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Bungee Cord counter balance system

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#1 SC ed

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Posted 12 October 2017 - 12:13 PM

I hope this is the correct group to ask, but just trying to learn about the possibility to use bungee cord as a counter balance for a front end heavy dob. I thought I had read that Hubble Optics was doing this but now cant find the link.

So, is anyone using bungee coed in this way, and if so how is it working for you, also pictures of your setup would be nice.

Thanks



#2 havasman

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Posted 12 October 2017 - 12:27 PM

For my solid metal tube XT10i I like these plastic coated magnets I found at the hardware store. They're quick & easy to move if needed to help balance. There's less variability than with an elastic solution.

 

fan_installation_small.jpg


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#3 vtornado

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Posted 12 October 2017 - 12:28 PM

If you get some welding magnets from harbor freight and wrap them in duct tape or cloth, so they don't scratch your tube, you can add them as needed to the back of the tube.  

 

I have also seen chain added to the back,

so the lower the dob points the more links the back has to raise.

 

A bungee cord in principle could work.  It might take trial and error to get the length, and stretch modulus to be right.



#4 bdawg6381

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Posted 12 October 2017 - 01:03 PM

I would start with this thread: https://www.cloudyni...-balance-a-dob/

 

Although most users use weights as solutions.  I've seen springs used but not bungees - I think springs would be a better option for longevity like here: http://www.mikehotka...nalSprings.html



#5 Starman1

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Posted 12 October 2017 - 04:41 PM

With heavy eyepieces (only), my scope dives when pointed below about 45° (good reason to not look that low, I suppose, and everything is better higher up anyway).

A small mini bungee provides just the right amount of lift, and the lift increases as the scope points lower.  That's also a good thing.

 

But a variable, or "virtual" counterweight has a couple problems:

--it raises the scope when changing eyepieces

--it can only truly balance one weight of eyepiece (and mine range from 6 ounces to 36 ounces, which is a problem)

 

The issue of imbalance can even affect the "perfectly-balanced" scope when an eyepiece is changed.

 

So the desirable accompaniment to the "virtual" counterweight is a brake of some sort that holds the scope in place while the eyepiece is being changed.

 

But, I like the virtual counterweight because as the scope moves lower, it effectively increases the weight of the counterweight.

Otherwise, I'd have to be diligent to change the weight on the scope depending on the altitude of the target.



#6 SC ed

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Posted 12 October 2017 - 06:33 PM

Don, thanks, the risk of pull up off the target is something I hadn't considered. Anyone figured out a temp break to eliminate that problem?



#7 SC ed

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Posted 12 October 2017 - 09:04 PM

As I read more information, I found one on person who by the way he ran the bungee it actually had a breaking effect on the rocker box. But he noted it was not effective in changing from a large 2 incher to the much lighter 1.5 eyepieces, this solution was out of Italy so may have missed some things in the translation.



#8 Starman1

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Posted 13 October 2017 - 12:59 AM

Don, thanks, the risk of pull up off the target is something I hadn't considered. Anyone figured out a temp break to eliminate that problem?

You mean "brake", but yes, I did find a solution once: https://www.cloudyni...ake-that-works/

I also liked the Meade Lightbridge method with the small pressure plate against the trunnion.

Attached Thumbnails

  • lb.jpg

Edited by Starman1, 13 October 2017 - 12:59 AM.


#9 SC ed

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Posted 13 October 2017 - 06:49 AM

Don, thanks for the link. Very interesting discussion, and solution. As is the Mead system.



#10 Cotts

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Posted 13 October 2017 - 07:20 AM

I saw a fellow use a chain on his dob.  It was a sturdy chain with, trying to remember, about 1" links. It was attached at 1 end to the bottom of the rocker box.  When the scope was vertical the chain was curled up on the ground.  As the scope was lowered in azimuth more and more links were lifted into the air by the rising rocker box, becoming 'counterweights'.   Variable counterweight system!   Clever.

 

Dave


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#11 havasman

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Posted 13 October 2017 - 12:33 PM

Don, thanks for the link. Very interesting discussion, and solution. As is the Mead system.

Knowing what scope you're trying to balance would, as always, help us better address your specific issue!


