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Can't reach focus with a 2" mirror diagonal.

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#1 Guest_**DONOTDELETE**_*

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Posted 23 September 2003 - 11:26 AM

I've got a refractor project going. 83mm x 900mm f11. I started with an old Meade 310 (by Vixen) refractor with a damaged lens. I replaced and added additional knife edge baffles, ultra flat blackened the tube interior, installed a new Japanese muticoated achromat and installed a finely machined and baffled 2", black anodized aluminum, rack & pinion focuser.

I now have a very high contrast, tack sharp, image and chromatic aberration is almost invisable. You have to look really hard to find any! This is what I was after; to push this design to it's theoretical limits. Now, I would dare to put this "project" scope up against anything in it's class and nearly identical to high quality semi-APOs.......but!?

Using my Panoptic and Nagler 6 lenses, I can reach focus with a Lumicon 1.25 mirror diagonal but not with a Meade 2" mirror diagonal.

Assuming the draw tube starts out fully retracted, the 1.25 mirror requires about 2 1/2" of back travel to achieve focus but the draw tube won't travel "in" far enough to achieve focus with the Meade 2" mirror.

With the draw tube fully retracted, the distance from the objective lens to the centers of each mirror size seems to be equal. I'm confused!

Anybody have any ideas about what is going on here and how to fix it?

John C.

#2 Guest_**DONOTDELETE**_*

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Posted 23 September 2003 - 12:50 PM

have you considered the fact that the eyepiece sits higher on a 2" diagonal, maybe even a whole inch higher than in a 1.25". this could be the cause, though i'd guess that the distance from the objective to the mirror in the diag is a little longer in the 2".

if it indeed is, why don't you just chop off an inch or so off the back & remount the focuser?


ds


#3 Jarad

Jarad

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Posted 23 September 2003 - 01:29 PM

If the eyepieces that aren't reaching focus are 1.25" eyepieces (since you used them in the 1.25" diagonal), you could try buying a low-profile 2"-1.25" adapter. This is an adapter with the center partly drilled out (or if you have the tools, you could drill a 1.625" diameter depression in the adapter you already have). This will lower a 1.25" eyepiece about 0.5" (Astrosystems sells them - see http://www.astrosystems.biz/focm1.htm near the bottom of the page). If 0.5" is enough, this may solve your problem, assuming that you can already get to focus with any 2" eyepieces you have (and if you have no 2" eyepieces, just use the 1.25" diagonal.... :foreheadslap:).

Hope this helps,

Jarad


#4 Guest_**DONOTDELETE**_*

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Posted 23 September 2003 - 01:53 PM

Thanks. A friend suggested I cut the optical tube! How much is anyones guess!

I must admit that I haven't tried my 2" eyepieces yet. The fog rolled in before I had a chance to test them.

The low profile 1.25 eyepiece adaptor sounds like a viable option to try, "before" I cut the optical tube!

To cut my tube, I'd also have to move my finder (2) mounts that are bolted into place, not to mention, plugging the old holes...and...geting the tube cut straight.

Guess I'll search for an adpator.

John C


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