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Balance weights

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#26 stargazer193857

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Posted 10 November 2018 - 12:18 PM

Well, nationalbalsa suddenly stopped carrying the basswood sizes I want. Maybe they will restock. Maybe they do custom requests. But one thing for sure is plywood costs 1/3 as much per volume, and makes a much simpler structure. It's only downside is the weight, being very over engineered. But the simple pieces are faster to construct. Great material for someone trying to have a profit margin, assuming the customer is ok with the weight.


So, my advise to you all is to move the pad angle up, widen then and their backing to reduce pressure, and use thinner depth and bevels. Although, on heavy tubes, that still might cause a pinch. I think 8" will be ok.

#27 brentknight

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Posted 10 November 2018 - 08:28 PM

Ahhh, I see.  Very long scope.   I would just add a push stick to the magnets so I could manipulate them more easily from the focuser.  A wood dowel would work well.  Include it in the duct tape wrap around the magnets grouped in whatever number you find convenient.  One dowel per group. 

 

Not trying to convince you of anything, but that is how I would do it. 

Not a problem at all Ed.  This is what the forum is for and I appreciate advice and suggestions - thanks!

 

It may not be apparent from the picture, but the tube is not metal either - it's sonotube cardboard.  The only steel is in the mirror cell and the various screws and such attached to that.  It's really not that hard to snap in my punch tool and remove/replace the magnets.  I can get a picture to post here soon.  A simple wedge or shim would also work I suppose, but the nice "click" I get from the punch tool lets me know it's in there solid.


Edited by brentknight, 10 November 2018 - 08:29 PM.


#28 brentknight

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Posted 10 November 2018 - 08:35 PM

I see the metal corner brackets you use. That is what I would expect on such a base. The rear top of the base is susceptible to side to side bending. Also, the large bearings apply a 45 degree sideways force to it. They are ok on a short box, but bend a tall one.

My rear base will slant forward, so less leverage can bend it. Also my smaller bearings will apply force tangent to the wall, only pushing nearly straight back, the direction it is strongest.

The steeper pad angles will provide the needed friction, and I'll widen them and curve and bevel them and use thin depth to avoid pinching issues. Pinching is more of an issue for heavier scopes.

The rocker box sides did flex quite a bit when I first got the scope, but the metal brackets help a lot with that.  Also, I added the shelf and an additional section of wood to the front and that stops the flex on that end.  The top back corners can still flex, but that does not have any effect on the stability of the tube.  My Teflon side bearings were wider apart, but I thought there was too much friction in the movement so I moved them closer together by about an inch.  At the time I had no problems with large eyepieces and balance - note to self: everything changes...



#29 stargazer193857

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Posted 12 November 2018 - 08:37 AM

Interesting, the friction. The alt motion always gives you at least as much friction as the az motion. Also, the alt bearings have less weight on them than the az pads. Finally, the az pads are on an 18" circle. From all that, I would think the az pads could be a bit farther apart for some multiplication. A goal of dob design is for alt friction to match az friction.

I do see the pinching risk, but I suspect good be bevels on 16th inch pads that are made wider should ease that. Your experience has me curious.

As for the scope front, is there a big lightening hole up there?

I admit though even my design might need a 1 pound Velcro weight, in case I get indecisive about whether my finder gets a 68 deg eyepiece or a Plossl. Also might use an ES 24mm.

Nice thing about a high center of gravity is less counterweight is needed, and changes are not as dramatic.

Edited by stargazer193857, 12 November 2018 - 08:40 AM.


#30 brentknight

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Posted 12 November 2018 - 06:17 PM

On the front, it's two solid pieces of plywood.  I've thought about cutting a hole, but the weight is not really a problem in terms of moving the telescope.  The friction/striction on both axis now are pretty much how I like them.  I'm able to track the scope, with at least a medium power, with just my hand without causing any jerky motion.  If it starts to get jerky at all, I can clean the bearing surfaces pretty easily...



