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Richard Berry Refractor tripod - leg angle?

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#1 starcanoe

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Posted 16 February 2019 - 01:46 PM

Hi all.

 

Working on my version of Richard Berry's refractor tripod. The one feature in Making Telescopes.

 

I can't find my darn book now !

 

Does anybody know the angle he used the tripod legs? The angle formed between one of the legs and a vertical line.

 

I'm thinking it is 15 degrees. That is looking a bit shallow...but 30 degrees is looking too wide..

 

Thanks !



#2 roscoe

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Posted 16 February 2019 - 02:23 PM

I usually build mine at 25 degrees....  perhaps you could set up one of your tripods without any hardware to hold it in place, and tweak it till it 'looks' right, and measure and average your three leg angles.   Do that somewhere not on a slippery floor, though... or you might get the dreaded runaway leg syndrome.....



#3 starcanoe

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Posted 16 February 2019 - 02:25 PM

I designed the rest of the scope based on Berry's design...so I need to stick to his angle..I just can't recall what it was..15 degrees or 30.


Edited by starcanoe, 16 February 2019 - 02:25 PM.


#4 Bob4BVM

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Posted 16 February 2019 - 02:38 PM

Highest stability would be 45*, which of course means a huge footprint.  Start wide and bring the legs in til you are comfortable with how it looks and stability, then build your spreader to fit.  That's how I do it.

CS

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#5 ScottW

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Posted 16 February 2019 - 04:16 PM

Hi,

 

I have the book and both of the top angles are 15 degrees and the one at the bottom of the legs, 75 degrees.

 

Have fun,

 

Scott



#6 starcanoe

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Posted 16 February 2019 - 04:43 PM

Thanks very much Scott...

 

Fun isn't what I'd call it though...but I can see the light (heh) at the end of tunnel finally !



#7 Jeff Morgan

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Posted 17 February 2019 - 12:22 AM

Highest stability would be 45*, which of course means a huge footprint.  Start wide and bring the legs in til you are comfortable with how it looks and stability, then build your spreader to fit.  That's how I do it.

CS

Bob

 

Indeed, with such a tall mount 45 would be impractical.

 

Which is probably why Berry settled on 15 degrees.



#8 jtsenghas

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Posted 19 February 2019 - 10:39 AM

I find that, in general, the taller a tripod AND the lower the center of gravity is the narrower an angle suffices.

 

When you think about it, the greater the splay angle, the more you are lifting the assembly as it tilts about the line through two feet and the further you have to push it before it overbalances.  A taller tripod with a smaller splay angle could have the same sized footprint. It would push over a little easier if the center of gravity were higher, but still would have to be pushed just as far to overbalance. 

 

I've seen others hang weights below their tripods to greatly improve their stability including battery packs to power the mount as well.

 

One trick I've done on my modified Berry Mount is to screw a pet stake into the ground and to attach it to the tripod with a bungee cord at outreach events. Even the most blundering of visitors won't knock THAT over.


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#9 John Miele

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Posted 19 February 2019 - 01:03 PM

I built mine at 25 deg. Using 15 deg. seems far too shallow for my comfort level. I say that because a Berry style mount has the CG of the scope well off of the centerline of the tripod. And when your scope is oriented in between two tripod legs, the tip over stability margin will be low. Especially if you are on a little bit of uneven ground.


Edited by John Miele, 19 February 2019 - 01:07 PM.


#10 starcanoe

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Posted 19 February 2019 - 03:30 PM

In a Berry design the "rocker/cradle" thingy is supposed to have a counterweight on the other end from the scope tube end such that the CG is located over the center of the tripod.

 

I'm more worried about stuff being way out of balance as the counterweights and tube are placed up there.



#11 jtsenghas

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Posted 19 February 2019 - 03:59 PM

In a Berry design the "rocker/cradle" thingy is supposed to have a counterweight on the other end from the scope tube end such that the CG is located over the center of the tripod.

 

I'm more worried about stuff being way out of balance as the counterweights and tube are placed up there.

I'm not sure I follow you. When the counterweight and tube are in place, things are, as you first said, balanced with the COG over the center of the tripod.

 

I don't understand your last sentence. I was proposing additional weights hanging on the tripod to further stabilize it and not to affect at all the balance of the OTA. What exactly are you worried about? 



#12 starcanoe

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Posted 19 February 2019 - 05:29 PM

Okay....lets say the rocker/cradle has the counterweight built in...

 

When you put it up there there ( without the tube)...its CG will not be over the center of the tripod.

 

Or the counterweights are NOT built in (more likely). Now you put the tube up in there. Again the CG is not centered. Until you put the right amount of counterweights up there.

 

The only way the CG is centered the whole time is if you put the tube in cradle....and the counterweights...and lift the whole thing up there and put it in already asssembled.


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#13 jtsenghas

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Posted 19 February 2019 - 05:43 PM

Got it  Thanks. I guess this effect can be minimized by rotating the mount in azimuth so that the counterweight is above a leg before the scope is hefted onto it, but I understand you now. 



#14 starcanoe

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Posted 19 February 2019 - 06:12 PM

That would certainly help. But it would be nice if it works out where no matter which way you assemble it...it is at least somewhat stable. But I'm not counting on it. Will just have to see.

 

I may end up spreading the legs a bit farther apart. At some point you just have to quit thinking, calculating, pondering, what iffing, and worrying and just build something and THEN you can actually see whats what.

