G’day, I am thinking of getting a new focuser for my Australian geographic 150mm x 750mm Newtonian reflector, the focuser on it is very clunky, so much that the collimator laser moves a good 10mm when going in or out.
Would I be wasting my money modifying such a cheap scope or could it make enough difference for a while until I can afford a new scope?
Googled it, what scope as nothing comes up, a photo perhaps?
many cheap scops can not have parts replaced
Never heard of the scope, my only advice is when considering a scope look to the future and buy one that is a leading make that accessories are made for, also as most people use 2" and 1.25" eyepieces get scopes that take both
You don't say if "Australian geographic 150mm x 750mm Newtonian reflector" is a scope or the shop
I have a similarly modest focuser on an inexpensive scope I have. I too found the draw tube had way too much movement. Modifying it is not difficult to improve its action and reduce this play.
In a nut shell, the hollow in which the draw tube moves through, it is too large, and the spring mechanism that holds the pinion may not be adjusted enough to reduce the play the draw tube has. There are two things you can try.
* First thing to try is to SLOWLY tighten the screws of the tensioner cover plate. It will make the draw tube a little harder to wind in and out, but it may be all you need to do. Do LITTLE, 1/8th turns of one screw, wind the focuser in and out, checking for reduced play, and if not enough, then tighten the other screw a TINY 1/8th of a turn, and repeat this process. Just be careful not to over tighten as this can damage the spring inside the cover plate. The action of the focuser should at most be firm but not stiff.
* Second is doing a modification to the focuser if tightening the tensioner spring is not enough. In this mod, you will need to remove the draw tube from the focuser, and dismantling the tensioner of the pinion so the draw tube can be pulled out. Just keep note of how the tensioner spring is put together when you remove the cover plate so you know how to put it back together. Do all of this with the scope lying flat on a table, NOT on its mount!!!
With the draw tube removed, you place a strip of adhesive backed Teflon tape down the whole length of the draw tube on the side opposite the rack. This tape will provide the necessary filler to reduce the gap between the draw tube and the frame of the focuser. This tape is approx 1/2mm thick, which is plenty enough for this mod.
Replace the draw tube into its frame and reassemble the tensioner assembly. You can carefully tighten the cover plate screws so that the pinion pushes harder against the rack, and hence the draw tube up against the focuser frame. I just needed the one thickness of tape to remove 90% of the play the draw tube had before. The action of the focuser is also a little firmer, but that's OK as it is needed to reduce the play in the focuser.
I'm happy to send you a piece of this Teflon tape is you like. Just tell me how long your draw tube is.
The pic below is a simple diagram on what needs to be done.
I hope this helps,
Alex.
Last edited by mental4astro; 21-07-2019 at 10:39 AM.
Not plumber's tape. That stuff is not made for working as a skid.
The stuff I mean is more like Teflon coated fabric. Has one side with adhesive that is attached to the draw tube. The yellow 'wax' paper backing comes off the adhesive surface. See pic.
I had a quick look for the stuff I have here at home, but it's not where I thought it was. Will need to have another look for it. What I have was given to me as an off-cut.
Thought I should join in this thread about shimming the cheap focuser drawtube with the adhesive teflon tape, as I have done it to my 150mm f8 Saxon newt. It had a horribly wobbly focuser which spoilt an otherwise very good scope and though the drawtube is a little bit tight now the image doesn't move halfway across a low power field when changing focusing direction.
I think I may have given Mental the tape he has as I used it a lot in my area of employment a few years ago and always had scraps left over that would otherwise been thrown out.
As Mental also said, if you want some tape, it is quite expensive to buy, send me a P.M. and we can work out a deal. Just the cost of postage that is.
I thought maybe as such.