#31 Sarkikos

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Posted 13 November 2018 - 09:06 AM

 I have two finders on my Dob, a 9X50 RACI and a RDF or Laser.    But in my very light polluted sky I often can't see the guide stars or the target in these.  So a 2" 38 mm is my next level up finder eyepiece.    I can use this in the scope by tightening down on the Alt bearing or adding a few magnets by the mirror, or a combination.   

 

Removing that big eyepiece and going to a heavier or lighter one has never been a problem.  Adjusting the balance or the friction dial happens easily.  But should the scope move because I was clumsy changing the eyepiece, it is just a matter of lifting it slightly till I regain the target.  Not a big deal. 

 

I usually confirm the target or some other reference in my finder scope so, should I lose it I can get it back quickly after the change. 

 

naturally I don't have this issue with my GoTo scopes. 

 

Naturally your smileage may vary. 

The trick to star hopping with an optical finder is to not be concerned about seeing the target object in the optical finder.  You don't need to see it.  You just need to see the location of the object.  Match the background stars in the finder with the stars in SkySafari Pro.  Center the location of the object behind the crosshairs.  You can see the object's location without seeing the object.  Then look in the eyepiece in your telescope.  

 

My home is in a bright red zone.  This method works for me there.  Maybe if I were in the middle of a city in a white zone, I would need to use a "finder eyepiece."  Maybe not.

 

Mike


Edited by Sarkikos, 13 November 2018 - 09:30 AM.


#32 Sarkikos

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Posted 13 November 2018 - 09:19 AM

Thanks Mike...

 

I'm finding more and more that from my orange front yard, I can't see quite enough in my finder.  Maybe it's the weird way of looking down to find something above me, or maybe I'm having a little extra trouble identifying the field using my SkySafari.  It just seems to cause me more frustration than anything else when I'm "looking" for something.  At a darker site I think it would be much more useful.

 

Right now I find using a pair of binoculars and a green laser much easier to get the scope to the correct field.  That method will usually get the object in the field of my target eyepiece.  I can use the finder then to "confirm" the correct field.  My next iteration in finding methodology is going to be adding an AZ circle to my angle gauge and seeing how that works. But then again the goal there is to get the object in a wide-field eyepiece.

You need to think in terms of the map in your head, rather than how your head is oriented to the sky. 

 

Do you have a laser finder and a RACI on your scope?  Using the laser finder, point the scope to the visible star that is nearest to the intended object.  Then compare the view in your optical finder to the view on SkySafari Pro.  Star hop from field to field, looking for asterisms or bright stars to verify that you are on the correct path, until you reach the object. 

 

Of course, make sure that SSP is oriented the same way as your telescope view.  Set the size of the display circle on SSP to match the view in your telescope and eyepiece.  Also, set the display circle to indicate N, S, E and W. 

 

For efficient star hopping, it is very important to closely align your finders to the view in your telescope.  Spend a little time on this step before you look for anything.  Align on Polaris.  Play with the RACI finder until Polaris is exactly behind the crosshairs.  When you turn on the laser finder, that beam should be visible in the RACI finder, pointing exactly at the center of the crosshairs.  Also verify that the laser beam shows at the center of a moderate power eyepiece in the telescope. 

 

My home is in a bright red zone.  I never use binoculars or a "finder eyepiece" to find objects for my telescope.  My main telescope is a 10" f/4.8 Dob.  So ... grin.gif

 

Mike


Edited by Sarkikos, 13 November 2018 - 09:33 AM.

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#33 Sarkikos

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Posted 13 November 2018 - 09:29 AM

For a finder eyepiece, with that as its main reason, the widest TFoV and brightest images the better. You're only looking for stuff, not studying it.

 

As an example, M57 in a 40mm will glow out as a tiny ring between gamma and beta Lyrae, rather than larger and fainter, with dimmer stars in your FOV.

And you don't even need to see the object in the finder.  This is where good charts - I like SkySafari Pro - come in handy.  Just center the location of the object behind the crosshairs of the finder.  You don't need to actually see the object itself.

 

Why limit yourself to objects that you can see in the finder?

 

Mike


Edited by Sarkikos, 13 November 2018 - 09:29 AM.


