Just shimmed my 2inch focuser. Wow!

[SteveBz]

Hi guys,

I've just shimmed my focuser. It took about 45 mins.  And it's completely fantastic.

Here's a photo.

Just a tiny little sliver of 4mm thick ptfe off eBay in the chanel where the lock nut is.

Cost a few quid. No glue or adhesive.

All the slop has gone and it moves like silk.

I should have done it ages ago.

I guess plans for a 2 speed crayford are now on hold.

If you have done it yet, it's  a complete game changer.

Steve

[happy-kat]

Ptfe tape is handy stuff. Was that for your C8?

[SteveBz]

Hi HK,

Yes for my 8" but it's not tape. I bought a 4mm slab of ptfe and cut about 1cm by about 5 cm off the edge and then filed it down until it fitted. I loosened the lock screw and associated grubs and slipped it in. Then tightened the screws a little. Really it's held in place by the screws.

But I cannot tell you how much improved the motion is.  It was never so smooth and wonderful.

And if I take it out, you will never guess that it was ever there.  No mods to the basic focuser .

Steve.

[bobro]

By co-incidence I've just finished doing the same thing to my 1.25" focuser as there was obvious side-side movement with the camera attached, affecting flats which were meant to line up with images. I used self-adhesive skived teflon tape, building it up to the required thickness. No side-side wobble left! I wonder if images will be a little better without the slightly slanted camera sensor?

Bob

[SteveBz]

5 hours ago, bobro said:

By co-incidence I've just finished doing the same thing to my 1.25" focuser as there was obvious side-side movement with the camera attached, affecting flats which were meant to line up with images. I used self-adhesive skived teflon tape, building it up to the required thickness. No side-side wobble left! I wonder if images will be a little better without the slightly slanted camera sensor?

Bob

Cool.  I hope you've been impressed by the difference. 

I've since found out that the sliver of PTFE I inserted in fact replaced a similar block of black plastic (not PTFE), that had been in the same place before and had fallen out, so much of the improvement I've noticed must be down to the fact that is was apparently broken in the first place.  However, the block I inserted was thicker, so allowed less side-to-side wobble, and so much smoother that the focus lock almost doesn't lock.  The action is surreally smooth.  I may have to drill a hole in the block to allow the lock to operate.  But the before and after actions are incomparable.

I also have a Skywatcher focuser that I have added and I'm planning to run it off the Arduino providing remote focusing and paving the way for using focus metrics like FWHM, or just straight contrast maximisation if it's easier, to do real autofocus.

I hope if I can get the focus really spot on to take some of those really dramatic photos.

Steve.

[frugal]

What does the PTFE rub against? Is it the metal draw tube? I presume that the roller connected to the focus wheel does not touch the PTFE?

[SteveBz]

2 minutes ago, frugal said:

What does the PTFE rub against? Is it the metal draw tube? I presume that the roller connected to the focus wheel does not touch the PTFE?

Yes, the draw tube on one side and the housing on the other.  It creates a triangle of itself and the two pre-existing shims already there.

But also it's the regular R&P focuser that came with the telescope, so there is no roller, much to my chagrine.

Regards

Steve

[frugal]

1 minute ago, SteveBz said:

Yes, the draw tube on one side and the housing on the other.  It creates a triangle of itself and the two pre-existing shims already there.

But also it's the regular R&P focuser that came with the telescope, so there is no roller, much to my chagrine.

I probably wasn't clear, by "the roller" I meant the bar that runs from one focus wheel to the other. My ED80 focuser works by having the metal bar that the focus wheels are connected to rub against the focus tube. The whole reason that the focuser seems to work is because there is friction between the metal bar and the metal draw tube. Obviously you don't want PTFE to make this bit slippery

[SteveBz]

10 hours ago, frugal said:

I probably wasn't clear, by "the roller" I meant the bar that runs from one focus wheel to the other. My ED80 focuser works by having the metal bar that the focus wheels are connected to rub against the focus tube. The whole reason that the focuser seems to work is because there is friction between the metal bar and the metal draw tube. Obviously you don't want PTFE to make this bit slippery

No, you were clear. But I have a cog where your roller is. That means there is some periodicity to my focusing and it's not totally smooth and continuous like yours.

I'm about to try the Skywatcher motor on my updated focuser this morning. The un-modded one was so stiff it didn't work.

I'll keep you updated.

Steve.

 

[jacksdad]

One thing I've considered on the sw r+p focuser I'm working on is modding the actual rack and the pinion.

 

My thought is to take the rack off the tube (it's held by a couple of screws) and replace it with a bit of random metal (probably brass) that's sized to fit in the anti-rotation slot better, and then to put a rubber 'tyre' on the pinion and give non-notchy movement.

[SteveBz]

Nice idea.

I want to test what I have first, but that sweet spot is so narrow, that I can only imagine that smoothing the action will help.

Regards

Steve. 

[jacksdad]

Yep, getting the pinion preload 'just so' is a challenge on the sw one - a smidge loose makes it feel sloppy and going the other way makes it feel indexed... Neither is good. And, I don't exactly trust the spring plate screws to stay put for long either.