Focuser Slop - Sky-Watcher Evostar 120

[Ian McCallum]

When I was out a few evenings ago with my SW Evostar 120, I noticed some play when I was trying to focus on Jupiter.  I'm guessing this is focuser slop.  Any ideas how to cure it , without buying a whole new focuser?🤔

[vlaiv]

In my case - solution was easy

Instead of buying a new focuser I fitted old one that I was left with after upgrading focuser on one of my imaging scopes

There are a few videos on youtube of stripping and re-greasing synta R&P focusers - like this one:

https://www.youtube.com/watch?v=K89k3U9mPe8

You'll need to adjust it and you'll probably need to replace PTFE pads that are in there

[doublevodka]

As Vlaiv rightly pointed it it's probably worth a strip and regrease anyway

But in the majority of cheaper focusers (and probably more expensive too but I haven't taken any of them apart) there are some fairly easy adjustments you can do without a total strip down (although it's worth it if you are confident enough)

Most focusers I've played with have three points of contact with the draw tube, two non adjustable ones at the bottom and one at the top that can be adjusted

On the top edge of the focuser you will see the locking knob, in front and behind that (looking from the rear of the scope) you'll see two small holes, these are grub screws with a small allen head which adjust the angle and pressure of the top adjustable contact plate

Firstly give the draw tube a little wiggle to give yourself a baseline as the idea is to improve on the amount of play and not make it worse

Then start adjusting the grub screws a quarter turn at a time, wiggle the draw tube in between and make sure you can rack the focuser in and out

If you have more play pushing down adjust the furthest away grub screw, more play up the closest one and rinse and repeat

The important note here is with a standard focuser you are unlikely to remove all play from it completely as it'll be too tight to move the focuser at all, but don't worry if that happens, just gradually back off both grubscrews a bit at a time until it moves nicely again

Hopefully the explanation helps, I'm a bit of a tinkerer and this is what worked for me 👍

[John]

When I've had Skywatcher scopes with that focuser I've found that they can be improved and adjusted to perform quite well, at least for observing purposes.

The rack and pinion focuser on the Evostar 120 is a close copy of the Vixen focuser. A method for adjusting the Vixen focuser is given here and this does work for the Skywatcher versions as well:

vixenfocuser.pdf

Edited November 30, 2021 by John

[Ian McCallum]

4 hours ago, vlaiv said:

In my case - solution was easy

Instead of buying a new focuser I fitted old one that I was left with after upgrading focuser on one of my imaging scopes

There are a few videos on youtube of stripping and re-greasing synta R&P focusers - like this one:

https://www.youtube.com/watch?v=K89k3U9mPe8

You'll need to adjust it and you'll probably need to replace PTFE pads that are in there

I'm dubious of putting any type of grease on Teflon (PTFE), as it's already very slippery.  It doesn't look anymore difficult that when I did the Crayford style focuser on my SW 200P.

[Ian McCallum]

3 hours ago, doublevodka said:

As Vlaiv rightly pointed it it's probably worth a strip and regrease anyway

But in the majority of cheaper focusers (and probably more expensive too but I haven't taken any of them apart) there are some fairly easy adjustments you can do without a total strip down (although it's worth it if you are confident enough)

Most focusers I've played with have three points of contact with the draw tube, two non adjustable ones at the bottom and one at the top that can be adjusted

On the top edge of the focuser you will see the locking knob, in front and behind that (looking from the rear of the scope) you'll see two small holes, these are grub screws with a small allen head which adjust the angle and pressure of the top adjustable contact plate

Firstly give the draw tube a little wiggle to give yourself a baseline as the idea is to improve on the amount of play and not make it worse

Then start adjusting the grub screws a quarter turn at a time, wiggle the draw tube in between and make sure you can rack the focuser in and out

If you have more play pushing down adjust the furthest away grub screw, more play up the closest one and rinse and repeat

The important note here is with a standard focuser you are unlikely to remove all play from it completely as it'll be too tight to move the focuser at all, but don't worry if that happens, just gradually back off both grubscrews a bit at a time until it moves nicely again

Hopefully the explanation helps, I'm a bit of a tinkerer and this is what worked for me 👍

I was just going to say that my focuser is no where near as bad as his is, in the video.

[doublevodka]

2 hours ago, Ian McCallum said:

I'm dubious of putting any type of grease on Teflon (PTFE), as it's already very slippery.  It doesn't look anymore difficult that when I did the Crayford style focuser on my SW 200P.

Yeah the PTFE does not need greasing, in my case I removed all the old sticky glue grease that the manufacturers like to use and then used some white lithium grease on the moving parts apart from the draw tube

[Ian McCallum]

I found that the rack was a bit loose, as the small screws holding it needed tightening. 

I ended up removing the pressure plate cover that holds down the focuser knob, removed the draw tube and degreased the rack and pinion, away from the tube.  It was the usual Chinese glue grease affair. 🙄

Once I had cleaned them up, I put the rack back on the draw tube and screwed it back on. 

I regreased the rack and pinion, with small amounts of grease, wiping off the excess. 

I put it all back together and it seems to be okay, but I'm unsure how much to retighten the pressure plate too. At the moment, it's just a light pressure, making sure all the screws aren't too loose, incase they fall out. 

 

 

Edited December 1, 2021 by Ian McCallum

[doublevodka]

Looks good to me 👍 Tight enough so it doesn't wiggle around is usually fine and the replacement grease always makes a nice difference

I just need to getting round to finding some suitable PTFE tape myself now to redo mine, if anyone ready has any recommendations I'm all ears 👍 It really niggles me the hard plastic used as bearing surfaces that leaves marks on the draw tube as it has on yours and mine

[Ian McCallum]

1 hour ago, doublevodka said:

Looks good to me 👍 Tight enough so it doesn't wiggle around is usually fine and the replacement grease always makes a nice difference

I just need to getting round to finding some suitable PTFE tape myself now to redo mine, if anyone ready has any recommendations I'm all ears 👍 It really niggles me the hard plastic used as bearing surfaces that leaves marks on the draw tube as it has on yours and mine

It's aluminium, which is a soft and lightweight metal - I doubt there's anything that can be done about that.🙄

[doublevodka]

15 hours ago, Ian McCallum said:

It's aluminium, which is a soft and lightweight metal - I doubt there's anything that can be done about that.🙄

For now I've peeled mine off and replaced it with sticky backed felt which seems ok, certainly no worse than it was but I do need to get round to using PTFE tape.

You can just about see the cause in your 5th photo, the white plastic bearing surfaces have ridges in them that leave the marks behind, although they rubbed away fairly easily

[John]

Some really quite expensive focusers have bearings that leave marks on the drawtube, eg: Moonlite focusers. At least the marks that the Skywatcher plastic strips leave on the drawtube usually rub off quite easily. With the Moonlite focusers (and some others) the marks are worn into the anodising and can't be removed.