 

I'm sure when I'm done I see errors and not so good ideas and how to improve this.



#15 Jeff Morgan

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Posted 19 February 2019 - 07:09 PM

The azimuth bolt has a nut on the bottom side, holds everything together even if you intentionally unbalance.

 

The role of the counterweight is to even out the pressure across the azimuth pads so the mount turns nicely.

 

There is a mod Chuck Hards did, the article was in Sky & Tel titled "Weightless Woodshop Refractor". Basically, on the side opposite the OTA cradle you have a hook with a teflon pad. The hook catches the underside of the azimuth hub assembly and glides along formica.

 

I built a Berry mount using that idea, works well. Here is a photo of it. The tripod was scaled about 2/3 of the Berry design to carry a 6" f/5 refractor. If you zoom in on the side of the cradle opposite the OTA you can see the "hook".

 

5664503-Jaegers on Berry Mount.jpg

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#16 starcanoe

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Posted 19 February 2019 - 07:29 PM

Nice looking scope. The hook idea is a good one.



#17 Jeff Morgan

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Posted 21 February 2019 - 04:22 PM

So I am sketching a mini-version for seated viewing using a flat to feed a 300mm telephoto lens and NV eyepiece, and thought I would try a 30 degree leg angle.

 

The goal is an eyepiece height of 46" when viewing the zenith, and the central hub only needs to be about 31" off the ground. The hub is 5" radius and the legs join 3" from center.

 

Of course a little math and one can figure out the footprint.

 

But to lay out a scale drawing and see it ... yikes!

 

Think I be using something much closer to 15 degrees in the finished version.



#18 John Miele

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Posted 21 February 2019 - 08:03 PM

In a Berry design the "rocker/cradle" thingy is supposed to have a counterweight on the other end from the scope tube end such that the CG is located over the center of the tripod.

 

I'm more worried about stuff being way out of balance as the counterweights and tube are placed up there.

Ahhh...that's right. I forgot about using a CW to offset the OTA weight. But I still think 15 deg. is too shallow...but that's just me I guess...



#19 Steve Allison

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Posted 22 February 2019 - 04:13 PM

My own Berry-style 6 inch F/15 refractor (long since sold) worked pretty well with the 15 degree leg angle for observing. But...

 

One night I was standing on a chair fussing with something on the yoke when I slipped and my feet hit the tripod. I may have grabbed at the yoke but in any event, the entire telescope came down on top of me. The 30 pound counterweight missed my head by inches. A wider spread to the legs might have saved me from my own clumsiness.

 

Something to think about, I think.

 

Steve

 

P.S.- Don't try this at home.


Edited by Steve Allison, 22 February 2019 - 04:15 PM.


#20 starcanoe

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Posted 23 February 2019 - 09:22 AM

Eeek !



#21 Pinbout

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Posted 23 February 2019 - 09:40 AM

Indeed, with such a tall mount 45 would be impractical.

 

Which is probably why Berry settled on 15 degrees.

just for engineering stability - base should be around 1/3 the height.... like they do in scaffolding

 

 

so a 40" high tripod would have a 13.22" radius legs that sets it at 18deg.

 

tripod.JPG



#22 Jeff Morgan

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Posted 23 February 2019 - 10:38 AM

just for engineering stability - base should be around 1/3 the height.... like they do in scaffolding

 

 

so a 40" high tripod would have a 13.22" radius legs that sets it at 18deg.

 

attachicon.gif tripod.JPG

 

just for engineering stability - base should be around 1/3 the height.... like they do in scaffolding

 

 

so a 40" high tripod would have a 13.22" radius legs that sets it at 18deg.

 

attachicon.gif tripod.JPG

 

 

 

Does that base distance include the radius of the central hub?

 

The legs join the hub at the edges, not the center. A small bit of distance (perhaps 5-6") is gained there.



#23 Pinbout

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Posted 23 February 2019 - 10:46 AM

 

Does that base distance include the radius of the central hub?

sure

 

so if your hub is 6", you'd wide up with 13.22+[6/2] overall radius.

 

its a concept - 1/3 the height that will give you a stable footprint, angle is what it is.



#24 John Miele

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Posted 24 February 2019 - 06:21 PM

Well even 10 deg. is stable for vertical loads! But you don't  know when you might accidentally push or pull on the scope or bump the tripod. Even a small lateral load can be dangerous. And the above figure is misleading. You have to worry about the worst case tip-over line which is drawn between any two of the legs. So that 13.22 inches is the best case number. The distance shrinks to 13.22*cos60 = 6.61 inches between the legs. 


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#25 starcanoe

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Posted 25 February 2019 - 08:15 AM

Good point John.

 

My scope is going to be a mixed blessing in that regard. It is gonna be pretty darn heavy, so an accidental bump seems unlikely to cause a tip over. On the other hand...if it does tip over....its a lot of weight landing on somebody so it is more dangerous when if it does.

 

I suspect what I may end up doing is having a fair bit of  "dead weights" in some container that sits on the ground under the tripod and is attached to it with something like a chain that has just a tiny bit of slack in it. Once the tripod tilts more than a bit that will stop it. I'll also probably have some type of swivel connection on the side bearing that attach the tube more securely to the rocker/cradle so that in the event of a major upset it can't come out either.


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