Hopelessly lost with collimation

[Jimtheslim]

I'm probably not the first, but I am terribly lost with collimation of my skywatcher 250. I have read every guide and watched every video and know what I need to do. I have been up for hours trying to get the secondary in the right place, it just won't sit where I need it. I loosen the 3 allen key screws so the centre screw can be adjusted to tilt it, but as soon as it's in the right place and take the screwdriver away it just turns back to where it was. Should I keep the screwdriver in place and try to tighten the 3 allen key bolts? I can get the secondary looking centre and round by eye, but putting the cheshire in I can then see it's way off. 

I can see why people love their refractors now!

[Ally8446]

The centre screw is to move the secondary closer to or away from the primary, and to adjust rotation.  The other 3 are for tilt. It's just trial and error I'm afraid. 

Ally

[Owmuchonomy]

The centre screw moves the secondary mirror up and down the tube so loosen the three allen screws and use it to get the 2ndry roughly central when viewing through the focuser with your Cheshire or equivalent.  Then turn the 2ndry gently so that you get as close to a circle reflection as possible.  You then use the 3 allen screws to tilt the 2ndry (not the centre screw) and fine tune the 2ndry position.  Finally when you are happy gently tighten the centre screw.

The best non-video instructions are on Astrobaby's website.

[Jimtheslim]

When I turn the centre screw it goes horribly loose and I fear the secondary mirror dropping from the mount. When I loosen the 3 small bolts and start turning the centre screw it doesn't move back and forth, it just tilts and sags down

[brantuk]

Are you using a collimation cap? When you do - all you need to do is get the 3 primary clips in view round the circumference and the secondary will be in approximately the right position. Then adjust the view for roundness and centralisation and lock it off with the center screw in that position. That will be 99.9% correct - then use the three small screws to tilt the laser beam onto the primary center spot and it's done. The rest is done at the primary end then onwards.

Btw - don't loosen the three small bolts. If the secondary is sagging then it's close to falling off - do it back up pronto or it'll drop on the primary. You're only supposed to slacken the center bolt a quarter turn so you can twist it to the correct position facing the focuser tube at the correct height to give a "round mirror" appearance.

[Moonshane]

My guide may help. Think of the secondary screws as push and pull. The central bolt pulls on the secondary holder and the three grubs push on it. You can only move the secondary position up and down the tube by loosening carefully and either turning the central bolt clockwise to move up or counterclockwise to send it down the tube. You MUST take care to avoid loosening the central bolt too much as the mirror can fall off and hit the primary - collimate the secondary position always with the scope at an angle not vertical.

Assuming all bolts are tight and you want to move the secondary up the tube (I should stress that having to move the secondary either up or down the tube is very unusual unless you have taken the scope to bits) then paying attention to the warning you would need to :

• Loosen the three grubs by about a full turn then tighten the central bolt until it all just holds. Check the position and repeat if necessary. Then collimate the secondary with just the three grubs screws - they will need just a tweak probably. To move the secondary down the tube a little you need to do the same but turhn the central bolt counter clockwise.

This is so much easier to show than describe. If you are near Stockport, I'd be happy to help.

 

 

[Jimtheslim]

16 minutes ago, Moonshane said:

My guide may help. Think of the secondary screws as push and pull. The central bolt pulls on the secondary holder and the three grubs push on it. You can only move the secondary position up and down the tube by loosening carefully and either turning the central bolt clockwise to move up or counterclockwise to send it down the tube. You MUST take care to avoid loosening the central bolt too much as the mirror can fall off and hit the primary - collimate the secondary position always with the scope at an angle not vertical.

Assuming all bolts are tight and you want to move the secondary up the tube (I should stress that having to move the secondary either up or down the tube is very unusual unless you have taken the scope to bits) then paying attention to the warning you would need to :

• Loosen the three grubs by about a full turn then tighten the central bolt until it all just holds. Check the position and repeat if necessary. Then collimate the secondary with just the three grubs screws - they will need just a tweak probably. To move the secondary down the tube a little you need to do the same but turhn the central bolt counter clockwise.

This is so much easier to show than describe. If you are near Stockport, I'd be happy to help.

 

 

Thanks for this, I was finding it counter-intuitive to turn it clockwise to move it 'up' the tube. Once I had my head round this I was able to keep making fine adjustments. I couldn't get the top of the circumference of the secondary into the top of the view in the Cheshire so I adjusted the spider veins slightly and this pulled it 'up' the view in the eyepiece which is what I was struggling to do. 

I think I am now there. The secondary appears round and the centre dot was in the centre of the secondary. I have now adjusted the primary so everything lines up. However the in the black circle formed by the focuser tube in the centre it looks as if the eyepiece hole is slightly toward the front as it's not quite centred in the central 'black hole'. Should I stress about this and go about trying to pull/push the secondary up and down the tube?

I live in Hexham btw. Quite some distance away but I appreciate the offer.

[Jimtheslim]

and of course, I had the scope level. I have a habit of dropping things.

[Moonshane]

Personally I'd not worry too much at all. The secondary position unless wildly out will have no perceptible effect on the visual image. The primary though if slightly out, will. It's a fortunate aspect of collimation that the easiest to adjust has the biggest impact.

Leave it for a bit, enjoy the scope and then try again in a few weeks. In reality though, you won't see any difference even if you get the secondary 100% perfect.

[Jimtheslim]

You mention the secondary twice in your opening statement. Did you mean to say the primary doesn't matter too much?

If I wasn't off to work I would go and lie down in a dark room. 

[Moonshane]

Sorry, me too obviously!! The primary matters, the secondary not really.

I have edited to the correct position.

[Moonshane]

This might help too.

 

[Alonsos]

I feel your pain ,my dob was out of collimation in transit from new , I read & I read and puzzled it thro, set reset set again, there comes a time when you tell yourself ( that's ok) and try it out ,but I don't think I will ever be happy with it

i try to defocus and get the donut shapes that still does not happen 

So this is no good for you ,it just hit a chord when I read your input

take care and good luck

[Jimtheslim]

1 minute ago, Alonsos said:

I feel your pain ,my dob was out of collimation in transit from new , I read & I read and puzzled it thro, set reset set again, there comes a time when you tell yourself ( that's ok) and try it out ,but I don't think I will ever be happy with it

i try to defocus and get the donut shapes that still does not happen

So this is no good for you ,it just hit a chord when I read your input

take care and good luck

I am really bad for stuff like this. I'm not sure I bought the scope because I wanted to look into the sky with it, or stress over trying to perfectly line everything up in my garden room. I cannot help but fiddle, even when things are perfect. I will try it on the next clear night. I take it if you defocus and get good, symmetrical donuts around stars it means it's in good collimation?

[Alonsos]

That's what I gather , although I get donuts they never look like the ones in books ha 

regards Alonsos