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what weight for magnetic counter-weights for 10" dob?

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#1 laurelg9

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Posted 13 April 2021 - 08:35 AM

I notice that all my "stuff" (telrad, finder, EP) on the top end of my collapsible dob makes it so the entire thing sinks if I have it at less than a 45 degree angle. I clearly need some counterweights on the hind end. 

 

What sort of weight am I needing?  How do y'all manage counterweights on a dob? Will it help me?  

 

 


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#2 happylimpet

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Posted 13 April 2021 - 08:40 AM

test it with things you know the weight of ....tins of food etc, until you get a rough idea.



#3 jmillsbss

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Posted 13 April 2021 - 08:56 AM

As much as all that stuff out on the tube seems heavy, it doesn't necessarily take an equal amount of weight to have a balancing effect.  I'm sure there's a mechanical engineer on here that can do the physics, but you may have noticed most dob counterweights are either 1, 3 or 5 pounds.  I have a 10" metal tube Orion dob and I have had, at one time, a dual finder stalk with a RDF AND a RACI finderscope, an ES coma corrector, and replaced the stock single speed focuser with a motorized Moonlite.  If you had all that stuff in a pile it'd probably be 5 pounds, or at least 3, but I can balance it with a 1 pound magnetic dob counterweight I got from Orion.  If you have a truss tube it will be different but I can't imagine it would be much.

 

All that weight on my nose can give me the sags if I view at very low altitude targets, so I got a conduit strap and about 15 qty 1" galvanized washers.  All together it weighs 10 ounces.  I put a hook on the bottom edge of my tube, sure avoid the collimation hardware, and I can add or remove weight one washer at a time, though I usually just hang the whole thing on there.  It sits perfectly level with the ground if that's where I put it.

 

One other thing.... at least with a tube dob, you can adjust the counterweight effect by sliding the magnet in different places on the tube.  That's a science too, but sort of an art at the same time.   I'd start with a 1 pound weight.  It does more than you'd expect.  I hope that helps.


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#4 Jon Isaacs

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Posted 13 April 2021 - 08:58 AM

Laurel:

 

Depending on the finder, the eyepieces and whether or not I'm using a Paracorr, I use either 2lb or 5 lb Harbor Freight retrieving magnets.

 

 

https://www.harborfr...pull-36904.html. This one weighs 2.0 lbs.

 

https://www.harborfr...pull-36905.html. This one weighs 5.0 lbs

 

I wrap them in duct tape to prevent damaging the scopes finish.  Two 2 pound weights is probably better than one 5 pound. You could start with one.

 

The scope needs to be balanced front to back and top to bottom so placing the weight at 45 degrees on the backside of the scope is best, it fits in the corner of the base when viewing near the zenith. 

 

You can move it up and down to find the best balance and just leave it there for the night.

4564976-Balancing a DOB.jpg
 
Jon

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#5 Jon Isaacs

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Posted 13 April 2021 - 09:11 AM

you had all that stuff in a pile it'd probably be 5 pounds, or at least 3, but I can balance it with a 1 pound magnetic dob counterweight I got from Orion.  If you have a truss tube it will be different but I can't imagine it would be much.

 

 

I am a mechanical engineer.  Balancing a Dob is like a teeter-totter, if you want it to balanced from the horizon to the zenith without just cranking down on the friction, you will need at least as much weight as you add, probably more.

 

Important distances in this teeter-totter are the distance between the altitude pivot and the focuser/finder and the distance from the pivot to the bottom ofvthe scope where the weight will be attached.  I call the ratio between these the Balance Ratio.  A balance ratio of 1 means for every pound you add up top, you need 1 pound down below.  

 

Commercial 10 inch Dobs have a balance ratio of about 1. Large truss scopes generally have the same pivot height but are much taller so the balance ratio is much higher, my 22 inch has a balance ratio of 5.5 to 1.  

 

Jon



#6 Ehanc

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Posted 13 April 2021 - 09:40 AM

When using my heaviest eyepiece (ES82 30mm), coma corrector, and finder scope, I find (2)  5# magnets work well to counterbalance my 10" Lightbridge.  I added adhesive backed felt from the craft store to prevent the magnets from scratching the tube, and I kept the boxes to store them in. If you go this route be careful not to get your fingers or anything else you don't want flattened to get between the magnets!  I have to think there is a more elegant solution available but this works.