#34 Sarkikos

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Posted 13 November 2018 - 09:38 AM

Ahhh, I see.  Very long scope.   I would just add a push stick to the magnets so I could manipulate them more easily from the focuser.  A wood dowel would work well.  Include it in the duct tape wrap around the magnets grouped in whatever number you find convenient.  One dowel per group. 

 

Not trying to convince you of anything, but that is how I would do it. 

IME & IMO, the optimal way to balance a Dob is to balance it beforehand for the heaviest setup up you will use, which usually means the heaviest eyepiece.  This can mean placing a weight toward the bottom of the OTA, on the upper surface, and another weight toward the top of the OTA opposite the focuser and finder. Keep those weights in their locations during the entire viewing session.  You don't need to change or move them at all.

 

Then when you switch to a lighter eyepiece, place a weight near the focuser to compensate for the weight lost at that location.  That's all. 

 

You're moving a weight on and off the tube right where you are.  Nothing could be simpler or easier.  You might need to use a heavier weight, or a couple weights stacked, to make up for the weight difference between the two eyepieces.  

 

Of course, even better is to start out with a good mount with large altitude bearings.  But most consumer scopes are pitifully lacking in this regard.  Don't we know it.  grin.gif

 

Mike


Edited by Sarkikos, 13 November 2018 - 09:42 AM.

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#35 Sarkikos

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Posted 13 November 2018 - 09:45 AM

Not a problem at all Ed.  This is what the forum is for and I appreciate advice and suggestions - thanks!

 

It may not be apparent from the picture, but the tube is not metal either - it's sonotube cardboard.  The only steel is in the mirror cell and the various screws and such attached to that.  It's really not that hard to snap in my punch tool and remove/replace the magnets.  I can get a picture to post here soon.  A simple wedge or shim would also work I suppose, but the nice "click" I get from the punch tool lets me know it's in there solid.

Your tube doesn't have to be ferrous metal to use weights.  You don't have to attach them with screws.  Put Velcro on the weights and on the tube where you will place them.  The weights will be secured, but still easy to remove and move around.

 

Mike


Edited by Sarkikos, 13 November 2018 - 09:46 AM.


#36 stargazer193857

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Posted 13 November 2018 - 09:49 AM

Using a 30mm 82 deg eyepiece sure would give bright wide views. But it would be heavy too, even just carrying it in the pack. And the counterweight.


There is an alternative to weights though. Balloons. And adjustible monopods.


I think the best cure for balance issues is to just aim the scope higher when using low power.

Edited by stargazer193857, 13 November 2018 - 09:52 AM.


#37 Sarkikos

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Posted 13 November 2018 - 10:33 AM

I think the best cure for balance issues is to just aim the scope higher when using low power.

That's like the guy who was looking for something under a lamppost at night.  A passerby asked him what he was looking for.  "My keys."  "You dropped them here?"  "No, over there.  But the light's better here."

 

grin.gif

Mike


Edited by Sarkikos, 13 November 2018 - 10:35 AM.

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#38 brentknight

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Posted 13 November 2018 - 03:46 PM

Your tube doesn't have to be ferrous metal to use weights.  You don't have to attach them with screws.  Put Velcro on the weights and on the tube where you will place them.  The weights will be secured, but still easy to remove and move around.

 

Mike

I really don't want to put any more sticky stuff on my tube.  I recently pulled the double-sided tape off my old Telrad finder base - it left "most" of the black paint still on the tube.


Edited by brentknight, 13 November 2018 - 03:47 PM.


#39 brentknight

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Posted 13 November 2018 - 04:01 PM

<snip>IME & IMO, the optimal way to balance a Dob is to balance it beforehand for the heaviest setup up you will use, which usually means the heaviest eyepiece.

 

Mike

 

When I did that for my ES30, all of my other eyepiece combo's were out of balance.  I decided this was kinda a backwards way to approch this problem since 90% of the time I use 1.25" eyepieces that balance just fine on the scope without ANY weights.

 

My solution (at this point) is to leave the tube balance as it is.  When I want to use the heavier (normally 2" eyepieces), I lock the altitude bearing so it won't slip, put about 8 magnetic weights at the back of the tube attached to the mirror cell screw, pop-in the larger eyepiece, and finally remove the lock on the AT bearing.  Same process when swapping back in the smaller eyepieces except I pull the weights off before removing the AT lock.  So far seems quick and easy-peasy...

