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Berry mount vs fork mount

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#1 Araguaia

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Posted 23 February 2019 - 05:40 AM

I am starting to plan the mount for the 127mm wood refractor I am building.  It will be alt-az.

 

I wanted to avoid the counterweights required to balance a Berry mount, but I am afraid a fork mount will be jittery.  The tube is only 70 cm long (total focal length 952mm), so the fork would be around 90 cm high.

 

I can make a stiff fork, but how to keep it from vibrating at the az bearing with such a long lever arm?  One thought is an az bearing as wide as possible (without getting in the way of the observer when pointing at the zenith), and perhaps add a handle to rotate the az bearing directly, rather than by pushing the scope.  A broom handle used to do that trick with our old rickety dob mount.

 

Thoughts and experiences?



#2 TONGKW

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Posted 23 February 2019 - 06:28 AM

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I have made a fork mount for my Meade AR5 refractor OTA (127 mm f/9.4) for school children to find the sky objects themselves.
I use a 1” vertical shaft with 2 ball bearings for the AZ axis.
There is no need for a counterweight and zenith can be reached.
There is no appreciable vibration at up to 100x.

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Meade AR5_fork mount.jpg

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#3 Tucker512

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Posted 26 February 2019 - 02:36 PM

I'm almost done with a 127mm f/7.5 binoscope build (and will post a build thread shortly) that originally used a Berry mount.  I discovered two things: it was pretty wobbly, and using it with or without the designed 22 pounds of weights made no difference in performance (stability or smoothness of motion).  So I rebuilt it with an angled fork arm.  The fork is only 8" tall but tilted at 45 degrees.  This allows the scopes to clear the tripod in almost every position.  I lose the very zenith if the OTAs line up with a tripod leg.  But it is more stable than the Berry mount was, and doesn't require the counterweights.  It isn't technically balanced, but it runs just fine on Teflon pads.

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#4 figurate

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Posted 28 February 2019 - 05:17 PM

This is a alt/az vertical fork mount I built some thirty years ago, actually my original ATM project which was used back then with a 3.5" homemade rft refractor. Since that time it has evolved considerably, but I spent a lot of effort adding rigidity and stiffness to the entire structure, and all that work did pay off because it is one of the more solid mountings of all the ones I own. I had started with a very high quality bearing I found to use for the azumuth bearing, perhaps a former computer tape drive unit, that coincidentally was a close fit into an aluminum flange I had, and it just grew from there, including a DIY tripod with the necessary leveling screws at the leg tips- and with a fork that tall you're going to need really accurate leveling.

 

This one is a bit more than 16" high from the altitude pivot point to the top surface of the bearing, or more than 400mm, and I think you would have problems going a lot taller than that (depending on your tolerance for vibration and stability), and 900mm is fairly extreme. In part, I was forced to take this approach by the size of the maple lumber I originally had on hand to make the tripod legs out of; I needed to add some height and the vertical fork was a solution. As you see here, the one I built has aluminum x-bracing incorporated into the fork and lots of hardwood inside and out, and I incorporated friction brakes for altitude and azimuth into the mix. The 80mm triplet it is pictured with here is quite heavy mounted in its cradle but with these proportions this is a robust, convenient, and portable mount for it. A final aesthetic touch was some experimental inlay work I did (the wood is not as fluorescent colored as the camera pictures it here).

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Edited by figurate, 28 February 2019 - 05:25 PM.

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