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#12 Jon Isaacs

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Posted 13 October 2017 - 02:07 PM

I saw a fellow use a chain on his dob.  It was a sturdy chain with, trying to remember, about 1" links. It was attached at 1 end to the bottom of the rocker box.  When the scope was vertical the chain was curled up on the ground.  As the scope was lowered in azimuth more and more links were lifted into the air by the rising rocker box, becoming 'counterweights'.   Variable counterweight system!   Clever.

 

Dave

 

A scope that's balanced top to bottom and front to back should be balanced at all altitudes. 

 

In my experience,  there are two factors that can cause a scope to become unbalanced at lower angles.  The first is that the actual imbalanced forces are a function of the cosine of the elevation .Since the altitude bearing friction is constant, as the elevation angle is reduced, less than optimally balanced scope may work well at higher elevations but as one approaches horizontal,  it will begin to drift with heavy eyepieces. 

 

The second issue is front to back balance.  If the scope is not balanced front to back, then the balance shifts as one moves in altitude.  Using counterweights to counterbalance finders,  eyepieces and focuses should be done with this in mind.  These typical go on the front side of the scope at the top so counterweights need to go on the bottom of the scope at the back side. 

 

3912615-Balancing a DOB.jpg

 

I believe the Summerian Dobs from Europe use a bungee cord as a virtual counterweight.  Theyre lightweight compact designs and balancing the normal way is not possible. 

 

My own experience is that a scope that is balanced top to bottom and front side to backside and has an optimal amount of friction for tracking with still be balanced within the range of normal (0-1 kg) eyepiece weights.  That means about +/-500 grams or one pound of force will be needed for tracking.  

 

As an aside,  many wonderful  object never reach higher elevation , Omega Centauri culminates at 10 degrees,  on a good night,  it's not to be missed.

 

Jon


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#13 DaveJ

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Posted 13 October 2017 - 02:16 PM

Exactly as Jon described and graphically demonstrated. Exactly.
I clicked on the "Like" button but wish there were a "Hurray!!!" button.
This is exactly how I balance in 3D with a refractor and SCT  on a GEM, too. And with side-by-side on the GEM of whatever configuration.
Balance is balance. I love physics. ;^)


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#14 SC ed

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Posted 13 October 2017 - 02:52 PM

This discussion at least from my point of view is about learning and not about balancing a specific scope. For whatever reason my scope, currently, does not have a balance problem but it may be because I currently don't own any really heavy eyepieces and don't have much riding on the scope.  

 

But, I may and would like to learn so thanks for the continued efforts to educate me in general sort of way.

 

Jon the lat graphic was great thanks

 

Ed


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#15 Starman1

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Posted 13 October 2017 - 05:19 PM

 

I saw a fellow use a chain on his dob.  It was a sturdy chain with, trying to remember, about 1" links. It was attached at 1 end to the bottom of the rocker box.  When the scope was vertical the chain was curled up on the ground.  As the scope was lowered in azimuth more and more links were lifted into the air by the rising rocker box, becoming 'counterweights'.   Variable counterweight system!   Clever.

 

Dave

 

A scope that's balanced top to bottom and front to back should be balanced at all altitudes. 

 

In my experience,  there are two factors that can cause a scope to become unbalanced at lower angles.  The first is that the actual imbalanced forces are a function of the cosine of the elevation .Since the altitude bearing friction is constant, as the elevation angle is reduced, less than optimally balanced scope may work well at higher elevations but as one approaches horizontal,  it will begin to drift with heavy eyepieces. 

 

The second issue is front to back balance.  If the scope is not balanced front to back, then the balance shifts as one moves in altitude.  Using counterweights to counterbalance finders,  eyepieces and focuses should be done with this in mind.  These typical go on the front side of the scope at the top so counterweights need to go on the bottom of the scope at the back side. 

 

 

 

I believe the Summerian Dobs from Europe use a bungee cord as a virtual counterweight.  Theyre lightweight compact designs and balancing the normal way is not possible. 

 

My own experience is that a scope that is balanced top to bottom and front side to backside and has an optimal amount of friction for tracking with still be balanced within the range of normal (0-1 kg) eyepiece weights.  That means about +/-500 grams or one pound of force will be needed for tracking.  

