Balancing a Dob for high power

[NorthernLight (Max)]

Hello Everybody,

I recently bought a Dob and have some trouble with high power observing as no matter how tight I make the clutches either side - the scopes keeps moving upwards when viewing Jupiter.

I adjusted the studs on either side and found equilibrium for a 45 deg angle of the scope with my high power barlow/EP assembly but when I go for Jupiter (being not 45 deg) it still moves upwards.

What is the trick? Or do I need some sort of counterweight mechanism?

[RB (Andrew)]

Some people use various sized magnets stuck onto the tube to gain balance.

[mswhin63 (Malcolm)]

Is the drift from the mount tilting or the earth rotating. Earth rotation is a issue when viewing high power and usually requires a motorised mount to keep it in track.

[NorthernLight (Max)]

wow, that was quick.

it is definetly the scope moving, I can see it going up.

What sort of magnets are people using? The thing is that most available magnets are flat, hence the connection is just a single tangetial point and that isn´t going to stay in place. I am not keen on getting magnets manufactured, was more looking for commercially available stuff that doesn´t cost the world.

Has someone ever tried magnetic vinyl sheets? How many can be stacked before adhesion is lost?

[RB (Andrew)]

I don't have a dob, but I have seen people use round flat magnets of suitable size to combat their balance issues.
It may be a minor 'weight' but it's enough to bring it into balance, and the beauty is that you can place them anywhere needed on the tube.

Someone who has used them hopefully can add more.

[Robh (Rob)]

Max,

I have five old 6 cm diameter speaker magnets, roughly 300g each = 1.5 kg total. They don't sit well on the tube and tend to slide around. I stuck them on the frame in the bottom of the tube as the metal parts of the cage are exposed. This also gives you the maximum radius to fulcrum distance.

You must have added some extra weight to the upper tube for this to happen. In my case I replaced a lightweight red dot finder with an 8 x 50 finderscope. I'm surprised it moves with the clutches tightened. Is there some way you can adjust or modify these?

Just had another thought. My scope drifted downwards with the extra weight up top. Yours is drifting upwards because the barlow and EP are too far off axis. Can you try a shorter barlow (e.g. Orion Shorty Barlow) if you are using 1.25 inch EPs? Or just get a lower focal length EP and don't use the barlow.

Regards, Rob

[NorthernLight (Max)]

Hi Rob,

I added a 5x powermate and a 25mm plössel - all 1.25" barrel (scope initially adjusted for 30mm 2" barrel GSO EP). I would have expected that the tube is rather going down due to the increased weight.

The clutch-knobs on either side are both tightened as fast as I can reasonably do (did the same to the centre screw on the base). I would have thought that once all is tightened no movement would be possible but the scope is still moving at the slightest touch.

Now, with the magnets - I am not sure if I got you. Did you attach the magnets to the outside of the mirror cell?

BR

[Robh (Rob)]

Originally Posted by NorthernLight

Hi Rob,

I added a 5x powermate and a 25mm plössel - all 1.25" barrel (scope initially adjusted for 30mm 2" barrel GSO EP). I would have expected that the tube is rather going down due to the increased weight.
...

Now, with the magnets - I am not sure if I got you. Did you attach the magnets to the outside of the mirror cell?

BR

Max,

The scope is most likely moving up because of the increased weight off axis due to the long barlow + EP at high angles. If it was just a weight problem the tube would also tend to drift down at low angles. Does it do that as well?

The magnets stick themselves to the metal frame holding the mirror. I have a 12 inch DOB as well, so there is plenty of space for them.

Rob

[NorthernLight (Max)]

Rob,

I actually haven´t tested the possible downwards movement when pointing lower than 45deg but will do next WE.

I think I now see where you are coming from: you increase weight at the back and take it off when necessary. I think I have some old speakers in the house that aren´t used anymore (apart from the housing-to hold other speakers ). Good thinking: thanks for that, much appreciated!!!

Cheers

[Robh (Rob)]

Originally Posted by NorthernLight

I think I now see where you are coming from: you increase weight at the back and take it off when necessary. I think I have some old speakers in the house that aren´t used anymore (apart from the housing-to hold other speakers ).

Max,

Because your top weight (barlow+EP) is way off axis, you may have to stick the magnets on the outside of the tube at the bottom on the side opposite the focuser. At vertical, clearance with the DOB base could be an issue, so you may need to slide them to the side a bit. You could try putting some magnets in the bottom of the scope on the opposite side of the focuser first. Depends how much weight you need.

Good luck, Rob

[RAJAH235]

Hi Max,
You can't see exactly what I did to counter-balance my scope but directly opposite the
finder, on the outside, I attached/screwed a suitably sized lead weight.
Mine used to keep going "up" when it got to ~60˚, until I added it.
Only needed ~ ½ lb flat sheet, folded to about 2" x 2" square.
That....& the extra 4lbs I added to the centre & 2 lbs to the bottom of the t'scope base.
(But the finder & brackets are very heavy.)

Easy peasy.
Solid as....now.

See image > http://www.iceinspace.com.au/forum/a...owse.php?a=551

Also added an adjustable brake band to one side of the tube bearing, which you can just make out in the piccie..

[shane.mcneil]

Hi

My GSO Dob had a similar problem. I thought it was the locking mechanism but turned out the mechanism fit into the timber cut out of the base was sloppy. Everything moved. I packed it with more felt and fix it.

Shane

[desler]

I went for a walk and found when they were doing one of the hard waste collections and scored some old speaker magnets, to protect the tube I filed them smooth and then hot glued felt to them so as not to scratch the tube.

You may also find you get some flexure in the mount box, which can cause some odd movements at the eyepiece. I solved this with a couple of el cheapo clamps out of a hardware store sales bin. Worked a treat.

Good luck.

Darren

[jakob]

On my LB16 I use led blocks, with screw half way in. The protruding head hooks in the tube. The tube has holes to take the head of the screw. Wokes well for me.

[cahullian]

I glued some felt to the bottom of my magnets to minimize sctatching to the surface of the bod.

[NorthernLight (Max)]

thanks guys for all the input. I think I`ll take some old speakers apart, take the magnets out and glue some felt to them to use them as counterweights. If it works for you -it sure works for me.

[jenchris (Jennifer)]

Why not use an adjustable strap with a bag of fishing weights? then you can adjust it as neccessary.
The weight placed the same distance from the bearing as the mirror is (on the opposite side of the bearing of course) will enable a state that stays in balance. Once you know what that is in weight, you can make up a nice neat counterbalance pod that won't ugly you out every time you see it.
My camera and chunky diagonal need 1.6kilos to balance on my SCT, it hangs like a little scrotum under the front of the OTA. Looks ever so cute...

[NorthernLight (Max)]

Well, I ended up using two magnets that usually hold the big advertisment in a supermarket on the (metal) ceiling in conjunction with a kitchen towel so they don´t scratch the tube. for the very lightweight 4mm plössl I added a 13mm double screw-wrench. And what can I say - it´s beautiful in terms of balance.

Only draw back now is the yellow tinted mirror but apart from that: gosh, 12" is such a difference when you come from 6!!! Even the moon makes more fun.
The only things I now need is a complete set of Naglers and one or two zooms...and a new and much better paid job!