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Old 30-03-2015, 05:09 PM
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MattT
Reflecting on Refracting

MattT is offline
 
Join Date: Jul 2011
Posts: 1,217
I have a 4" f5 Refractor too.
This is what I found with mine.
The R+P it comes with is 120mm long. I replaced that with a GSO Crayford that is 90mm long. The measurment is only the focuser and flange without any adaptors.

With an ES 30 82º eyepiece and 2" mirror diagonal focusing on a tower in the distance the measurment on the focus tube said 55mm, adding a 2x GSO 2" ED Barlow I needed to rack the focuser in toward the objective and focus was there at 27mm out. Did the same with an ES 24mm 68º and got the same amount of in travel needed.

My only other 2x Barlow is an ES telecentric and the focus position stayed the same at 45mm with the 24mm eyepiece, no in/out travel at all.

I presume the f5 achro's were not designed to be used with barlows?

A mirror diagonal will help but I'm not sure it's enough. From memory there is a difference of 9mm between a prism and mirror diagonal. A new focuser will, I got mine from Andrews Commiunications, as will using an 2x ES Focal Extender. Myastro shop sell ES FE's.

Matt
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