Jay Emirzian
member
Reged: 11/04/05
Posts: 13
Loc: Wilbraham, MA
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Hello,
I have a quick question for everyone........ How much counterweight have you needed for your Dobsonian mounted Newtonians to keep eyepieces, encoders, finders, etc. from tiping over?
Thanks much,
Jay
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Jon Isaacs
Postmaster
Reged: 06/16/04
Posts: 12070
Loc: San Diego, California
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Jay:
I think that really depends on the particular scope and how it is balanced. My GSO 10 inch F/5 with the Correct Tension springs handles an 8x50 finder plus a telrad plus a Paracorr plus a heavy eyepiece without a problem.
I have built two bases for DOBs and made sure the scopes were mirror end heavy to that I didn't run into balance problems. So far it has worked though I figured my 12.5 inch Discovery to use about a pound on the focuser end for safely. Since one pound on the focuser end takes 3 or more on the mirror end, making the mirror heavy makes balancing easier.
Some DOBs use adjustable altitude bearings to handle this problem. The old Celestron Starhoppers and many of the Discoveries are done this way and I find it works quite well.
jon
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Tom Karpf
super member
Reged: 11/18/05
Posts: 162
Loc: Newington, CT
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Quote:
Some DOBs use adjustable altitude bearings to handle this problem. The old Celestron Starhoppers and many of the Discoveries are done this way and I find it works quite well.
I'm a big fan of adjustable bearings. I built my dobsonian with adjustable bearings and a rotatable tube. Seems silly NOT to do either.
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Thomas Karpf
Secretary - Astronomical Society of Greater Hartford
8" f/6.8 homemade Dobsonian with home-made mirror
Dynamax 6" Schmidt-Cassegrain
ETX-90 RA Maksutov-Cassegrain
ETX-70 Achromatic refractor
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Bob W6PU
Carpal Tunnel
Reged: 12/23/04
Posts: 2191
Loc: Springer-N.E.NM
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Heads Up!
Regarding The Celestron Dob Base.
I purchased a new Celestron Dob base to remount my Treckerscope 10" OTA. The tension adjust on the altitude bearings, which appear to be plastic, use "Pressed In" threaded bushings.
After about two weeks of use, both of these pressed in threaded bushings were PULLED OUT of the plastic altitude hubs!
As you tighten the altitude tension adjust grips, you exert a pulling motion on these steel bushings that are pressed into the plastic hubs, IMO, this is a Very Poor design!
Jay, I mounted a adjustable slider at the mirror end and under the OTA, where I use eight pounds of counter weight.
Bob
10" Coast Inst. Treckerscope Newt,circa 1956
32mm Vixen Plossl
21mm Stratus, 13mm Stratus, 17mm Hyperion
10mm Speers Waller, 7mm and 5mm BO/TMB
2x Shorty Plus, 2.5x Televue, 3x Coast Inst. barlows
Bob
Edited by Bob W6PU (05/05/07 12:00 AM)
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2457
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jay, a good rule is you need 1.2 pounds on the back times the f/ratio of the scope for each pound you add the front of a balanced scope.
so if you add a 3 pound bino on a f/4.5 scope you may need 16.5 pounds of counterweight.
(1.2*3)4.5 = 16.5
the larger your alt bearings are, the more they want to "stay put" even when out of balance, so if you are making your own dob, dont put little alt bearings on like you see on the import scopes. put the largest you can but at least 1.3 times the size of the primary mirror.
a 20" mirror gets 26" alt bearings.......
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Bob W6PU
Carpal Tunnel
Reged: 12/23/04
Posts: 2191
Loc: Springer-N.E.NM
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Balance seems to be much more effective if you add the back end weight UNDER the OTA, opposite the side of the eyepiece!
Bob
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Jay Emirzian
member
Reged: 11/04/05
Posts: 13
Loc: Wilbraham, MA
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Thanks everyone!!
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guyinthesky
sage
Reged: 04/13/07
Posts: 330
Loc: western ma.
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hi jay, im your neighboor up in goshen, near northampton. welcome ! john
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Zhumell 12" dob
celestron c100ed-r gt cg-5
meade 5" telestar reflector
just enuf naglers
astro-tech 66ed
celestron C9 1/4
oberwerk 100-45
ALVAN CLARK and SONS 3 inch refractor
canon 350d
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Jay Emirzian
member
Reged: 11/04/05
Posts: 13
Loc: Wilbraham, MA
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Thanks John!! Wish I had skies like yours here in Wilbraham.
