How to eliminate altitude play in a dob

[Shashikiran Kolar]

Hello,

A newbie question.

I would like to minimize the loss in altitude that occurs when I rotate the OTA of my 8-inch DOB about its altitude bearings. When I have an object centered in the cross-hairs of the finderscope, I would like to let go of the OTA and have it not slip to a lower altitude. Currently, I somewhat over tighten the clutch knobs and point the finderscope to an area a little higher than the intended one, let go the OTA and gently guide it with short down-strokes to the intended target.

Also, once at the target, I find that OTA loses altitude in time (particularly with heavy eyepieces). How can I eliminate this play and bring in a smoother experience to my viewing?

Thanks for any suggestions.

-Shashi

[Ajay Talwar]

Hello Shashi,

A true Dobsonian telescope has the unique feature to avoid this very problem. When you move a Dobsonian, the movement is smooth because of the use of altitude teflon pads. When you stop moving the dobsonian, it stops right there, because it is balanced. There are no locks and tightening knobs in a true dobsonian.

In your case the telescope is unbalanced towards the eyepiece side, and you have to tighten the knobs. Ideally you should not have to use the tightening knobs.

There are several solutions to this problem:

1. If there is a possibility of sliding the telescope tube towards bottom, this will solve your problem immediately.

2. You can install a small weight towards the bottom of the telescope to balance. Think velcro!

3. You can increase the angle of the teflon pads on the side bearing to increase friction, so that slight imbalance does not move the telescope. Let me explain - if you take the centre of the side bearing, and draw a vertical line, then draw a line from the centre of the side bearing to the teflon pad, that is the angle I am talking about. You can increase the angle by 5°-8° (on both sides, i.e. 4 pads) to increase the friction with the side bearing. When I say increase the angle, I mean bring the teflon pads higher, but maintain the same distance from the centre of the side bearing. You have to do this quite accurately. If you make the pads on one side lower then the other side of the telescope, then the telescope tube will be tilted w.r.t. the rocker assembly and will touch the side board.

4. You can install a sliding weight on the bottom part of the tube. (think velcro at 2-3 places) This will have the added benefit that if you change eyepieces, then you can just slide the weight a bit and get the telescope balanced perfectly, as ideally you should not be using the tightening knobs at all.

Ajay Talwar

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[Shashikiran Kolar]

Hi Ajay,

Excellent, thank you.

Yes, the OTA can be offset with the altitude bearings on my Dob, and I will try this tonight (#1 below). I'm not sure this is a scalable/user-friendly method and I wonder why the manufactures do not include a sliding counter-weight mechanism (#4 below), or a variation, as standard.

#3 below will not work for me as the OTA does not have the standard bearing-on-teflon-pads seating, but a "sealed ball-bearing" one (picture also attached, sourced from Tejraj). But, for the sake of completeness, should not the teflon pads be bought lower (i.e. towards the bottom of the semi-circular cutouts in the rocker assembly) to increase friction (since the weight of the OTA that falls on them is increased)? I guess this is also why the tensioner spring works: pulling the OTA harder against the teflon pads to increase friction.

I think I will end up with #4 as a long-term solution.

Thanks for the help.

-Shashi

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