 

As an aside,  many wonderful  object never reach higher elevation , Omega Centauri culminates at 10 degrees,  on a good night,  it's not to be missed.

 

Jon

 

Jon, you're right.

Unless the scope is balance without an eyepiece.  Then it's out of balance with a heavy eyepiece in the focuser.

And if it's in balance with the eyepiece, it'll be out of balance without it.

I have a choice when using a 1kg eyepiece to view something down low: perfect balance with or without the eyepiece, but not both.


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#16 Jon Isaacs

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Posted 13 October 2017 - 06:10 PM

Jon, you're right.
Unless the scope is balance without an eyepiece.  Then it's out of balance with a heavy eyepiece in the focuser.
And if it's in balance with the eyepiece, it'll be out of balance without it.
I have a choice when using a 1kg eyepiece to view something down low: perfect balance with or without the eyepiece, but not both.

 

 

If there is no friction that would definitely be the case. But if the scope is fully balanced with a 500 gram eyepiece and there's at least 500 grams (actually 4.9 Newtons or about 1 pound) of force required to overcome the friction,  then the scope will not move whether there's no eyepiece or a 1000 gram eyepiece in the focuser. 

 

I find ~ 1 pound tracking friction about optimal for both balance and tracking.  With my 10 inch Dob and it's small bearings,  there's not enough friction and I need to use counter weights with my heaviest eyepieces but my other all have large bearings and sufficient friction to not require additional counterweights. 

 

Last night , after getting some flack about tracking at high magnifications in another thread,  I thought I'd try the 3.5 mm Nagler + 2x Barlow in my scope which operates at a 2820 mm  effective focal length. Without the 2x Barlow that's 805x.  I'll leave the math to the reader. Things wizzed by pretty darn quickly but I was able to manage it.. I don't recommend it but it was doable.. 

 

Maybe you find lower tracking forces work better for you,  my above digression was mostly to say that a scope that's setup with ~ 1 lb tracking friction can track reasonably well at higher magnifications. 

 

But it's also possible that your scope is not quite balanced front to back or even end to end.  

 

I can't say but I do spend some time with weight placement. I believe your scope has the focuser flat so it and the eyepieces are on the center line so they're balanced front to back. If any counter weights for the finder are not on the backside,  that might be an issue. 

 

Jon



#17 Starman1

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Posted 13 October 2017 - 11:43 PM

It's a typical dob, with the COG above the altitude axis.  When the scope is horizontal, a 31Nagler + Paracorr dives for the ground.

When the scope is at 45°, it doesn't move whether eyepiece is present or not.

It's easy enough to apply a mini-bungee cord for the low position.



#18 Jon Isaacs

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Posted 14 October 2017 - 01:19 PM

It's a typical dob, with the COG above the altitude axis.  When the scope is horizontal, a 31Nagler + Paracorr dives for the ground.

When the scope is at 45°, it doesn't move whether eyepiece is present or not.

It's easy enough to apply a mini-bungee cord for the low position.

 

Don:

 

If you're happy with it,  that's what counts. 

 

My point is that with some attention to the various factors that affect balance,  many Dobs can be in balance at all elevations.  

 

Jon



#19 Starman1

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Posted 14 October 2017 - 03:34 PM

With one eyepiece.

When your eyepieces vary from 6 ounces to 40 ounces (about the range I use in the scope), that's not possible.

You can come closer than most do, however. 



#20 SC ed

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Posted 14 October 2017 - 09:27 PM

If the scope is balanced, for a given eyepiece (or take a more significant approach) a 2" wide angle with a Barlow . And you pull the eyepieces out, how do you keep the scope from drifting up in altitude. Do you use a brake of some kind? I mean when you pull 2 lbs off the front of the scope it becomes tail heavy and will want to rise until balance is restored.



#21 izar187

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Posted 16 October 2017 - 01:59 AM

If the scope is balanced, for a given eyepiece (or take a more significant approach) a 2" wide angle with a Barlow . And you pull the eyepieces out, how do you keep the scope from drifting up in altitude. Do you use a brake of some kind? I mean when you pull 2 lbs off the front of the scope it becomes tail heavy and will want to rise until balance is restored.

