Goto Dob Balance

[Widefield]

I have started to try to balance my GOTO dob for use with 2" eyepieces. It is quite stiff to move manually (I assume that is because of the GOTO - it tracks fine so it seems OK) so it is actually quite hard to tell when it is in our out of balance just by moving the tube. It certainly does not drop at the front when you put the larger eyepieces in and the little extra force you need to apply to move it makes it quite hard to tell if its balanced or not - certainly anything close to fine balance is going to be very hard to judge.

In that situation should I just be looking at how it tracks to see if its out of balance or is that potentially going to damage the motors/bearings if there is a gross misbalance there? Is this something others have experienced?

[SeedyF]

I have the 250 flextube goto dob. I notice the balance is not right when I try to slew up and the scope doesn't move! But generally if I am adding anything odd to the end of the scope - and by odd I mean heavier than normal, I put a magnetic weight on the bottom of the scope. So when I put my solar filter or webcam on it's one of my weights (welders magnets) and if it's the SLR its two magnets!

I haven't noticed any tracking issues. I do planetary imaging at high magnification and keep the object centred for 3 mins with no problem.

[tteedd]

Hi SeedyF

I have the same 250 flextube goto Dob. Aquired recently 2nd hand from Astronomia.

It had difficulty increasing in azimuth when I first got it but this went away when I replaced the finder with a lighter R angle model.

However I bought a wide angle lens at the IAS and, this being heavier, my problems came back.

On testing it I found the scope to be nose heavy without any EP's fitted.

I am at present awaiting Astronomia's comments.

Interesting to know that others have the same problem. I looked at some ordinary Skywatcher Dobs for comparison but they all apear to be perfectly ballanced.

I have put some weights on the screws at the back of the mirror. This has solved the problem at the cost of dragging the collimation out. Soon sorted but I need a permanant solution.

Tteedd

[JRWASTRO]

My solution.

Firstly I balanced the dob on a separate jig with the heaviest and lightest eyepieces in place and divided the weight into 3 portions:

I then melted some lead and poured it into a home made mold and when everything had cooled I attached a powerful neodymium magnet to the lead billet. I can provide detailed drawings.

3 weights are placed displaced by 120 degrees and this improves the balance considerably.

The third weight can be slid up and down the OTA depending on what accessory is in place. BTW the bottom of the weight is lined with a piece of chamoise leather. 3 pictures follow

I hope this is of use.

Jeremy

[tteedd]

It does not seem like 3 years+ since this conversation. I was prompted to revisit it after reading the latest (Feb 18) Sky at night - 'Scope Doctor'.

Part of my response to Steve was:

My solution was to purchase two magnets of the type used to stick signs to taxi cab roofs (Ebay).  These are of sufficient weight to place on the rear of the OTA and move to the point where the rig is balanced.

 

I have also found the bolt on the top of these magnets extremely useful as it allows you to attach other instruments to the OTA without damage or modification and putting them at the position where the rig is perfectly balanced.

 

It seems appropriate to copy it here and thank Seedy & Jeremy for starting me in the right direction.

 

Ted