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Where to get a no-undercut extension?

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#1 dcoyle

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Posted 13 November 2012 - 02:06 PM

I have a 2" Lunt Herschel solar wedge, with an additional ND filter, which is on the eyepiece side of the diagonal. I used a 35mm tall, 2" dia. extension after the diagonal to achieve focus without racking the focuser out any farther than necessary.

Since I added the extra filter, the extension won't seat correctly.

I need a decent 35mm tall, 2" dia. extension without an undercut. I am sure there are others that would find it useful, too.

Are there any vendors?

Thanks

Dan

#2 Binojunky

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Posted 13 November 2012 - 05:07 PM

Check out Agena Astro and Scope Stuff, they have a good few of various sizes , maybe something there that will get you going,DA.

#3 DaveJ

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Posted 13 November 2012 - 05:34 PM

Check out Agena Astro and Scope Stuff, they have a good few of various sizes , maybe something there that will get you going,DA.


I've got every 2" extension that Agena sells. They all have the $(#*@ undercuts. Can you tell I'm not a fan of the undercuts? Most useless "invention" of all time as far as I'm concerned.

#4 junomike

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Posted 13 November 2012 - 06:39 PM

Is It possible to use a TV 2" Ext. instead?

The Orion Self Centering Ext. Is only 8mm longer than the 35mm you're after.

This Ext would be perfect, but only available in the UK :(

Mike

#5 dcoyle

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Posted 13 November 2012 - 08:58 PM

I can't find one anywhere, Agena, included.

To me, there is no rational reason for an undercut on an extension.

While I'm at it, there should be TWO thumbscrews, having only one allows rocking perpendicular to it.

One sees many improvements in the optical train geared toward astrophotography. I hope it spreads to extensions.

Dan

#6 dcoyle

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Posted 13 November 2012 - 09:08 PM

Juno Mike, you're the Man. Thank you. The 43mm Orion may fit.

Dan

#7 junomike

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Posted 14 November 2012 - 06:01 PM

Dan It should be ok unless you're having In-focus Issues with your current set up.(needing to move the eyepiece closer to the objective).

The self centering is an added bonus as well. The one's I have used seem more secure than a compression ring and easier to use as well.

Good luck!
Mike

#8 DaveJ

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Posted 14 November 2012 - 07:03 PM

I've got every 2" extension that Agena sells. They all have the $(#*@ undercuts. Can you tell I'm not a fan of the undercuts? Most useless "invention" of all time as far as I'm concerned.


STOP THE PRESSES! I just checked and AgenaAstro has this one that's exactly what you're looking for. And no *$^& undercuts, either.

#9 dcoyle

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Posted 14 November 2012 - 09:06 PM

Why, Thanks DaveJ, that looks like the one. :)

Dan

#10 Jon Isaacs

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Posted 15 November 2012 - 04:48 AM

While I'm at it, there should be TWO thumbscrews, having only one allows rocking perpendicular to it.



In my experience, that depends on the alignment of the undercut with the compression ring. If it misaligned, rocking is possible, otherwise, a single screw is all that is necessary with a compression ring.

Jon

#11 dcoyle

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Posted 15 November 2012 - 12:37 PM

Jon, you say:

"otherwise, a single screw is all that is necessary with a compression ring."?

That "all that is necessary" phrase needs some 'splainin.

My experience and the last 10 years of industry production would disagree. Even my lowly Skywatcher 100/900 Synta focuser has two screws on the "compression ring".

They are not really "compression rings", they are brass bearing surfaces to spread the point load of the single screw over a slightly larger area. The brass is chosen because it is easy to deform and so does not mar the EP barrel.

If you have never had an EP rock when held by one screw, brass 18 gauge "compression ring" buffer or not, great. Others have.

If there is clearance between the EP barrel and the focuser, as there must be for it to be removable, then there are at least two degrees of freedom that have to be restrained. One point doesn't do that.

Dan






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