TS monorail spring-like action: how do I eliminate it?

[Draconid]

Hello,

This is my first real topic here on SGL, but it is about a problem that's been bugging me for a while...

I have a TS monorail focuser 1:10 (fine focusing) for my Skywatcher skyliner 250px dobson and it has an annoying "feature" : if you turn the fine focus knob, it will go back a little bit after releasing, just like some kind of spring action is pulling it back. Although this action is small, it is very annoying during planetary imaging. (an electronic focuscontrol is in development, meanwhile I use hand focussing)

While turning the fine focus, the main focus knobs are also turning so I don't think it is problem of the fine focus mechanism. The action gets worse when the tension screw is more tighten.

Given that the focus tube is rolling on a lineair ball bearing with many small bearing balls, I don't get what could mechanically go wrong, except that maybe the ball bearings have flattened.

Does anybody has an idea what could cause this?

Any help is highly appreciated.

[Mike Hawtin]

If you remove the coarse focus knob (not the one with the fine focus) you will see that there is about 2mm clearance between the knobs smallest stepped diameter and the part of the focuser assembly it fits into.  What is happening is that when you turn the focus knobs the focuser spindle is actually pivoting due to this excessive clearance which gives you that springy feeling, as you stop turning the tension on the spindle is released and it unwinds.  Just look at the spindle move when you turn the focuser and you will see what I mean.  This is a very badly designed item.

Hope this helps.

[Macavity]

These focusers, although excellent, do seem to require a bit of tweeking to get the best out of them.

I slackened off various (most?) of the screws and bolts, "wiggled things", and re-tightened them...

Any unevenness and slight binding of the "monorail" is now gone, still with zero side-play.

If you get fed up with designing / making an electric focuser, I do rather like TS Motofoks!

http://www.teleskop-express.de/shop/product_info.php/info/p4851_Rigel-Systems-nFocus---DC-Motor-with-controller-for-2inch-and-3inch-GSO-M-LR.html

Though I notice they've become "fatter" now? -- Perhaps not quite as neat as they were... 

[Draconid]

Thanks Mike, I will look into the thing you mentioned.

I did noticed that there is some play on the spindle if you pull down on either side on the focuser knobs. Maybe a custom-made bush is the answer to this.

[Mike Hawtin]

No problem, one other thing I forgot to mention concerns the tension screw, if you need to hold a heavy load without slippage be very careful how much tension you apply as the focus knob housing has a tendency to fracture under pressure from the screw.

[PeterW]

That explains things, looks nice, but not as good as it could be function wise!

Cheers

Peter

[Draconid]

Just had a look at the focuser and it's indeed a wobble the spindle has when turning because there is no proper support on the lefthand side. (righthand is the side with the fine focus knob. )

The only thing I can think of is to find a small bearing or bush  to fit into the hole where the lefthand knob is.

It is realy a shame that this focuser has such a design fault, I wonder how other manufactures have this sorted...

[alan potts]

I am glade i read this thread, I have two of them on various scopes and whilst only visual it would be nice to put right. Tend to agree, bad design but look at the cost of Moonlite and Feather touch, same as the scope.

Alan.