Help with collimation

[E_ri_k]

Hey guys, just received my glatter collimator and TuBlug! Very nice and easy to use, got my reflector collimated in about 5 minutes....I thought.

I thought I would look through the focuser and see if everything was nice and lined up, it wasn't really. (I'll try to explain this so it makes sense...and add a picture) Got the laser right in the centre of the primary mirror, and the shadow perfect in the TuBlug. When I move the focuser in and out the dot moves a fair bit away from the centre and then back perfect again. This makes me think the focuser is not square on the tube? If so, how do you fix that?

The picture is what I see when I look through the focuser without anything in it. To me it looks like the focuser is not sitting square.

What do I do?

[umadog]

Guess what? That's what it's meant to look like, you did it right! I attach two links which have really great information about collimation. IIRC they should go some way towards explaining why it looks like that. If you need to join the forum to see the info, then join. It's well worth it. There are photos in the first link which show exactly what you're reporting (just ignore the weird triangle reflections):

Telescope Reviews: Concise thread about autocollimators+improvements

Telescope Reviews: Useful info about secondary mirror alignment

The focusser issue is different. Here is a thread on that:

Telescope Reviews: Best method to square the focuser to the OTA

I have never never "squared" my focuser. As you can see, there is some debate on the issue. I centred the secondary in the OTA (allowing for the offset) and then found that the secondary wasn't centred in the focuser. I figured out that the focuser must be pointing slightly to one side. I therefore adjusted the focuser so that it pointed straight at the secondary. I did that with a sight-tube. Now everything is good. That's all I had to do to "square" the focuser.

I don't have a laser but use an auto-collimator (the device that generates those funny reflections in the first link). The auto-collimator is even more accurate than a laser (although it's less intuitive to use) and with it I also see a shift in the collimation when I rack the focuser in and out. I collimate at about 1/4 racked out. At 1/2 racked out collimation is slightly off and at 3/4 racked out it's good again. I can only attribute that to small imperfections in the focuser. It's behaved this way both before and after I adjusted it. I suspect that it's for this reason that one is supposed to collimate with the focuser racked out to where it would be if you were in focus with an eyepiece.

Just to expand on that, to make sure that the secondary is correctly positioned in the focuser you need to use a sight-tube. The laser won't help there. Chances are that you've done everything pretty well. I wouldn't mess with the focuser unless there is a compelling reason to do so.

[E_ri_k]

Ok, had a read on the net, and played around with the adjusting screws on the focuser. Started to collimate again, with the focuser focused in the middle and fiddled with the screws on the focuser. Now when I focus in and out, the laser only moves minimally from the centre spot on the primary. It moves off the spot by about 2 or 3 mm when the focuser hits the bottom, and is ALL the way in, which I guess isn't a huge issue?

Think I may have solved it!

[E_ri_k]

Thanks for those links! Yeah I played around with the laser with different focus distances. At first the laser dot moved about 10-15 mm of the center when I changed focus! The dot now stays pretty much dead on centre when the focuser is moved in and out between the usual focusing range.

[umadog]

Woah! 10 or 15 mm is a lot! What I was seeing was not likely of that magnitude. You seem to know what you're doing and correctly diagnosed that the issue was the focuser: good stuff, you're well ahead of the game.

I reckon you're probably good as you describe it. How much do you rack the focuser when changing eyepieces? Probably not very much so you're unlikely to be shifting that laser point by more than 1 mm. Remember that you only need to be collimated over the range of conditions under which you use the scope. Furthermore, collimation doesn't have to be exactly, exactly, bang-on. There are tolerances and it sounds like you're well within them for this measure, although I don't know the exact numbers.

[Jason D]

The diagram looks correct. First two attached are photos from my setup. Also check my avatar. The secondary mirror shadow is supposed to look shifted towards the primary mirror.

As far as the laser shift when the drawtube is racked in and out, that does not sound correct. This has NOTHING to do with squaring the focuser. Does your drawtube have some play (i.e. loose)? If yes, that could explain it. Basically, the laser beam does not sound to be parallel to the focuser axis. Check 3rd attachment.

Jason

[E_ri_k]

Thanks guys, Jason, I have no play in the drawtube. The problem was that whenever I focused in or out, the laser spot would move off the center of the primary. I figured that the focuser was not aimed correctly at the secondary mirror.

Read some stuff on the net and maybe I used the wrong word when I said "Square" the focuser.

After adjusting the screws on the focuser however, the laser dot stays centred on the primary when moving the drawtube in and out.

Unless I am missing something though..., I am assuming that it is correct for the laser to stay centred on the primary, no matter where the position of the drawtube is? Otherwise I hope I haven't messed anything up by adjusting the screws...

[umadog]

Hmmm.. Good diagram, Jason. I'm now wondering what was going on to cause that 10 mm movement to go away.

[E_ri_k]

It must have been the drawtube on the focuser not being perfectly at right angles to the primary mirror? Sorry for the poor diagram, I'm not able to take a photo of my setup at the moment... By adjusting the 3 screws, the collimation is the same with the drawtube in different positions.

[E_ri_k]

Ok, I think I am an idiot.....I got it right finally. Needs a bit more tweaking but I think I'm on the right track.

[Jason D]

Glad to hear your problem has been solved.

You can't go wrong with Glatter laser collimator and TuBlug.

[E_ri_k]

Thanks Jason. Yeah the glatter is great. Did a star test last night, and it looks pretty perfect to me. Got some good images also!

[umadog]

Awesome!