 

The biggest challenge with this setup though is removing the lock and weights without accidentally moving the telescope.  Removing the lock is not usually much of a problem, and I discovered that if I just quickly pull the weight "pack" straight back off the cell screw - I get minimal movement.


Edited by brentknight, 13 November 2018 - 04:10 PM.

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#40 pgrunwald

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Posted 13 November 2018 - 04:32 PM

https://smile.amazon...e?ie=UTF8&psc=1



#41 aeajr

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Posted 13 November 2018 - 04:43 PM

IME & IMO, the optimal way to balance a Dob is to balance it beforehand for the heaviest setup up you will use, which usually means the heaviest eyepiece.  This can mean placing a weight toward the bottom of the OTA, on the upper surface, and another weight toward the top of the OTA opposite the focuser and finder. Keep those weights in their locations during the entire viewing session.  You don't need to change or move them at all.

 

Then when you switch to a lighter eyepiece, place a weight near the focuser to compensate for the weight lost at that location.  That's all. 

 

You're moving a weight on and off the tube right where you are.  Nothing could be simpler or easier.  You might need to use a heavier weight, or a couple weights stacked, to make up for the weight difference between the two eyepieces.  

 

Of course, even better is to start out with a good mount with large altitude bearings.  But most consumer scopes are pitifully lacking in this regard.  Don't we know it.  grin.gif

 

Mike

Sounds like a reasonable approach.  Let me walk through this so I understand.

 

When  I got my Orion XT8i  I did not have 2" eyepieces or a 2" barlow.   I would have balanced it for the 26 mm plossl that came with it.  I presume that is how they set it up at the factory.  And, until I added 2" eyepieces I never had a balance issue.

 

Now I have a 2" Meade 20 mm 82 that weighs 26 ounces.   I sometimes drop this into a 2" GSO barlow that weighs 13 ounces.  39 ounces total. 

 

So I presume you would advise that I balance the scope for that 39 ounce load.   This would involve adding weight to the mirror end permanently.   Would that be about 39 ounces?   

 

When I remove the 39 ounces for the eyepiece and the barlow and drop in a 10 ounce ES 82 8.8. and 1.25" adapter, I would hang a 29 ounce balance weight off the front of the scope?   

 

Is that how it works?   

 

Now, when I have my 38 mm 70 degree, 21 ounces, I would remove about 21 ounces of balance weight from the front of the scope.

 

Do I have it right?



#42 brentknight

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Posted 13 November 2018 - 05:26 PM

<snip>Sounds like a reasonable approach.  Let me walk through this so I understand.

I think I like the balloons and monopole idea better...  What kind of balloons work best?


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#43 Sarkikos

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Posted Yesterday, 07:47 AM

Sounds like a reasonable approach.  Let me walk through this so I understand.

 

When  I got my Orion XT8i  I did not have 2" eyepieces or a 2" barlow.   I would have balanced it for the 26 mm plossl that came with it.  I presume that is how they set it up at the factory.  And, until I added 2" eyepieces I never had a balance issue.

 

Now I have a 2" Meade 20 mm 82 that weighs 26 ounces.   I sometimes drop this into a 2" GSO barlow that weighs 13 ounces.  39 ounces total. 

 

So I presume you would advise that I balance the scope for that 39 ounce load.   This would involve adding weight to the mirror end permanently.   Would that be about 39 ounces?   

 

When I remove the 39 ounces for the eyepiece and the barlow and drop in a 10 ounce ES 82 8.8. and 1.25" adapter, I would hang a 29 ounce balance weight off the front of the scope?   

 

Is that how it works?   

 

Now, when I have my 38 mm 70 degree, 21 ounces, I would remove about 21 ounces of balance weight from the front of the scope.

 

Do I have it right?

Keep in mind that the moment of inertia of a weight will be greater the farther it is from the fulcrum.  In other words, the farther the counterweight is from the altitude bearing, the greater will be its force to move down that end of the OTA.   There is a formula for this, but I just figure it out by trial and error. 

