Can I improve on a basic rack and pinion focuser?

[Jasonb]

Hi all...

I have a beginner's scope, the Starwatcher Star Quest 130P. It comes with a simple rack and pinion focuser, with no adjustments that I can see, just two screws that hold the focuser wheel in place (and I suppose could be used to loosen it/tighten it a little). I've attached a pic.

I've noticed that there's some play in this focuser. Not in and out (i.e. in and out of focus) but side to side. If I put in something like a heavy Cheshire and bring the focuser out fully, the weight of the Cheshire causes the focus tube to tilt slightly to the side. 

So, I was wondering is there something simple I can do to help improve this? I'm not looking down the path of replacing the focuser with a better one etc., just wondering if I can steady it a bit. I know the type of focuser that comes with the Heritage scopes (Helical I think it's called) can be improved with some plumbers tape, but I don't know if that would work around the teeth of the rack and pinion focuser? It also has some grease in it, and I don't know if that would be a good idea to cover with tape? Could I put something inside the focuser tube itself to decrease its diameter ever so slightly to make it more steady?

Any advice appreciated, thanks!

[NGC 1502]

You’re on the right track with what you’ve already suggested.

To reduce the wobble with the focuser tube you could try the following.  If you could obtain some Teflon tape (low friction) or at a pinch you may improve things with clear tape like Sellotape.  Be careful not to let anything drop into the tube and damage the mirrors, carefully dismantle.   If you can position the tube horizontally with the focuser pointing down that’s safest.

You need to experiment with narrow strips along the length of the tube opposite the rack.  You may find narrow strips work best.  Reassemble and check, you may need several tries.  If you use too much tape the focuser could be locked solid.  When doing this it’s often a compromise, accept a tiny bit of slack rather than too stiff an action.

You could experiment with adjusting the screws next to the rack and pinion, again it’s trial and error, and a tiny bit of slack may have to be accepted.  If you lubricate the rack and pinion use just very tiny amounts of light grease.

I’ve sorted some absolutely dreadful focusers into something usable.  If you make it better not worse then consider that success 😁

Good luck, Ed.

 

 

[jonathan]

I would look at getting an A4 (or A5 if available) sheet of very thin polystyrene, it can be bought from model shops or online.  It will have a white translucent look to it, probably a fraction of a mm thick.  I've used this (sometimes building up one layer at a time) to shim moving parts that had too much slack in them, created a perfect fit.  It needs to have a tiny fraction of play for it to operate freely, but it will be barely noticeable.  The advantage of the polystyrene is that it has a very smooth and even surface, usually one side is matt and the other is shiny so you can decide which works best (I'd probably say matt); apply using double-sided sticky tape or a tiny amount of strong contact glue (not plastic glue though, it won't stick to the metal).

Another alternative could be a narrow (like 10mm) strip of very thin stick-on velvet type material, probably get this from a craft shop or haberdashery department.

[Jasonb]

Thanks for your replies, and sorry for the delay in responding myself. I think I'll definitely give either the tape or the thin polystyrene a go, just to slightly thin the diameter of the tube. I did tighten the two screws on the focuser, and it has made focusing 'tighter' and stiffer to move, but I'm uncertain if it's having any affect on the 'give' in the focuser tube.

Thanks again...

[Zermelo]

I have an identical focuser on my 150i.

I removed it recently when I flocked the OTA, intending to follow Astro-baby's tune up.  After disassembing it I decided not to bother. The grease in the rack already seemed to be the lithium type that she recommended, and there were three teflon strips guiding the draw tube rather than the single felt pad shown on the website. So perhaps Skywatcher have made some changes.

You could remove yours and see what you have. If the draw tube is loose then some thicker shim as described above could fix it. I don't think adjusting the screws on the pinion housing will help. As you say, that just makes the action stiffer (I had to slacken mine off before I used it, they were too tight).

Just take care when removing the focuser. There are three screws that have nuts on the inside, so best to do it with the tube horizontal, but also I would recommend not having the focuser vertical when you do it, just in case the nuts drop onto the secondary.

Like you, I decided against replacing the focuser on an inexpensive scope with an upgrade (I think you might still get a single speed Crayford for a little under £100). But I do have trouble finding precise focus sometimes, so I've split the difference and ordered one of these instead, to use in tandem.

[Chaz2b]

There is a post from 12th August 2019 removing slop from a rack and pinion focuser by ICR-2019 in Getting Started Equipment help and advice. I’m no good in using links....woe is me!

chaz

 

Edited February 1, 2021 by Chaz2b

[Jasonb]

Very helpful replies again, thank you! 

