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RDF problems

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Dear Members,

ive been using an C925 evolution for 8 months or so. It has a 1.25 diagonal. All works well Rdf aligned spot on.

i recently bought an ES102CF apo triplet. It has a 2 in diagonal.

so I bought a 2in visual back for the sct so I could interchange the 2in diagonal. But the Rdf is now out of alignment. It is so far off that there is not enough adjustment to get it aligned on either axis.

Amy ideas of what I'm doing wrong or what the problem might be?



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Sounds to me like you've changed the focal-plane of the scope. I'm presuming the new visual-back and jump to 2" has added length to the scope. So you'd need to add length inside the scope to reach the same point where it was set before. That's not really possible. But this is just a guess. However this may help you.....

You could ask about this in the Yahoo Astronomy Groups. Here's a listing of telescope groups geared to Celestron:


And this one shows recent activity:


And telescopes in general:


And for SCT's in particular, there's always Doc Clay Sherrod. He's a Guru of All-Things-Cassegrain, and he maintains a website with forums for questions and ideas, as well as literature on everything from cleaning to tweaking the drives which is unobtainable anywhere else. Joining is free and easy. Just some questions to weed out the advertisers and robots:


Good luck!



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Good tips from Dave.

I'm probably out of my depth here, but can only say I have a Cel. 8SE.  I simply took off the VB and put the 2 inch diag. in its place, no problem.

I have a RACI finderscope mounted on the tube, and the RDF is secured to the arm of that finder.  Both of these devices are aligned with the 8SE.

One thing I might suggest - if you can't align your RDF  at all with the screws/knobs, try and improvise some way of giving it an angular boost in whichever plane it most needs it.  Just a thought - worth a try - after all, my RDF is way off from its original position now!


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People often need to shim one end or the other of the RDF to get them to align, actually reasonably common. Cannot help on the thickness of the shim or the placement.

I often wonder if this is a case that the popular RDF is the WO item, or clone of it, so it fits a WO scope well but not the others. Sort of thing that arrives when manufacturers take a mass produced item that although good was intended for a slightly different scope. Just a thought

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Thanks everybody for your suggestions, it's very strange as 1.25 is fine. I'm going to order the ultra short 2in visual back from Baader and see what happens. I suspect as has been suggested that the generic cheap 2 in adapter may too long or at fault.



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