Sloppy focus tube, what to do?

[Ro3bert]

My Celestron 114LCM has a very (to me) focus tube. When it is mostly to completely in it is very stable but when it is out where the 25mm EP focuses it begins to be wobbly. The tube doesn't fit snugly in the outer tube at that area.

This can be very annoying when collimating or viewing. It also means the tube is slightly offset from perpendicular to the line of the secondary mirror. Too, when focusing it will wobble a bit and it is very difficult to get sharp focus (hard to explain).

I'd like to do something to snug it up a bit. Thought of wrapping the far end with a thin sheet of something or strips along the inside of the outer tube but don't know what might work.

Has anyone modified this focus tube of the outer tube to address this situation?

Then on the other hand it may be almost a non-issue. I don't know but it seems to me something should be able to be done. I like Cellestron but China made?

[Alien 13]

I have often used aluminium cut from a coke tin for this purpose you can add amall tabs to hold it in place.

Alan

[Ro3bert]

I have often used aluminium cut from a coke tin for this purpose you can add amall tabs to hold it in place.

Alan

Thanks Alan for the quick reply. I can't believe the response time on this forum. I'm a member of two other astronomy forums and while they are just as friendly the response time seems to be very slow.

I'm confused, isn't the aluminum a bit thick? I just can't get my mind around how you use it and where or how the tabs work?

Any images showing how you do it? I had the idea of some sort of thin material to wrap around the furthest end of the focusing tube.

Maybe I'm not thinking outside the box.

[Dave In Vermont]

I think he's describing a simple shim one inserts into the tube to take up any space - making side-to-side motion impossible. You could also fold a piece of aluminium-foil to do the same thing. The idea being to make the fit in the draw-tube snug.

Burning boxes a specialty -

Dave

[ronin]

Unfortunately when assembled they are more or less thrown together and then it is on to the next one.

There are guides to improving them, usually means pulling one apart, cleaning, regreasing and reassembly with all adjustment made better.

When wound fully out there will not be a lot at the main focuser body end to prevent some slop.

There is also generally adjustment screws underneath that adjusts the tension and locks the focuser - it may be simply a need to adjust one of these to get a small improvement.

If I recall on the FLO website (top of each page) they have a guide to improving the Skywatcher Crayford focuser and Synta = Skywatcher + Celestron.

There is also this: http://polluxchung.com/gso_crayford_fix/index.html

Which is for a different focuser but the principles are similar.

[ronl]

I think this guide could be helpful http://www.astro-baby.com/Skywatcher%20Focuser%20Tune%20up/Skywatcher%20Focuser%20Tune-up.htm

this looks similar to the Celestron focuser.

That's if it's a rack and pinion and not a Crayford.

[Ro3bert]

I think this guide could be helpful http://www.astro-baby.com/Skywatcher%20Focuser%20Tune%20up/Skywatcher%20Focuser%20Tune-up.htm

this looks similar to the Celestron focuser.

That's if it's a rack and pinion and not a Crayford.

I is very similar to the Celestron. I've already had it apart to re-grease it but wasn't sure how to get the slop out. Now I will see what I can do with it. Images of the process I use will follow.

[ronl]

You just need to pad out as shown in the link, I did my frac and it worked really well, put felt

were it shows in the guide, and on the tube at the top at the far end, if you need to, put some

on the sides as well it, you need to try it first before doing the sides, I just did the bottom and

top, no slop at all now.

[Alan64]

I added self-adhesive felt to a 60mm refractor focusser earlier this year...

The knob-and-pinion assembly must be removed, the drawtube removed, then to line the entire inner length of the focusser body where the drawtube is inserted with the felt.  Three narrow lengths placed as shown did the trick.  I also shimmed the 1.26" visual-back with a thin sheet of clear blue PVC to convert it to a 1.25".

I used red felt for the second and final attempt.  It is now utterly slop-free, even when extended out to a rather surprising length.

[Alfian]

I had a similar issue with my "old" Celestron 130. When the focuser was stripped down, one of the nylon (?) guide strips had come adrift and was at an odd angle and the third was totally absent. I stripped the guides out, cleaned the sticky glue off and cleaned all surfaces. I replaced the guides with some made of a kind of fairly dense felt (supplied from my good wife's textile spares box) and glued them in place with contact adhesive. These were then almost impregnated with good quality lithium grease, with any excess carefully removed. Once reassembled the focuser was understandably much stiffer, but once "run-in" it was a vast improvement and even now I am surprised just how smooth it is.

[Ro3bert]

I'm going to post another thread with images of the work. Don't think it will ever become a sticky but maybe it will help someone.

Unfortunately the slop was not completely eliminated but it is much better than before.

I don't think the slop has anything to do with sloppy manufacturing, it has more to do with inadequate design and engineering.

[Alan64]

If felt proves to be too thick, a large shim of thin sheet-brass can be cut and formed to fit the focusser's drawtube cavity whilst ensuring a cut-out or space for the rack... http://www.hobbylinc.com/k-s-brass-hobby-and-craft-metal-sheets-metal-strips

It can then be glued to the cavity's interior.  The installation would have to be smooth and precise so as not to, oppositely, bind the drawtube.

I eliminated the slop in the azimuth portion of said refractor's alt-azimuth mount, but it required considerably stiffer phosphor-bronze instead...

The improvement turned out as expected, and with utterly smooth and slop-free rotation in the azimuth.

For the focusser, brass or aluminum sheeting would suffice.