Help with skywatcher 150/750

[Neil H]

Hi all I binned my laser collimator and got a Cheshire , when I collimator every goes well until it's time to do the main mirror the only way I can get  it in the centre is to hold mirror forward while adjusting then lock it in place , the rubber O rings are not wide enough to keep pressure on the mirror , is this normal ? Is there a way round it ? Is there a mod you do to the main mirror mount ?

[Alan64]

Yes, you replace the rubber o-rings with metal springs, and as I did for this reflector...

You can see the o-ring there, at left within the image at right, and after it was removed.  It's just sitting there, waiting to be cast into the bin.

The springs should be heavy-duty, like that used in industry, but not large ones.  You may have to cut them down in size even, and as I did.

Trial-and-error the undertaking, but just remember, you'll defeating the manufacturer in their wanton indifference towards the consumer.

Afterwards, it makes collimating so much easier.

Edited February 20, 2020 by Alan64

[Neil H]

Thanks for that do I need to get longer screws if I convert to springs

[Alan64]

1 minute ago, Neil H said:

Thanks for that do I need to get longer screws if I convert to springs

No, not for the adjustment-bolts at least.  You want them at the same threaded length.  The lock-bolts can be as long as you'd like, and stretching back into infinity even.  I replaced all of mine with socket-heads; before and after...

I just don't care for Phillips-heads.  To me, Phillips-head screws are for permanent fastenings only, and they're not all that great for that, either.  They were developed to make driving a screw easier, and faster.  

It didn't matter about the length of the lock-bolts when I changed over.  I wanted them a little longer.  

You will have to adjust the length of the springs to correspond with the length of the adjustment bolts; again, trial-and-error.  You can choose just the right tension in so doing.  

[Neil H]

Thanks They do compression springs on Amazon but need to work out what to get 

[RT65CB-SWL]

Replace the rubber 'O' rings, because in time they will lose their tension and split/perish over time, where as springs will retain the tension and a minor tweak now and again to ensure perfect collimation. I find the hexagon heads are better than slotted machine screws or 'phillips'/'pozidrive' as there is little or no risk of of the allen key slipping out when collimating. My 're-modded' ETX105 has them under the backplate and high-tensile hex-machine screws to secure the backplate to the OTA. (image below).

The three large blue discs on the backplate are anodized aluminium countersunk/fairing washers.

Edited February 22, 2020 by Philip R
additional images.

[joe aguiar]

I think u should go to a home depot or whatever u guys have there or large hardware store.

Buying spring on line will probably be a more miss.

You should test the springs. It shouldn't be too easy to compress as it be moving too much possibly move the mirror up and down.

Yet u dont want it so stiff it doesnt compress at all unless it needs a 100 lbs, that will be too stiff.

U gotta find a medium/medium stiff so it holds the mirror in place but doesnt move too easy, but also u dont want it too move at all.

Hope that kinda makes sence

Not sure if u should buy 2 or 3 pairs of 3 springs and see which tension works best.

Maybe u can return 2 of the pairs once u see which spring works best, assuming it doesnt look work or cut etc

 

[Alan64]

15 hours ago, Neil H said:

Thanks They do compression springs on Amazon but need to work out what to get 

You don't want the springs to be too large in diameter; not quite twice the diameter of the bolt.  You do want the springs to be difficult to compress with the fingers.  You should be able to squeeze the springs a little with the fingers, but only very little.

[Neil H]

Hi guys I think I may have found some ,so they are On there way to me will know when they get here but now reading what you guys have put they may be no good , I don't want to spend loads on this scope as I will be getting a new better one but here are the springs 

https://www.ebay.co.uk/itm/Compression-springs-5-pack/172494428588?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Edited February 21, 2020 by Neil H

[wookie1965]

They look to big think you only need around 16mm and they need to have more tension than what they look like they have, you need something more like this 

https://www.ebay.co.uk/itm/Compression-Spring-Small-Various-Sizes-Up-To-10mm-Diameter-Pressure-Compressed-1/222382066727?hash=item33c7010c27:m:mMijpHIbntB23tauIBEoyOw

If you look at these it will give you some idea,

https://www.cameraconcepts.com/bobs-knobs-primary-mirror-springs-for-many-common.html

[Neil H]

Hi Paul I looked at the listing if the others are no good I will order the biggest they do , I will pop over to my mates he may have some I can use 

[Alan64]

This the pack of springs I got at my local big-box hardware...

That is, the likely candidates for the job from the pack.  Note how substantial, how thick, they are.  Also, compare them to the adjustment-bolt laying before them.  I chose two from the third grouping from the left, and the only ones of stainless-steel.  I then cut three out of those two to the length required...

I then bent the cut ends towards the remainder of the springs...

That way, they won't try to slip to the side when compressing them.

Keep in mind that you're tensioning a 150mm primary-cell.  I don't think that the ones you ordered would tension that of a 76mm.

[Neil H]

That's my fault I was thinking it was just to hold mirror against the screws , I can't find a shop like you have , but Paul (wookie1965) pointed me to these 

25mm long ,1mm wire,8mm diameter 

https://www.ebay.co.uk/itm/Compression-Spring-Small-Various-Sizes-Up-To-10mm-Diameter-Pressure-Compressed-1/222382066727?var=521250561773

[Alan64]

8 minutes ago, Neil H said:

That's my fault I was thinking it was just to hold mirror against the screws , I can't find a shop like you have , but Paul (wookie1965) pointed me to these 

25mm long ,1mm wire,8mm diameter 

https://www.ebay.co.uk/itm/Compression-Spring-Small-Various-Sizes-Up-To-10mm-Diameter-Pressure-Compressed-1/222382066727?var=521250561773

My own, after I cut them down, were about half that length.  But cells do differ from one to another.  25mm is how long my springs were originally, therefore you might have to cut those down, as you don't want to set the primary-mirror back too far.

[Neil H]

Well I fitted the springs that I got  off eBay and they seem to work ok, I recollimated all seems good ,so only time will tell 

[Neil H]

Hi guys quick update Collimation is 100% , star test show Collimation 100% and springs are working well