#7 laurelg9

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Posted 13 April 2021 - 11:41 AM

so much good information here, thank you all.  I love Cloudy Nights. 


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#8 bbasiaga

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Posted 13 April 2021 - 11:59 AM

The best thing you can do is to test it and see, as noted above.  You don't need to add as much weight to the back as you did to the top....you just need to add enough to offset the current imbalance.  If your scope is falling like a rock, then the imbalance is large and you want to start with higher weights.  If it is just slowly travelling down, the imbalance is small and you can start lighter. 

 

An interesting option is chain.  If you hook a chain on to the bottom of the scope and let it pile up in the rocker box, more of the chain will be lifted up as the scope goes down, and less as it rises up.  This effectively makes a variable counterweight.  You may have to play around with the size of chain and the number of links always hanging. 

 

Lead shot, or BBs in a little bag also work.  Just keep adding to the bag until it does what you want, then weight it.  Even nuts and bolts you have lying around the house are fine for that.  Just keep going til you like it, and weigh the result.  Then you can fashion a permanent counterweight. 

 

-Brian


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#9 RSersen

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Posted 13 April 2021 - 12:58 PM

No pictures yet but I'm adding a cooling fan to my XT8 this week, which is a USB PC fan. Will be powering it with a rechargeable power pack that weighs ~1 lb, hoping it'll double as an effective counter weight. If not I can probably find a heavier one on Amazon.

 

Ripped a couple magnets from an old hard drive, used a liberal amount of electrical tape to attach those to the power pack, and the whole thing sticks to the OTA and can move up or down (as far as the fan cable will allow) as needed, without scratching. That part's all done and works great, just need the fan itself to show up in a couple days.

 

Have an XT10i on backorder, will plan to migrate the whole mod to that when I get it, but may need an additional weight for heavier EPs. We'll see.



#10 CowTipton

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Posted 29 April 2021 - 12:19 PM

I use these on my AD8.

 

https://www.homedepo...-206503428-_-N

 

They're ~1/2lb each, have a nice strong pull, and already have a no-scratch liner on the contact surface.

I even use one as a guide handle for the scope under the focuser (manual dob.)



#11 aeajr

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Posted 29 April 2021 - 02:40 PM

I notice that all my "stuff" (telrad, finder, EP) on the top end of my collapsible dob makes it so the entire thing sinks if I have it at less than a 45 degree angle. I clearly need some counterweights on the hind end. 

 

What sort of weight am I needing?  How do y'all manage counterweights on a dob? Will it help me?  

Everyone has their own approach.

 

I use hobby magnets that I can move around on the tube of my Apertura AD12 12" Dob.

 

These weigh about 3.5 oz per pair.  

https://www.harborfr...nets-97504.html

 

I wrap each pair in duct tape so they don't scratch the optical tube.  I have 8 pairs that ride on my optical tube all the time.

 

Two pair sit down by the mirror all the time, about 7 ounces.  This is to offset my Telrad. 

 

The other six pair sit on the pivot point where they have no impact on balance. When I drop in a large eyepiece, I just slide some or all of them down to the mirror.  When I am done with that eyepiece I just slide them back.

 

Super cheap and easy to use. And if I need more, they are $1 a pair. 


Edited by aeajr, 29 April 2021 - 02:43 PM.


#12 Dave McCrary

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Posted 29 April 2021 - 03:55 PM

I just started using my 8 x 50 RA finder and needed some counterweight. Ihave been using a Farpoint magnetic 1.5 pound bag that works for me. You can easily adjust it up and down the scope as needed. I like it.

Dave

 

https://farpointastr...tic-bag-weight/


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#13 kklei940

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Posted 30 April 2021 - 11:34 AM

There are a lot of different ideas for counter-weights. Here is a cheap one I came up with after my 10" dob took a nosedive the first time I used my ES 92 17mm and walked away.

 

It's a 3 lb. dumbell that cost a bit more than $3 with a bunch of strong little magnets I took off of electric toothbrush heads. I used JB Weld on the magnets and covered them with duct tape to prevent scratching on the tube. This amount of weight combined with the Correct Tension system on the Altitude adjustment seems to be working pretty good so far. No more accidents at least.

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