Help with collimation

[AndyWB]

I'm afraid that tonight while having a play with my telescope to try to fully understand collimation, I managed to move my secondary. I've spend the last 7 hours trying to fix this, and in all honesty, I feel like crying.

The tools I've got are a collimation cap, and a laser collimator. The laser collimator does appear to be collimated - rotating it in the focusser doesn't seem to make a difference to where it strikes.

I am really struggling to get the position of the secondary set. I use the collimation cap to get the secondary looking all circular. Following Astrobaby's guide, I've put coloured card opposite the focusser, and as a baffle in front of the primary, and adjust the secondary until it looks circular. I then use the tilt screws to get the image of 3 clips equidistant to the edge of the secondary - in my scope they aren't at the edge of the secondary. All looks good at this point.

However, if I then replace the collimating cap with the laser, the light beam usually doesn't even strike the secondary, and it hits the primary off to one side. Where does strike the primary seem to vary with attempts, which may suggest the error is mine in setting the secondary.

If I use the tilt screws to adjust the secondary so the laser strikes the center of the primary, and I then replace it with the collimation cap, I find that the image of the primary is off near one edge of the secondary.

I can only figure that I'm getting 1 of the degrees of freedom wrong, but I can't figure out which. What can I do?

[Jason D]

Since you have a collimation cap and a laser collimator, try the procedure I outlined in the last post of the following page

http://www.cloudynig.../o/all/fpart/21

Give it a shot and see if it helps.

Jason

[AndyWB]

"At this point frustration builds and no matter what they do, they just can’t reconcile between the collimation cap and the laser collimator."

- No kidding!

Jason, that is marvellous. The bit that got me was 4a) ; I'd undone the centre bolt slightly, and not returned it to the correct length. It was only slightly off - but enough that I couldn't get the collimation cap and laser to reconcile.

(Embarrassingly, I'd read that in other guides - but I'd not realised how such a small difference in the length of the bolt would make such a big difference. Once I'd finally realised that in your instructions I realised that if I breathed on the secondary I could see the laser spot was not centred on it - so of course the laser wouldn't match the collimation cap)

I followed your instructions, iteratively trying shortening the centre bolt, and got it to 'pretty damn close' in about 15 minutes, and I reckon if I had some milk bottle washers I'd have it spot on, just rotating the secondary is slightly notchy. Still, I reckon it's good enough; I can always come back to it if I have another day to kill . Thanks!

Oh, and it's a useful procedure for me also due to not needing to reference the focuser edge; I found that very hard to see with the Heritage 130's screw in focuser.

[Jason D]

Hello Andy,

Glad to hear things worked out for you.

Jason

[Moonshane]

Jason, that's a superb post and has almost converted me to laser collimation! It's certainly consolidated your position as resident newt collimation expert as far as I'm concerned.

Is there any chance you have a readily available collimation cap and Sight tube/Cheshire version in such terms? If so, please would you be willing to start a thread with both versions on it? I am sure that many (indeed probably all) our members would appreciate such a resource in one spot.

[Damo636]

Fantastic post Jason, I only wish I knew the half of what you have forgotten about collimation!

[blueroom]

What laser collimator have you got Andy?

[Jason D]

Fantastic post Jason, I only wish I knew the half of what you have forgotten about collimation!

[Jason D]

Jason, that's a superb post and has almost converted me to laser collimation! It's certainly consolidated your position as resident newt collimation expert as far as I'm concerned.

Thanks, Shane

Is there any chance you have a readily available collimation cap and Sight tube/Cheshire version in such terms? If so, please would you be willing to start a thread with both versions on it? I am sure that many (indeed probably all) our members would appreciate such a resource in one spot.

I don't at this time. With a sight tube/cheshire you do not really need a collimation cap but the collimation cap could be helpful if the sight-tube lower edge clips the view of the primary mirror reflection.

Jason

[Moonshane]

cheers Jason. I generally find that my Cheshire does indeed clip the view somewhat so I bought one of these. http://www.firstlightoptics.com/collimation/rigel-aline-collimation-cap.html

it's very cheap and in truth you can get the whole collimation pretty close although I just use it for initial alignment of the secondary position/rotation, then moving onto the sight tube/cheshire unit.

I thought those on a tight budget or with little experience might be happy to know about this tool which for the price works very well indeed.

[AndyWB]

What laser collimator have you got Andy?

I got one from Skys the Limit, I believe it's the 'Next Generation Laser Collimator', though I fail to see quite what is 'Next Generation' about it! I have checked it's collimation in scope, and it seems good; taking it out and replacing it in the focuser can result in a very slightly different result, but not much, and I put that down to the focuser. And it was a slight second, so cheap.

All that said, what this has shown me is that the laser isn't any damn use for the secondary (well, not by itself). Well, in fact, the link to Jason's procedure explains pretty clearly why it can't help. It seems good for the primary, though. I am considering a Cheshire in case I ever have to go through this again; it does seem the tool of choice.

Having done some reading - and Jason seems to have written a lot about collimation at Cloudy Nights (which I shall slowly digest)- it appears that my secondary has a slight tilt/rotate error, but the thread I'd found recommended leaving a worse error as it was minor. I'll probably try and fix it anyway tonight if it's cloudy.

