Collimation question

[Stonius (Markus)]

I've had my scope for years and made a collimation cap out of a film cannister way back when (remember those?). According to that, my scope never went out of alignment, even after a drive along bumpy dirt roads, so I never did it, though I did check it semi-regularly ('is it out of alignment yet?').

More recently I figured I should get a Cheshire eyepiece just to be sure so I can eek a little more performance out of my scope and found that it was every so slightly off.

So after years of owning a telescope, I decided to have a crack at collimating. Followed the instructions, after a few attempts I got it pretty exact.

BUT - no matter what I do, the Cheshire cross-hair in the eyepiece itself (not a reflection, in RED on the diagram) will not line up with the center. Every collimation I have attempted has wound up with the cross hairs displaced in the same direction.

http://markusstone.com/hosted_files/...tion_issue.jpg

What I don't understand is how the rest of the telescope's optical axes can be perfectly aligned, yet the cross-hair at the bottom of the eyepiece obstinately refuses to comply. If I rotate the eyepiece in situ, the center of the cross hairs stays put.

I've adjusted the tilt and rotation of the secondary and tilt of the objective; in short all the adjustments available to me. At this point I'm sufficiently stumped that it's overcome my embarrassment about asking such a basic question, even if I do get the wooden spoon award.

At this point, I wonder if I need to tweak the alignment of the focuser to the tube of the telescope? Has anyone else had this problem? I'm scratching my head here...

Thanks

Markus

PS, I honestly can't remember whether the Cheshire cross was off in the beginning before I started tweaking because I was following the instructions and looking at the reflections in the mirrors.

[janoskiss (Steve H)]

The Cheshire does not lie. Your collimation is off (if I read what you're saying and your diagram correctly). Just adjust the primary till the cross hairs line up. If that doesn't do it, your secondary is too far off.

Note that there should be some overall offset because of the way a Newtonian works (I assume you're talking about a Newt). But the x-hairs, its reflection and centre spot should all be aligned. I'll try and find a link to a good article for you that explains this and will edit my post when I do...

This article explains well how to do the collimation: http://www.skyandtelescope.com/astro...tor-telescope/

... but it does not show the final secondary offset inherent in Newtonian geometry. But if you follow the instructions you'll have a well collimated scope (the star test at the end is not normally necessary, but if you have enough seeing, it's a good way to check things).

This article explains secondary offset (which will pretty much take care of itself automatically if you follow the collimation procedure correctly): http://www.skyandtelescope.com/astro...ondary-mirror/

... but it does not show how things should look through the Cheshire when you're done. Maybe someone else has a better link.

[ausastronomer (John Bambury)]

Hi Markus,

Your problem is that the secondary holder is not centered in the telescope tube and your focuser is not properly squared to the telescope axis.

I posted a very lengthy post about 8 or 10 years ago on here explaining exactly how to do this but I have no idea where that post is now. Maybe someone marked it or can point you to it.

Cheers
John B

[Stonius (Markus)]

Okay. So I'm thinking it must be the gross positioning of the secondary in relation to the focuser. I'll have another play with it and see if I can get any different results. <sigh> Starting to wish I'd left well enough alone. Once up to speed, how long does it take an experienced collimator to get alignment?

Cheers
Markus

[doppler (Rick)]

Here's a step by step guide.

http://astro-baby.com/collimation/as...on%20guide.htm

ps I still only use a film canister with a 1mm hole in the cap, and fine tune with a star test.

[wayne anderson (Wayne)]

Hi Markus, These posts my be the ones John mentioned if not they may still be helpful.


http://www.iceinspace.com.au/forum/a...hp/t-6678.html

http://www.iceinspace.com.au/forum/s...?t=6429&page=2

[Stonius (Markus)]

So much reading....

I think the article John was referring to was this one; http://www.iceinspace.com.au/63-390-0-0-1-0.html

Still reading before rolling my sleeves up once more...

[janoskiss (Steve H)]

Originally Posted by doppler

Here's a step by step guide.

http://astro-baby.com/collimation/as...on%20guide.htm

That looks like a good one: thorough but not over the top detailed (and sticks to simple Cheshire). It also shows the (apparent) offset of the secondary: what should appear centred and what does not need to be once collimation is completed.

@OP. It's not hard but you've got to do it systematically from start to finish. Just so you know what to worry and not worry too much about: The positioning of the secondary is least critical (you can eyeball it and/or use a ruler), its orientation is second least critical (get it close enough looking straight through the focusser) and the orientation of the primary is most critical (be as precise as you can be with Cheshire).

[Stonius (Markus)]

I think I finally nailed it.

My problem was I was too scared to change the height of the secondary before but I felt more confident having read through the mentioned articles (thanks guys!). That was a big difference. I tweaked one of the vanes too, to center it, but the trick that got me back on track was to grab a laser pointer and point it into the Cheshire eyepiece.

It took me a few minutes to realise the laser while having a wide, flat face (which enabled me to hold it at 90 degrees to the eyepiece) wasn't, in fact, propagating at 90 degrees. After a few paper shims at the eyepiece (yes, dodgy, I know but it worked in a pinch!) I managed to get the laser hitting the crosshairs at the bottom of the Cheshire. From that, I fiddled with the secondary until it projected the laser dot onto the primary centre spot. Then I went and checked the Cheshire visually.

From there it was pretty much textbook. Can't wait to take it outside and see the fruits of my labour :-)

[janoskiss (Steve H)]

Excellent, well done! From here on it'll be like riding a bike: easy and you'll never forget how.

[Stonius (Markus)]

Oh and thanks Rick - that article is a beauty, though I only got around to reading it after the work was done. Printing it off now :-)