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paul hart
scholastic sledgehammer
Reged: 06/01/04
Posts: 913
Loc: Lawn Guyland, New Yawk, Earth
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Hi Jay,
Sorry I missed you at NEAF, I was there saturday, and broke Sunday.
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Paul
Long Island, New York
Orion Starmax 127 Mak
TScope 14" f/4.7 dob w/ F80 finder
10" f/4.8 dob
Orion 10x50 Ultraview Binoculars
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Tom Karpf
super member
Reged: 11/18/05
Posts: 162
Loc: Newington, CT
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Interesting question. I'm currently struggling with the question of weighting as well. Counterweighting the mirror end seems silly to me.
I've tentatively decided on the following:
1) Balance the scope with your HEAVIEST eyepiece.
2) Attach a hook of some sort up by the focuser.
3) Put together bags (maybe eye hooks?) with counterweights to attach up by the focuser.
I figure this will make it MUCH easier to swap eyepieces without losing sight of the target.
Remove the big eyepiece and add the weight that matches that eyepiece.
Choose your next eyepiece and grab the weight that represents the difference between that eyepiece and the weight of the big eyepiece.
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Thomas Karpf
Secretary - Astronomical Society of Greater Hartford
8" f/6.8 homemade Dobsonian with home-made mirror
Dynamax 6" Schmidt-Cassegrain
ETX-90 RA Maksutov-Cassegrain
ETX-70 Achromatic refractor
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Jon Isaacs
Postmaster
Reged: 06/16/04
Posts: 12070
Loc: San Diego, California
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>>>I've tentatively decided on the following:
1) Balance the scope with your HEAVIEST eyepiece.
2) Attach a hook of some sort up by the focuser.
3) Put together bags (maybe eye hooks?) with counterweights to attach up by the focuser.
----
I don't think it is quite that complicated, or at least in my thinking it is not..
The ideal DOB action is not zero friction but rather zero stiction. This means it takes some force to move the scope but that the force required to move it and the force required to keep it moving are the same.
Some friction is necessary and desireable. Those big bearings that one sees, sometimes 16 or 18 inches in diameter, they actually increase the friction over smaller diameter bearings. Correct Tension devices do this as well.
So, in my view, the goal is to balance with the heaviest and the lightest eyepieces and have smooth balanced motion between the two axis's. This is easier do do with a big scope because the eyepieces are a smaller proportion of the total mass.
jon
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Jay Emirzian
member
Reged: 11/04/05
Posts: 13
Loc: Wilbraham, MA
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Hi Paul,
I'm surprised I didn't see you there. Barry and I were there all day. I to spent the whole piggy bank that day. It was worth it though, I got some good deals.
Jay
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jayscheuerle
Carpal Tunnel
Reged: 01/16/06
Posts: 2989
Loc: S. Philadelphia, PA
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Unfortunately Jay, most Dobs appear to be balanced for having NOTHING up front. On top of that, Chinese and other tube dobs have undersized altitude bearings that are more sensitive to weight variance. If you have a tube dob with available clearance on the base, you can move the bearings toward the focuser a few inches to avoid needing much weight in the back.
When my 12" LB is completely decked out with shroud, light shield, 9x50RACI Finder, Telrad or Green Laser, and a 2" EP, I have 5 lbs. of weights on the underside, and up to 5 MORE lbs. on top. A properly designed scope shouldn't need that much counterweight. Someday... I'll put larger altitude bearings on this thing and shift the COG forward to the proper sweet spot.. - j
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12" Green Goblin (trusser w/Protstar secondary and OWL refigured primary)• 6" f/5 Eero2 ball-scope • 6" f/5 Frankenscope • Garrett Optical 10x50 binos • Edmund 8" yoke-mounted red-tube reflector • Edmund 6" GEQ red-tube reflector (on loan to Dad)
Gone, but with lessons learned:
Skyquest XT8 • NexSTar 8i • Eeroscope 6" f/5 ball(sacrifice was not in vain) • Vixen ED80sf • Edmund red-tube 4.25" f/10 • Edmund Astroscan
Facts are stubborn things.
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Tom Karpf
super member
Reged: 11/18/05
Posts: 162
Loc: Newington, CT
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Quote:
Those big bearings that one sees, sometimes 16 or 18 inches in diameter, they actually increase the friction over smaller diameter bearings. Correct Tension devices do this as well.
