Collimation help please

[rawhead]

Hi folks, I need some help / advice please. I fitted a new focuser yesterday (here) I’m now trying to get the collimation sorted. I have collimated with a laser and it all looks good. I have also made a sight tube with a film canister and have taken a pic through the hole : (Apologies for the quality.) The largest inner circle is the secondary Next is the bottom of the film canister Finally the paper reinforcement ring that marks the centre of the primary. Do I need to centre the secondary or shim the focuser? (Or something else) Cheers (I pasted this text from Word and cannot change the text back to white?) Jon

(click to enlarge)

(click to enlarge)

[rawhead]

I've now fiddled with

shimming the focuser, aligning the secondary and the primary.

I can tilt the secondary so the sight tube and the reinforcement ring is centred but then the primary is way off...

confused :?

[George]

If it was collimated OK before fitting the new focuser I'd say your focuser is not square. try slackening the focuser off a little and try jiggling around and see if this improves things.

[acey]

I can't see what's going on in your picture but it looks as though it's not centred. Important thing, as you probably know, is to do it sequentially and get each step right. I used to use the film cannister method, then recently I bought a Cheshire and realised that I'd never been collimating properly - you just can't do it accurately enough with a film cannister. A Cheshire makes it really easy, even if you're doing it single-handed and need to go back and forth from collimating knobs to eyepiece.

Also, a laser only works if you get each step exactly right: if your secondary isn't properly centred then you'll introduce an error when shining the laser spot onto the middle of the primary - and if that stage isn't sufficiently accurate then you'll introduce a further error when aligning the primary. And if there's any slop/wobble in the focuser then this introduces error right from the start. The barlowed-laser method is generally considered to be far better (but I quickly gave up on a laser altogether, except for checking the secondary, and have stuck with the Cheshire).

A very confusing thing, to me at least, is all the reflections you can see while collimating. Some people put a card in to block the reflection of the primary in the first stage of centering the secondary under the focusser. I like to shine fairly strong light into the tube (during daytime collimation) so that I can see the outline of the secondary holder clearly. If you've only changed the focusser then the secondary itself may well be centred within the tube and you only need concern yourself that the focusser is properly shimmed to centre the secondary. Then use your laser to adjust the secondary so that the spot is centred on the primary, then adjust the primary.

I've seen it stated that centring the secondary under the focusser may not be as critical as aligning the secondary and primary, the only resulting defect being a loss of some light entering the eyepiece.

Main thing I'd say is that if you want to get properly collimated then you need something more accurate than the film cannister method. But the important thing is the quality of the views you get, and if it looks good enough to you, then it is.

Andrew

[scotastro]

I saw your pictures of fitting the focuser - the base seems to match the curve of the tube well - but did you check if the focuser was square to the tube before fitting the spider and secondary?

Here is a link that may be useful -

http://www.umich.edu/~lowbrows/reflections/2003/dscobel.10.html

If the laser spot does not hit the ring on the primary it is the secondary that is out of alignent not the primary.

I did not understand you statement - "I can tilt the secondary so the sight tube and the reinforcement ring is centred but then the primary is way off..." - have you put a ring on the secondary cos you should not have done.

Scotastro

[rawhead]

If it was collimated OK before fitting the new focuser

Not sure that it was... Only just made the sight tube.

I have loosened/fiddled with the focuser. Didn't seem to make any difference...

[rawhead]

I can't see what's going on in your picture but it looks as though it's not centred. Important thing, as you probably know, is to do it sequentially and get each step right. I used to use the film cannister method, then recently I bought a Cheshire and realised that I'd never been collimating properly - you just can't do it accurately enough with a film cannister. A Cheshire makes it really easy, even if you're doing it single-handed and need to go back and forth from collimating knobs to eyepiece.

Also, a laser only works if you get each step exactly right: if your secondary isn't properly centred then you'll introduce an error when shining the laser spot onto the middle of the primary - and if that stage isn't sufficiently accurate then you'll introduce a further error when aligning the primary. And if there's any slop/wobble in the focuser then this introduces error right from the start. The barlowed-laser method is generally considered to be far better (but I quickly gave up on a laser altogether, except for checking the secondary, and have stuck with the Cheshire).

A very confusing thing, to me at least, is all the reflections you can see while collimating. Some people put a card in to block the reflection of the primary in the first stage of centering the secondary under the focusser. I like to shine fairly strong light into the tube (during daytime collimation) so that I can see the outline of the secondary holder clearly. If you've only changed the focusser then the secondary itself may well be centred within the tube and you only need concern yourself that the focusser is properly shimmed to centre the secondary. Then use your laser to adjust the secondary so that the spot is centred on the primary, then adjust the primary.

I've seen it stated that centring the secondary under the focusser may not be as critical as aligning the secondary and primary, the only resulting defect being a loss of some light entering the eyepiece.

Main thing I'd say is that if you want to get properly collimated then you need something more accurate than the film cannister method. But the important thing is the quality of the views you get, and if it looks good enough to you, then it is.

Andrew

Thanks Andrew. Looks like I may need to think about getting a cheshire...

I will try barlowing the laser. Never realised that was an option.