 

For the weight to balance the OTA longitudinally, you want it as close to the bottom end of the OTA as possible and convenient.  This will minimize the weight required. 

 

I put plastic bags of lead shot in a fanny pack (hip bag, whatever they call it) that I strap around the OTA near the bottom.  The weight is on the lower surface of the OTA.  There is an opening in my Dob mount that the weight fits through when the OTA is pointed toward zenith.  I position the fanny pack so that the weight swings through the opening in the Dob mount.  There is not enough room below the OTA to attach any weight to the bottom of the mirror cell.  I don't think I'd want to do that in any case.

 

For transverse balance, I strap a smaller fanny pack of weights around the upper part of the OTA.  I position this opposite the focuser and finder complex.  

 

My 10" f/4.8 Newt is in rings instead of permanent trunions, so I can move the OTA up and down in the rings.  I positioned the OTA as far down as possible without it hitting the bottom of the Dob mount.  Then I placed my heaviest eyepiece in the focuser.  Now I began experimenting with different weights in the two packs until I achieved balance both longitudinally and transversely.  

 

Finally I attached a patch of Velcro on the OTA near the focuser.  I keep a small bag of weights attached to a corner of the Dob mount.  When I switch from a heavy eyepiece to a light one, I reach down, get a weight (or two) from the bag and place it on the Velcro patch.  This weight compensates for the weight lost when switching to the light eyepiece. 

 

Thus balance is maintained without having to move any weights up and down the back of the OTA.  I merely attach or remove a small weight on the OTA near the focuser, right where I am.  I don't have to stand up and reach around the back of the OTA.  I don't even have to get up from my seat.

 

My Dob mount does not have springs or any other kind of altitude detention device.  If you have such a device on your Dob mount, maybe you can get away without having a three-counterweight system.  But the motion on my Dob mount is very smooth because it is very well balanced. 

 

Even without the longitudinal and transverse counterweights, it would probably still be a good idea to use a weight near the focuser to compensate for switching between eyepieces that have a large difference in weight.  This really does make more sense than sliding a weight up and down the upper surface of the OTA.

 

Mike


Edited by Sarkikos, Yesterday, 11:36 AM.


#44 aeajr

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Posted Yesterday, 07:55 AM

snip...

 

Thus balance is maintained without having to move any weights up and down the back of the OTA.  I merely attach or remove a small weight on the OTA near the focus, right where I am.  I don't have to get up from my seat.

 

snip...

 

Mike

Thanks for explaining the system you use.   Glad it works for you.



#45 Sarkikos

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Posted Yesterday, 08:02 AM

I really don't want to put any more sticky stuff on my tube.  I recently pulled the double-sided tape off my old Telrad finder base - it left "most" of the black paint still on the tube.

I've never had double-sided tape take paint off any of my OTAs or mar them in any way.  I use Goo Gone to loosen up the tape before I remove it.  

 

Years ago, there was a member of CN who removed sticky tape from his telescope by scraping it with a metal spatula.  Needless to say, the result was ugly scratches on the OTA.  Obviously the wrong tool for the job!

 

Mike



#46 Sarkikos

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Posted Yesterday, 08:03 AM

Thanks for explaining the system you use.   Glad it works for you.

Yes, it works for me.  It would work for others, as well.  It has to work.  Physics is physics.

 

:grin:

Mike



#47 brentknight

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Posted Yesterday, 11:00 AM

I've never had double-sided tape take paint off any of my OTAs or mar them in any way.  I use Goo Gone to loosen up the tape before I remove it.  

 

Years ago, there was a member of CN who removed sticky tape from his telescope by scraping it with a metal spatula.  Needless to say, the result was ugly scratches on the OTA.  Obviously the wrong tool for the job!

 

Mike

Taking tape off cardboard is a little more challenging.  I used a heat gun to losen the glue.  In one or two parts I got it a little too hot - my bad.



#48 Sarkikos

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Posted Yesterday, 11:37 AM

Right, a cardboard OTA would complicate things.

 

Mike




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