7 glass will work just fine. But, MOST IMPORTANTLY, the high index glass in more expensive prisms doesn't correct the prevalent color error (red defocus) which so many ED doublets display. In conclusion, it would appear that the optimal type of prism for use in many of today's ED doublet refractors, is one using BK7 glass. Inexpensive 1.25" star diagonal prisms by Celestron, Meade and Orion all use BK7 glass. -------------------- A few telescopes of dubious value. Understanding wife and three curious cats. "Semper ubi sub ubi" Post Extras:       Pages: 1 | 2 | 3 | (show all) Previous   Index   Next   Threaded   Extra information 1 registered and 8 anonymous users are browsing this forum. 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