I saw your pictures of fitting the focuser - the base seems to match the curve of the tube well - but did you check if the focuser was square to the tube before fitting the spider and secondary?

Here is a link that may be useful -

http://www.umich.edu/~lowbrows/reflections/2003/dscobel.10.html

If the laser spot does not hit the ring on the primary it is the secondary that is out of alignent not the primary.

I did not understand you statement - "I can tilt the secondary so the sight tube and the reinforcement ring is centred but then the primary is way off..." - have you put a ring on the secondary cos you should not have done.

Scotastro

Thanks Scot. That link looks very handy. Just printed it off. I haven't marked the secondary . I'm just peering through the sight tube gauging when it looks central. Shame the pics did not come out a bit clearer. :thumbleft:

[scotastro]

Looking at the view through the focuser with all the different reflections can get confusing. To really simplify collimation think of it that the secondary gets to laser light onto the centre of the primary (you always set the secondary mirror first) then the primary gets the light beam back onto secondary and so to the centre target of the collimator.

One point - have you checked the collimation of the laser - sorry for that last if you already have.

Mike

aka Scotastro

[Ajohn]

It sound like you have missed one or more of the steps and you 2ndry isn't at 45 degrees. The film cannister should be ok providing the hole really is central. I use an old eyepiece with the lenses taken out. Some old plossls are good for this as they leave a hole 3 or 4 mms dia but if larger one can move ones eye further away/rack the focuser out.

The 1st step is to set the 2ndry at 45 degrees by eye.

2nd step get the 2ndry central in the tube. (That may need to be changed later.) A tape measure can be used for that just get it as close as you can.

3rd step is to get the 2ndry central in the focuser. This is where the thing with a hole in it comes in. You need to adjust the height not tilt of the 2ndry and adjust the focuser and or the position of your eye so that the 2ndry mirror very nearly fills the view. That way you can centralize it accurately. You must keep your eye central to the hole you are looking through. That's why small holes are desirable.

4th step is to adjust the tilt of the 2ndry so that you can see the whole of the bottom of the tube central in the view. You may find that you can't and that the 2ndry holder looks a little odd and isn't holding it at the angle it's intended too. This means that the holder needs to be moved away from the focuser. So go back to step 2 and repeat all but offsetting say 1mm away from the focuser. Repeat till all comes ok. Sorry about this but that's one of the reasons I feel that they shouldn't make scopes like that. (But they do and 1mm may be too much)

You can now use your laser to finalise the 2ndry tilt. After that it pays to go back to step 3 and run through it all again.

Some lasers also project a grid. You can view that down the tube or project it onto a wall and use it to tweak the main mirror. The only problem is that your eye needs to be very central to the tube and at the distance needed that isn't easy or the wall needs to be square to the tube. You may find that the grid can be seen by putting some newspaper flat on the end of the tube and adjusting the focus. The aim is to tilt the main until there is no distortion in the grid.

This is a reasonable flash movie of newtonian scope collimation but it misses the important 1st step of getting the holder in the right place - there shouldn't be any need to do that on a new scope but there will be when the spider is replaced and may well be on a 2nd hand scope that some one has played with. He also uses the term beneath rather than central and the view he shows assume the scope is only slightly out. My method will get it to that state and the video can then be followed. Personally I would stick with the eyeball method and use the laser to finally tweak things. Perfect collimation if you want it will need a star test despite what many say.Expect to see a reflection of your eye in the scope too. If it's all confusing drop a tissue paper mask over the main mirror until the top bits are right.

He also shows the use of a more expensive laser.

John

http://www.andysshotglass.com/Collimating.html

John

[rawhead]

It sound like you have missed one or more of the steps........

Wow. Thanks for that John. I think that's the most concise and comprehensive list of collimation actions I've read. I actually think I'm beggining to understand it all. Once the mount is sorted I'll give it a bash.

Cheers

Jon

[Ajohn]

Hope it helps. I just wish that !!####*********'s would keep the 2ndry mirror central in the tube because if it isn't it can take a long time to sort that out. They could even clearly state what the offset off centre should be.

Hopefully the set up will make the focuser square to the beam of the 2ndry mirror. I've checked that by looking at distant roofs with a fairly high mag and waggling the eyepiece about it the image isn't even. May pay to look at packing the focuser if there's a problem. But you can think what that means about the mirror alignment and go back and tweak the lot again.

John

John

[rawhead]

An update, think I've fixed it.

With the Tal:

1st step OK. The secondary is mounted at a fixed 45 degrees. No adjustment possible.

2nd step OK. The spider automatically centres the secondary. No adjustment (or very little) possible.

3rd step. Fail! :? I thought the secondary was in a fixed position facing the focuser. It’s not. When you loosen the centre screw, the whole thing loosens and swivels. It had swivelled out of alignment! Now fixed.

I then used the laser to adjust the secondary to centre the laser dot on the primary, then adjusted the primary so it centred on the bulls eye in the laser collimator.

In the original pic, the reinforcement ring on the primary is now in the centre of the film canister. 8)

Now for some clear skies…. (I’m off this week so it’s bound to be cloudy)

Thanks again for all the help and advice.

Jon