[moffer]

cheers Jason. I generally find that my Cheshire does indeed clip the view somewhat so I bought one of these. http://www.firstligh...mation-cap.html

it's very cheap and in truth you can get the whole collimation pretty close although I just use it for initial alignment of the secondary position/rotation, then moving onto the sight tube/cheshire unit.

I thought those on a tight budget or with little experience might be happy to know about this tool which for the price works very well indeed.

Yep, definitely worth giving the FLO colli cap a go before forking out on more expensive gear. It's the only tool I use and I'm perfectly happy with the collimation of my son's 200p.

[AndyWB]

Right, I've tried to fix this, but somehow I keep coming back to this position:

You can see:

• The primary isn't centred on the secondary, though all of it is on the secondary.
  
• The long axis of the secondary isn't directly towards the primary (right)
  
• Also, you can see that this is a Heritage 130p, with only 1 secondary support.
  

I can't see any stars at the moment, but moon through the clouds looks, well, like the moon through clouds should.

What should I do? Call this good enough, or keep trying? I've spent ages on this, and seriously, that's as good as I seem to be able to get it - which is frustrating, 'cos I'm sure it was better. I just don't know what I'm doing wrong.

[Jason D]

What should I do?

Try the following:

1- Loosen the secondary mirror set screws just (and I mean barely enough) to rotate the secondary mirror

2- Rotate your secondary mirror clockwise as you look down the OTA. I would say maybe by around 2 degrees -- just by a little amount.

3- Recollimate using only the set screws -- no more rotations.

See if the above will improve the secondary mirror position for you.

Post another photo after you go through the above steps.

You ran into what is called "rotate/tilt" secondary mirror error. See that attached animation (click on it to see the animation). Specifically, see how the secondary mirror shadow changes.

Jason

[AndyWB]

Right, so, loosened bolts slightly, rotated secondary just slightly:

Seems much more centred.

I then placed the laser into the focuser. Laser seems to show secondary alignment off. It is to the left of the primary spot:

I then use the tilt screws to return the laser spot to the centre of the primary - and end up back where I started

[AndyWB]

Sorry, that is to say, the image through the collimation cap is exactly as previously posted

[AndyWB]

And, I don't know if it's relevant, but the laser spot is nearly always in the same position after I rotate the mirror.

[AndyWB]

Also, I notice that the laser spot isn't hitting the secondary remotely near it's middle:

I presume it should?

[Jason D]

If you rotated the secondary then adjusted the 3 set screws with the laser, you should not end up at the same spot.

Can you rotate more? I did say rotate by a little amount but give more rotation and retry.

[AndyWB]

I know, I can't figure it out either. I did find that I had to undo all 3 screws slightly more than I'd have liked before I could rotate the secondary; it was more than just taking the pressure off. I'm wondering if the back of the secondary could have notches in it where the screws were previously, and that's causing it to rotate as I use the tilt screws to realign the laser.

I'm not got to try realigning again just yet; I'm tired and pretty despairing just now. I mean, how bad is the collimation in the top photo? Would that do for visual work? Alternatively, I'd consider seeing if I can take it to a shop and get them to align it - I've spent the better part of 10 hours on this. (Do telescope shops do that?)

[Jason D]

. I'm wondering if the back of the secondary could have notches in it where the screws were previously, and that's causing it to rotate as I use the tilt screws to realign the laser.

That was my thought too. See if you can cut and insert washers made from a milk container as shown in the attachment. You can cut the washers radially to make it easier to insert without removing the mirror. Start with one washer then try two and decide which is better.

Do your best and accept the outcome. I do not believe your collimation looks bad enough to warrent you going to some shop. Just enjoy the views.

Jason

[Moonshane]

that's a good suggestion with the milk bottle washers. I use a steel washer myself for the same purpose. it prevents gouges in the soft monkey metal secondary stalk which do cause re-rotation back to an original point (which is usually the wrong place). it also allows more easy turning of the stalk.

[AndyWB]

Right, OK, I'll try that. I'm drinking milk as I write

It might not be right away, though, and for the first time in weeks we've clearish skies at the moment - presumably I should still be able to use the telescope even with it's slightly rotated secondary? I mean, I gave it a very rough test by seeing if the moon looked like I'd expect it to - and it did - but that's hardly the most scientific.

I guess my inexperience is showing - I don't know how good "good enough" is?

And Moonshane, thanks, good to have a theory corroborated.

[Moonshane]

to be honest the difference between perfectly collimated (basically not possible in practice without a lot of money being spent) and just slightly out (especially where the secondary is concerned) is unlikely to be detected at the eyepiece so do just enjoy the views. in fact, seeing conditions, local hot air plumes and light pollution are more likely to have a visible effect than collimation which as in your case is close to being very good.

[alan potts]

I have just read right through this thread and congratulate your efforts, I take my hat off to you, a scope of mine would have been in the skip by now. Newtion scopes seem a bigger problem than most of mine, though I still need to get something to help check and re-collimate the Mak/Newt. I am not looking forward to that.

Alan.