Stripped grub screw in a Williams Optics Star71 refractor. Help!

[oymd]

I've recently acquired a WO Star 71 5 element wide field APO.

The optics are pristine, but it's an old scope, and the focuser has the usual marks on it from years of imaging, and had an electronic focuser attached to it. Lots of marks and scratches on the underside.

The coarse knob was very loose when I received it, and on trying to tighten it, I noticed that the grub screw is stripped.

The coarse knob then just suddenly fell off, and I have been trying for the past 2 days to sort it out, but have failed.

Overall the focuser works from the FINE side, but is grindy and rough, but works.

I have ordered some new Imperial and Metric allen keys, tried WD40, GT85, pentalobe screw drivers, regular flat head screw drivers, but nothing catches on the grub screw. I even ordered a ful set of grub screws, hoping to get the old grub screw out.

The seller is extremely helpful, and offered to take it back, or get it fixed.

Can I get some advise before I make a decision?

I plan to attach a ZWO EAF in the future. 

1- Will the ZWO focuser, or any electronic focuser attach on the COARSE side or the FINE side?

2- Is it possible to attach to the COARSE side, even though the focuser arm is recessed deep in the housing?

3- Is there a solution to this stuck grub screw?

4- If removal of the grub screw is possible, Is there someone I can ship it to to fix it, at cost of course?

5- Is the whole focus assembly REPLACEABLE with a new one? This focuser looks different to othere refractor focusers I have seen, where the focus draw tube is inside the OTA, with a ridged bar screwed underneath. Is this a R&P design? Not sure.

6- The optics look clean. Is it worth keeping it, and getting it fixed, or its just not possible and I should return it?

many thanks

 

 

 

Edited April 26, 2021 by oymd

[Clarkey]

For the grub screw it should be possible to drill it our and re-tap the hole. It may need a size bigger but that should not be a major issue. If there is a local garage or engineering firm I'm sure they could do it for you. You can also buy stud removal tools but I do not know how small they go.

[vlaiv]

I think that easiest fix is to do following:

- take a drill bit that is slightly larger than current grub screw thread - if it is M2 - take drill bit that matches M3.

- drill out grub screw and old thread in one go with drill using above drill bit

- take tap that is used to cut larger thread size (M3 in above example) and cut new thread

- take new thread size grub screw to tighten coarse know onto focuser shaft

[Clarkey]

Just thinking about it, you could just leave the existing grub screw and put a new one in to replace it. Simple to do.

[JamesF]

I would be tempted to try to drill the grub screw out using a left-handed drill bit (yes, they are a thing :) of a smaller diameter than the grub screw.  Often the bit will bite into the screw and turn it out rather than drilling all the way through it.

James

[vlaiv]

2 minutes ago, Clarkey said:

Just thinking about it, you could just leave the existing grub screw and put a new one in to replace it. Simple to do.

It really depends how stuck grub screw is.

Putting another one on top of it should force it all the way down and possibly out if shaft opening is large enough if grub screw is not really that stuck - but question is  - why did it get stripped if it's not stuck.

[JamesF]

Actually, come to think of it you could probably even use a normal right-handed bit, as there appears to be room to drive it all the way through until it falls out.

James

[oymd]

8 minutes ago, Clarkey said:

For the grub screw it should be possible to drill it our and re-tap the hole. It may need a size bigger but that should not be a major issue. If there is a local garage or engineering firm I'm sure they could do it for you. You can also buy stud removal tools but I do not know how small they go.

 

6 minutes ago, vlaiv said:

I think that easiest fix is to do following:

- take a drill bit that is slightly larger than current grub screw thread - if it is M2 - take drill bit that matches M3.

- drill out grub screw and old thread in one go with drill using above drill bit

- take tap that is used to cut larger thread size (M3 in above example) and cut new thread

- take new thread size grub screw to tighten coarse know onto focuser shaft

 

5 minutes ago, Clarkey said:

Just thinking about it, you could just leave the existing grub screw and put a new one in to replace it. Simple to do.

Thank you all for your replies.

I am not that savvy with tools and drills to do it myself!!

Can you please answer my other questions? Especially regarding fitting a ZWO EAF to this COARSE side? If the ZWO EAF can go on this side, I can ignore the coarse knob entirely?

[JamesF]

1 minute ago, oymd said:

Can you please answer my other questions? Especially regarding fitting a ZWO EAF to this COARSE side? If the ZWO EAF can go on this side, I can ignore the coarse knob entirely?

I may well be wrong, but I think it's generally considered appropriate to fit a motorised focuser to the coarse side rather than the fine, because the mechanism in the fine side isn't really up to the kind of load a motorised focuser might put through it.

James

[oymd]

And if I manage to get the grub screw out, and put in a new grub and sort the Coarse knob out, which screws need adjustment to make the focus smoother and less grindy?

[vlaiv]

1 minute ago, oymd said:

 

 

Thank you all for your replies.

I am not that savvy with tools and drills to do it myself!!

Can you please answer my other questions? Especially regarding fitting a ZWO EAF to this COARSE side? If the ZWO EAF can go on this side, I can ignore the coarse knob entirely?

I don't have experience with electronic focusers (other than planning to DIY one for myself) - but I do believe they should go on fine focusing shaft.

That really depends on step of motors driving it and any reduction ratio in motor itself. Single step needs to be fine enough to hit critical focus zone.

Mind you - if you put motor focuser on fine focus knob on the other side - well, why do you need manual focusing then, especially if you are going to use scope for imaging?