Perhaps my bearings are a bit small. They're 8" diameter, for an 8" mirror.
I guess maybe I need to look at adding some springs to increase friction.
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Thomas Karpf
Secretary - Astronomical Society of Greater Hartford
8" f/6.8 homemade Dobsonian with home-made mirror
Dynamax 6" Schmidt-Cassegrain
ETX-90 RA Maksutov-Cassegrain
ETX-70 Achromatic refractor
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hokie1525
sage
Reged: 03/15/07
Posts: 340
Loc: Light Laden Northern Virginia
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I have yet to have any severe problems with balancing. If I add a large EP, I just tighten the tension bearing. Its still pretty smooth for tracking, as my biggest problems are with the azimuth bearings. I also have 2 fans on a plywood baffle with 9.6V battery packs to help balance things. But worst case, I guess you could add a couple of heavy magnets and maneuver them around when you switch EPs.
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Orion XT10i w/COL
20x80 Barska Binos
30mm Meade UWA (Big Boy!)
TV 12mm T4, 7mm T1
Baader Hyperion 17mm, 5mm
4mm WO/UWAN, GSO 2" ED Barlow
Crockett Park
River House
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Jon Isaacs
Postmaster
Reged: 06/16/04
Posts: 12070
Loc: San Diego, California
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>>>>Perhaps my bearings are a bit small. They're 8" diameter, for an 8" mirror.
I guess maybe I need to look at adding some springs to increase friction.
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8 inch bearings for an 8 inch mirror is about as big as they get... Have you played with the position of the Teflon pads? You can adjust the friction by moving the pad location, flatter/towards the center reduces the friction, more steeper/toward the top increases it.
jon
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Tom Karpf
super member
Reged: 11/18/05
Posts: 162
Loc: Newington, CT
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Quote:
Have you played with the position of the Teflon pads? You can adjust the friction by moving the pad location, flatter/towards the center reduces the friction, more steeper/toward the top increases it.
Aha! Thank you. I'll try moving the pads away from the center. This scope is still VERY much a work in progress (haven't even sanded anything yet), so it's not like a couple of more screw holes will even be noticeable.
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Thomas Karpf
Secretary - Astronomical Society of Greater Hartford
8" f/6.8 homemade Dobsonian with home-made mirror
Dynamax 6" Schmidt-Cassegrain
ETX-90 RA Maksutov-Cassegrain
ETX-70 Achromatic refractor
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guyinthesky
sage
Reged: 04/13/07
Posts: 330
Loc: western ma.
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here is my top secret counterweight  its the cheapest one ive seen, requires no holes drilled,you could use some paracord if you dont have the wire like i used. it has a soft pvc body that wont scratch the scope or base,it is instantly weight adjustable, it can put out a fire or be usefull in a survival situation, wash your hands, or fill your over heated radiator on the way back from a starparty. try THAT with a metal counterweight ! 
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Zhumell 12" dob
celestron c100ed-r gt cg-5
meade 5" telestar reflector
just enuf naglers
astro-tech 66ed
celestron C9 1/4
oberwerk 100-45
ALVAN CLARK and SONS 3 inch refractor
canon 350d
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jimmoscheck
Pooh-Bah
Reged: 01/08/07
Posts: 1058
Loc: Under the SE Michigan lightdom...
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Hi Jay,
Welcome to Cloudy Nights!
I believe you have a 15" Obsession, right?
I don't understand how DK can write that book and then every Obsession that I have ever seen needs counterweights to make them balance. I read and reread the chapter on balancing many times before I ever cut any wood for the mirror box. I spend a whole weekend in January doing torque equations longhand as I'm fairly computer illiterate and never have used a spreadsheet. As it is I got lucky and my scope balances perfectly with out any counterweight on the back end. I did have to move the mirror cell up one inch higher than my figures called for due to negecting to take into account the collimation bolt clearance above my azimuth encoder, but by doing this it ended up balanced perfectly. I do need to put some weight on the UTA for when I use 1.25" planetary EPs. I also keep my AN computer on a stalk instead of on the UTA which helps to keep the UTA weight down.
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Jim Moscheck
14.5" f4.3 home built Kriege stlye truss
Stevens primary
"A mind is like a parachute. It doesnt work if it's not open." - Frank Zappa
Edited by jimmoscheck (05/08/07 04:29 PM)
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