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Ab Umbra Lumen

Diagonals, visual backs and clearance

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Having an EVO 9.25 I am thinking of one classic upgrade path, a 2" diagonal, which would also help to share eyepieces with the planned buy of a big dob. I have read plenty of existing discussions, but am still lacking enough information on the right one to choose.

In particular, the missing point is about clearance and combinations: I have read a short diagonal that fits directly in the SCT thread (like WO) should allow for clearance (pushing up the OTA), therefore allowing the scope to work at its full potential. My doubts are:
1) does getting rid of the visual back cause big problems? I have read it makes hard to block the diagonal at the right position, and that it can make the diagonal dangerously swivel
2) if there are indeed problems, and keeping a visual back (like 2" Baader's click-lock, for instance) is better, wouldn't this prevent clearance?
3) I am tempted by the 2" baader click-lock dieletric for its no screws capablity: is there a combination that allows clearance for this diagonal?

Anybody help please?

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Ab, I am not sure about any of the above as the kit I bought for my old SCT screwed on just the same as the visual back. One point to note is whatever you decide make sure the length is compatible with the distance between the back of the OTA and the case of the mount when the scope is pointing at the Zenith as you may get a foul condition when everything is attached.

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2" mirror diagonal is still longer than 1.25" stock sCT fit diagonal, therefore longer light path and some slight icnrease of spherical aberration.

I've done some tests about impact on focal length of SCT of different visual backs, some pictures may answer parts of your questions.


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Hi Ab and welcome to the forum. :)

A 2" diagonal provides a much more stable fitting for your eyepieces/cameras/filter wheels etc than the smaller 1.25". I found the "swivel" problem was more with the smaller diagonal cos it's hard to get your hand in there to tighten it enough. You can get much more purchase on a larger piece. Just check the clearance on your mount when pointing at the zenith - I'm sure your 925 should be fine. You'll need to get one with an Sct threaded adaptor to attach to the visual back - or a Baader click lock and a normal 2" diagonal. Hth :)

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Click lock is certainly a great option, very secure.

Looks like you need this part:


plus ideally the 2" clicklock


Or actually I found this instruction which shows you can attach the 2" clicklock directly using a Baader locking ring.



This way will give the shortest light path and least obstruction at the zenith.

EDIT I see Yong lists the direct connection in his first configuration, although that was done with the Zeiss Prism, not the Baader Clicklock. 

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Thanks to everyone for the informative replies.

So summing up:
* the 2" diagonal does not suffer from "swivel"/stability issues, in fact it is more robust wrt eyepieces
* clearance should not be much of a problem, at least using configurations *without* the visual back...
*... which in turn, also improve on the focal lenght problems a 2" diagonal would bring (thanks Yong for the great thread, very useful and full of interesting data (for binoviewing too)
* regarding the Baader diagonal, in view of the above, the good news is that one can still avoid the visual back using their locking ring, so turning the 2" diagonal into an SCT-friendly one

I think all the doubts have been settled, thanks again!

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Thanks Dave: in fact I've seen Wiilliam Optics also does a rotolock version, which is similar to the Baader's and likely with less problems wrt visual backs, alas it looks to be hard to impossible to find (at least in Italy or even in the UK). Shopping from the US shouldn't be much of a problem in this modern world, but then there are extra import taxes (extra-EU) to be paid, and the procedure and costs (at least in Italy) are so heavy that people usually avoid it unless there is really no other option.

It is a strange world, where we are connected by the web, and disconnected by "protection" (sigh) taxes :-/

Then, of course, a vacation trip to Vermont would solve any problem ;-)


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