Newtonian collimation question

[doug]

     I have a question!  Collimation of a reflecting telescope!  Very easy for those who know how and have done it, but for a no-no like moi, I have a technical blank in my head. 

     I bought a little collimation device which was recommended by the "Sky at Night" magazine, thinking ..at last I can do it.. so I followed  the instructions, unscrewed screws on the small mirror (secondary?) and tried to adjust it. Hmm!!  I still get funny oval Airy rings around my starfield, so now I just use the refractor. I must be doing something wrong, but I know not what. So come on, all you techniks, assist me in my noble quest. I can always bring the telescope tube and associated bits along to an observing session and, if somebody shows me where I went wrong and demonstrates the CORRECT way to collimate it, I shall always be in their debt.

     Thanking you all in anticipation.

[Ian]

collimation is one of those things that sound really simple. Just line all the bits up, bob your uncle...

But, as you've found when you're staring down the tube with one eye (so you get no depth perception) twiddling little screws trying to work out what difference you're making.

I would assume the bit of kit have is a Cheshire eyepiece, basically a plain tube with a very small hole one end and a crosshair at the other. It's probably got a wedge shaped bit at the top as well that you can shine a light on to give yourself a brighter target to look at. This is the best, single tool for collimating because it allows you check every component.

Before twiddling any screws, make absolutely certain you know what each bit is that you're looking at. This is not trivial. Peering down the cheshire you'll see the reflection of the little hole you're looking through, then the end of the tube with the crosshairs, the end of the focusser tube, then in various orders, the reflection of the primary in the secondary, the edge of the secondary itself and possibly the telescope tube. There will also be the reflections of the eyepiece and the secondary in the primary too, if you mistake these for anything else, I would say your collimation is about right.

The next question is, does the primary have a centre spot? You'll struggle to complete the job without it...

Aligning the centres of the elements

The first thing to check, which should only need to be checked once, is that the focusser, secondary and primary are all centred correctly. To establish this, use the edge of each bit (that's why you need to know what you're looking at). Check using the edge of the secondary that it is centred in the view through the bottom of the cheshire (I'm going to assume that it is, and unless it's massively off, it will be good enough). The check the edge of primary the same way. (this is more important, and worth trying to get right).

Aligning the axes

Now it's the twiddling screw bit. Again, starting from the focusser, looking through the cheshire align the centre dot on the primary with the crosshair on the cheshire (not it's smaller reflection, that bit'll come). If it is not, then adjust the secondary until it is.

Now, the final adjustment, line the reflection of the crosshairs up with the actual crosshairs and once that's done, you're scope is collimated.

Couple of tips. While looking through the Cheshire, shine a small torch into the opening in the side of eyepiece, you'll get a clearer view of the crosshairs, secondly, shine it in and around the business end of the scope. It can be easier to identify the edge of the secondary by shining a light on it's back. They usually have a ground edge that isn't as silvered, so it glows when illuminated that way.

However, having written that, if you're getting oval Airy discs that is a symptom of astigmatism. This may be inherent in the mirror, but are more usual and better diagnostic is a pinched primary. Bad collimation usually causes the diffraction rings to be off centred but I'd check that first. If the primary is pinched, this is usually caused by overtightening any clamps holding the mirror. Anything that squeezes or holds the mirror should be tightened up enough to hold the mirror, nothing more...

[Ian]

or failing that, bring it along to a meeting, and we can show you, it might be easier (now I've re-read wot I rote)

[doug]

     

     Thank you, Ian, for your invaluable advice.  You are a man with obvious talent, so I shall have a go,as Wilfred Pickles used to say.  (You won`t know who that was so don`t worry)

     Cheers from Doug.

     (I notice that I am no longer "stardust" but "nebula".  Do I have spiral arms or am I cloud?)

[Ian]

Wasn't he in Steptoe and Son?

When you're scope's properly collimated, you'll be able to tell the difference between a cloud and spiral