@ZermeloYep, I checked, and my focuser has what looks like three teflon strips as well. I do think that I'll add some more to tighten it up, the post @Chaz2b referred to seemed to have good success with this.

That Helical focuser looks interesting, so you're basically attaching the helical focuser directly to the focuser of the telescope, and using it as a fine focus and the main telescope focuser as a coarse focus? Definitely seems like a good cost-effective upgrade for only £31, thanks! How heavy is it, do you know?

First step is getting the default focuser sorted, I tried a local shop today but they didn't have an Teflon tape, reckon I'll get some online instead.

Edited February 1, 2021 by Jasonb

[Zermelo]

That was the plan, yes.

I don't know how heavy it is (still en route), but I imagine not significant compared with the weight of an EP.

[Jasonb]

3 hours ago, Zermelo said:

That was the plan, yes.

I don't know how heavy it is (still en route), but I imagine not significant compared with the weight of an EP.

Ah sorry, I thought you had it already. Hope it arrives for you soon, let me know how you get on with it.

[DaveL59]

I used this stuff or one like it to sort the slop in the focuser of the LT70AZ
GTIWUNG 2PCS High Temp Tape, PTFE Tape/Teflon Tape, High Temperature Adhesive Tape, Vacuum Machine Sealer Tape for Vacuum Hand Impulse Sealers, Heat Resistant Up to 300℃ (10m / 39.37inch): Amazon.co.uk: DIY & Tools

Details here:

 

[Jasonb]

Yep, that's exactly what I was looking at on the web, Amazon.de have it (works for me as I'm in Ireland), but gonna try another shop locally here tomorrow before ordering it.

Thanks...

[Jasonb]

Well, I've had no luck with finding that tape locally, so Amazon it is, it should be here next Tuesday (though my last few orders all arrived days earlier than expected).

[Jasonb]

So, Amazon.de continue to impress me, as the tape arrived today. 

I took a couple of pics of the scope through a Cheshire tube, the first one is the view with the focuser tube all the way out, and the second one is the same view but with me lifting the end of the Cheshire, showing how much play there is.

Then I took out the focuser, and inside the tube you can (just about) see three white strips of tape. They have ridges on them, that stand proud of the tape itself. I originally was going to put the Teflon tape between these strips, but I realised I'd probably need 3 or 4 strips of Teflon tape to get up to the level of the ridges, so instead I just put the Teflon tape over the existing tape. The Teflon tape is wider, so I kept the excess away from the channel where the teeth run.

Once I'd added three strips of Teflon tape, one over each existing white strip, I put the focuser back in again, and you could immediately tell it was tighter, with a lot less play. Screwed it all back together and I'm quite happy with it. Took less than 5 minutes in all.

Still interested in how you get on with that other focuser @Zermelo, let me know if it does the job for you!

 

[Zermelo]

On 01/02/2021 at 19:31, Jasonb said:

Ah sorry, I thought you had it already. Hope it arrives for you soon, let me know how you get on with it.

OK, it arrived quite quickly, but on its first outing I experienced inability in finding focus with some eyepieces, as previously reported here.  So I went for the same solution as it describes, buying a step-down ring, removing the SW outer black piece and screwing the ring directly onto the silver draw tube.

I also found that, as delivered, the helical focuser had a very tight action - so tight that the step-down ring was unscrewing from the draw tube when I turned it.  So I removed the helical focuser and spent ten minutes operating it back and forth, and eventually it loosened up a bit (though still a bit tighter than I would prefer). I'm not sure if this is normal.

In use, it did what it said on the tin - I can focus more finely with it than I could with the focuser knob alone. I imagine a replacement focuser would be noticeably better, but also noticeably more expensive.

[Jasonb]

Excellent, thanks for the update Zermelo, I appreciate it. Hope it works a bit smoother for you over time.

[Cosmic Geoff]

If you like the idea of adding a helical focuser to your rack & pinion focuser, there is a design available from Svbony or from China that does not rotate the eyepiece/camera and is available in various variants with M42 (T2) outer threaded end, 1.25" eyepice holder, and on the inner end a variety of options - T2, M35, M32.5,  and adaptors for female T2,  2" barrel, 1.25" barrel.

I have one that I bought a while ago with the M35 x 1mm thread inner end, that fitted into the wider half of a Skywatcher focuser-to-1.25" adapter.

[Zermelo]

12 hours ago, Cosmic Geoff said:

does not rotate the eyepiece/camera

yes, that would have been a consideration if I'd been into imaging