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Any benefit in changing to a 2" dielectric diagonal


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Hi, I have an 8" celestron sct. So far the views have been good, however is there any benefit to moving up to a 2" dielectric diagonal and will this still fit directly into the back of my OTA?

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If you only have a 1.25" Diagonal at the moment and you have 2" EP's then the answer is obvious. If you have a 2" diagonal at the moment then the answer would be no unless the mirror diagonal you have is corroded.

HTH

SPACEBOY

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The larger diagonal is only needed when using 2 inch eyepieces. Otherwise it has no effect.

There is a lot to be said for a two inch diagonal with widefield EPs in an SCT because otherwise the long focal length restricts your field of view at low powers.

Olly

Edited by ollypenrice
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You can get an SCT 2" diagonal that simply screws directly onto the back of the OTA or buy an inexpensive SCT to 2" visual back that screws onto the OTA and then you can use any regular push-fit diagonal. The later also means you can use the diagonal on a refractor as well.

John

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Read a report a while back from a well known pro who said the way the 2" ones are made means the center part is bigger and clearer/better quality and benefits 1.25" users.

I read that as well, the theory being that the optical quality of a mirror or coatings degrade a bit towards the edges so with a 2" diagonal you tend to be using the central, and, if the theory holds, better optical quality, area of the mirror.

In practice though, I can't say I've noticed any difference between a good quality 1.25" diagonal and an equivalent quality 2" one. In fact I've struggled at times to see much of a performance difference between low cost diagonals and much more expensive ones :)

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I can see the reasoning between a 1.25" and a 2" and if you are in the market for one you might just want to go for the higher reflectivity and better surface accuracy* of the dielectric anyway but I wouldn't upgrade for the dielectric coatings alone (I'm quite happy with my Meade 2" UHTC).

*Dielectric is, apparently, easier to apply in a more uniform fashion giving, I have read, a better surface accuracy - hence why even the cheaper ones can quote 1/10th pv.

Edited by haitch
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One really good reason to upgrade from the standard diagonal that comes with most scopes is the mechanical build quality. I wouldn't want to put an expensive eyepiece in a cheap diagonal that may not clamp it in place properly (the sound of expensive glass hitting the ground is not a good thing :)) or a nose-piece that can come loose and let the diagonal rotate.

John

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