[JamesF]

Looking at the pictures of the protruding end of the grub screw again, is it just me who thinks the screw might be cross-threaded?

James

[oymd]

2 minutes ago, JamesF said:

I may well be wrong, but I think it's generally considered appropriate to fit a motorised focuser to the coarse side rather than the fine, because the mechanism in the fine side isn't really up to the kind of load a motorised focuser might put through it.

James

But how would I fix the EAF to this side? The focus arm  (what ever the brass coloured arm is called) is recessed in the focus housing? How will I fix the EAF to the arm?

[oymd]

3 minutes ago, vlaiv said:

I don't have experience with electronic focusers (other than planning to DIY one for myself) - but I do believe they should go on fine focusing shaft.

That really depends on step of motors driving it and any reduction ratio in motor itself. Single step needs to be fine enough to hit critical focus zone.

Mind you - if you put motor focuser on fine focus knob on the other side - well, why do you need manual focusing then, especially if you are going to use scope for imaging?

exactly, that's why I am asking. If the ZWO EAF goes on the COARSE side, then this whole problem would not really be an issue. It will be a decisive factor in me keeping the scope or returning it?

[JamesF]

1 minute ago, oymd said:

But how would I fix the EAF to this side? The focus arm  (what ever the brass coloured arm is called) is recessed in the focus housing? How will I fix the EAF to the arm?

Well, if you have a coupler to join the two shafts, with the grub screw removed you could line the grub screw in the coupler up with the hole and put an allen key through the hole to tighten it (or even drill another hole).  I'm guessing though, I'm afraid.  I'm not familiar with the ZWO EAF.

James

[oymd]

5 minutes ago, JamesF said:

Looking at the pictures of the protruding end of the grub screw again, is it just me who thinks the screw might be cross-threaded?

James

Sorry, what does that mean?

What I have noticed is that the protruding part of the screw looks TILTED to one side, and not pointing straight at the centre of the circle as it should do

[JamesF]

Just now, oymd said:

Sorry, what does that mean?

What I have noticed is that the protruding part of the screw looks TILTED to one side, and not pointing straight at the centre of the circle as it should do

It means basically what you've said.  The threads of the screw and the hole are not aligned, so the screw goes in at an angle and the threads cross each other rather than interlocking.

James

[vlaiv]

5 minutes ago, oymd said:

exactly, that's why I am asking. If the ZWO EAF goes on the COARSE side, then this whole problem would not really be an issue. It will be a decisive factor in me keeping the scope or returning it?

Have a look here:

https://www.cloudynights.com/topic/670574-autofocuser-slipping-and-not-working-zwo-eaf/

There are couple of pictures of setups with EAF.

First - you can put it on coarse side if you get needed flexible shaft coupler.

Second - EAF needs to be bolted down to the scope - which prevents manual focusing once it is installed (so you might as well put it on fine focusing shaft)

Third - it is recommended for it to go on fine focusing shaft.

Actually - scratch that last part - have you seen manual for ZWO EAF:

https://astronomy-imaging-camera.com/manuals/EAF_Manual_EN_V2.2.pdf

[Clarkey]

I assumed the stuck grub screw was the one on the first photo - part of the focus knob. If it the screw in the 8th photo you should be able to get a pair of pliers on it to screw it all the way in until it falls out.

[JamesF]

4 minutes ago, vlaiv said:

Third - it is recommended for it to go on fine focusing shaft.

I'm confused   The instructions appear to say to fit it to the coarse side.

James

[oymd]

9 minutes ago, Clarkey said:

I assumed the stuck grub screw was the one on the first photo - part of the focus knob. If it the screw in the 8th photo you should be able to get a pair of pliers on it to screw it all the way in until it falls out.

Tried that. I have a set of fine pliers, but there just isnt enough torque in my hands to turn the grub screw

[oymd]

5 minutes ago, JamesF said:

I'm confused   The instructions appear to say to fit it to the coarse side.

James

Yes, I had a look at the manual just now.

It seems the EAF goes on the COARSE side.

Thats good news.

So, if I plan to use the scope SOLEY for imaging, and with a permanent EAF attached, will this work?

Also, when electronic focusers are used, am I right in thinking that the focus does not need to be buttery smooth?  Because it certainly is NOT smooth?

[vlaiv]

1 minute ago, JamesF said:

I'm confused   The instructions appear to say to fit it to the coarse side.

James

I did correct myself - few lines below that:

8 minutes ago, vlaiv said:

Actually - scratch that last part - have you seen manual for ZWO EAF:

When I researched DIY motor focuser - recommendation was to put it on fine focusing shaft due to precision needed. One of comments in the thread that I linked to - also pointed that out.

However, ZWO EAF has rather decent resolution - of 5760 steps. Even if one revolution of coarse knob is 10mm of focuser travel - that is still 1.7µm per step. More than enough (critical focus zone is in tens of µm for fast systems).

[vlaiv]

1 minute ago, oymd said:

So, if I plan to use the scope SOLEY for imaging, and with a permanent EAF attached, will this work?

I think it will work without issues.

[JamesF]

If the grub screw is cross threaded (and I think that's the most likely state of affairs now), it may well be pretty much impossible to move, hence the stripped head.  Drilling it out may be the only solution to remove it.

However, if you fit the EAF to the coarse side then you could just drill another hole for access to tighten the coupler and ignore the grub screw.  Unless of course it fouls the coupler, in which case it will probably need to be removed or ground off anyhow.

James