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ParaCorr on a PortaBall

Started by rhranac, 05/02/2006 04:34PM
Posted 05/02/2006 04:34PM Opening Post
I recently took delivery of a new 12.5-inch PortaBall, and am enjoying the views through this marvelous light bucket. Having been spoiled by the edge-to-edge near-pinpoint stars in my Takahashi TOA-130, I had forgotten that coma is something that happens in fast Newtonians (the PortaBall is f/5).

The coma isn't objectionable by any means, but I thought I'd pursue the ParaCorr route to eliminate most or all of it. What I discovered is the PortaBall doesn't have quite enough in-focus travel to bring images to focus when the ParaCorr is installed in the focuser. It's close, but the Feathertouch bottoms out just before the eyepiece reaches focus.

My question is whether anyone on the forum has tried moving a PortaBall's primary mirror up a slight amount to move the focus point back a bit. I think this will be fairly easy to do, but I'm concerned about proper illumination of the secondary when the primary mirror is moved closer to it--say 1/8 inch or so.

Thoughts?

Ron
Posted 05/02/2006 05:21PM #1
The 1/8 inch will have no impact on the illumination. The
diameter of the beam at the secondary will increase by
0.125/5 = 0.025 inch which will have no practical effects.

Another approach, which I have not tried, but should
work in theory, is to screw the Paracorr to the bottom
of the FeatherTouch. I believe most FT have a 52mm
thread at the bottom end. So you get a 52mm to 48mm
step down ring from a camera store, and a 48mm to T
adapter (part T2-29 I think it is) from Baader or
AlpineAstro, and then screw the Paracorr lens into the
T-thread. And drop eyepiece directly into the
FeatherTouch (no Paracorr tube). You will probably
need to run the focusser the other way now -- out
pretty far -- to reach focus.