It can rise. or drift down. As suggested, movable weight(s) work. More static friction in the altitude bearings works. Adjustable friction in the altitude bearings works. Not barlowing big honkin' 2" ep's works. Lighter weight biggest low power 2" ep works. Adjusting the position of the altitude bearings works, if possible. Often, it's some combination of several things, that one adapts to over time.



#22 Shneor

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Posted 17 October 2017 - 01:15 AM

You might want to look at a C-clamp that's the right size to prevent movement when changing eyepieces.



#23 Sarkikos

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Posted 18 October 2017 - 10:55 AM

If starting from scratch - whether making your own Dob mount or shopping for one - wouldn't it be best to maximize the size of the altitude bearing?  These counterweight systems are only after-the-fact measures to improve a Dob mount that was not ideally constructed to begin with.  It might also be a good idea to maximize the total area of the altitude bearing that is in contact with the mount.  

 

I should know.  My Dob mount - I bought it for my C10NGT 10" f/4.8 Newt nine years ago - has a relatively small altitude bearing and is set too high off the Dob mount.  A friend has a Dob mount that is similar except that the altitude bearing rests half-way down into the side of the mount.  Only about 1/4 of the edge of my bearings contact the side of the mount at any time.  Of course, if the altitude bearings are made very large, like the typical boat anchor bearings in bigger Dobs, you won't be able to set them very far into the side of the mount.

 

My Dob mount has ebony-star and teflon, so these aspects are covered.  

 

To balance my mount, I used the two counterweight system illustrated in this thread.  But I added a third counterweight in the form of a magnet that I place/remove on the OTA near the focuser.  I have the setup balanced with the first two weights for my heaviest eyepiece, the Terminagler.  When I replace it with a lighter eyepiece, I place a magnetic weight near the focuser to compensate for the loss of weight.  It works great, but I still need to hold onto the OTA when I'm switching eyepieces and placing the weight, or the OTA will move.

 

Here's a pic of the Dob mount.

 

Mike

Attached Thumbnails

  • IMG_1105.JPG

Edited by Sarkikos, 18 October 2017 - 11:05 AM.


#24 Starman1

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Posted 18 October 2017 - 04:23 PM

Typical dob construction places the diameter of the altitude trunnion at 1.5x the mirror diameter, which would make the 

trunnion 15" in diameter for a 10" scope.

People usually cut the trunnion in half and use semi-circles, but a full circle is kind of interesting and

makes for all kinds of possibilities:

http://www.wildcard-...s1/fullum_4.jpg

http://starizona.com...ullum Booth.jpg

https://www.astromar...99/801225-1.jpg

http://www.discmount...es/IMG_3578.jpg

http://www.apm-teles...er-4124NF_4.jpg



#25 Sarkikos

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Posted 18 October 2017 - 05:28 PM

I had to look up "trunnion."  It is what many Dob users mean when they say "altitude bearing."   Trunnion is a term from artillery:  "a supporting cylindrical projection on each side of a cannon or mortar."  https://www.google.c...chrome&ie=UTF-8  That's a good description for what we're talking about here, except replace "cannon" with "OTA."  grin.gif

 

So the trunnion should be 1.5x the mirror diameter?  The trunnions on my Dob mount are only 8" across, quite smaller than the 15" they ought to be.  They are nearly half the "typical dob construction."  I don't know about "typical."  I've seen many under-sized trunnions on Dob mounts. The big companies keep churning them out ... as well as some of the one-man operations who ought to know better by now.  Maybe "ideal" - or even "merely adequate" - would be a more appropriate word here than "typical."

 

But this doesn't explain my friend's Dob mount.  His mirror is actually 11".  The trunnions on his mount are certainly not 16.5" across.  They look only a little wider than mine.  And yet he says the mount balances fine, with no movement when he switches eyepieces.  It has no retaining springs or tension knob.  It has ebony-star and teflon, like my mount.  The only obvious difference is that his trunnions set half-way into the mount, while mine are only one-quarter in. 

 

Mike


Edited by Sarkikos, 18 October 2017 - 05:28 PM.



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