FAQ about Collimating a Newtonian telescope

by Nils Olof Carlin (updated oct -00)

Well, maybe the questions here are not as frequently asked as they ought to be. After all, a lot of amateur astronomers own and/or use Newtonian telescopes. If you are one of us, you should ask these questions, and find out for yourself how important the answers are to the performance of your instrument.

What is collimating, anyway?

Collimating a telescope is lining up its optical components (lenses, mirrors, prisms, eyepieces) in their proper positions. This should be accurately done, or else the image quality will suffer.

Different telescope types, like Newtonian, Schmidt-Cassegrain, or refractors all need good collimation. However, they have quite different optical components, and here I will only talk about Newtonian telescopes, the simplest mirror telescopes.

Newtonian? My telescope is supposed to be a Dobsonian!

Don't worry. A Dobsonian telescope is a special kind of Newtonian, with a simple but very efficient mounting that distinguishes it from other Newtonians. Optically, and as far as collimating is concerned, they are the same.

I have bought a telescope, and it is factory collimated. Do I have to bother about collimating it?

Yes, most likely. If you have a factory-collimated refractor, Schmidt-Cassegrain, or Maksutov, you could very well leave the collimation alone and have a good chance of enjoying the excellent performance of your telescope for years to come. With a Newtonian, chances are less, for several reasons:

The main mirror must be held in place without stress that could bend it and change the optical figure, and cannot be rigidly held - it may shift slightly whenever you transport or shake the tube. The secondary mirror is also held by a "spider" that may change its position ever so slightly, and as we will see, it doesn't take much to disturb the collimation enough to really matter. If you move your telescope to darker skies and back, and particularly if you have one with truss tubes that you assemble and disassemble, you must be able to check the collimation each night out, and you must be able to tweak it whenever needed.

Even if your telescope was factory collimated before shipping, it may have been on its longest journey ever before it reached you, and chances are great that it has lost much of its collimation. If you learn how to check the collimation, you will know whether or not your telescope is ready to deliver its best.

If the situation is that bad, maybe a Newtonian isn't for me. Should I trade it in for something better?

My advice is: think again. There may be other good reasons for you to prefer another type of telescope. But a good Newtonian is a great performer when it is well collimated, and can come out close to or maybe ahead of any other instrument of the same size (aperture). Before you decide to trade it in, ask how much extra you would have to pay to get an alternative.

Suppose you have bought a fine guitar with a lovely note, and you are learning to play it. Now you notice it seems to get slightly out of tune. What do you do - learn how to tune it, or trade it in for a piano?

I believe that the reason Newtonians have a dubious reputation for critical performance is that too many Newtonians are never even collimated at all. Poor optics may not be easy or cheap to fix. Poor collimation, however, is something you can learn how to handle, and chances are good that you will be able to turn your scope into a star performer.

Don't forget - a complete collimation of all the optical components is a bit of work - but the nightly checking takes a few seconds, and the tweaking, if needed, may take a minute.

OK, I am willing to give it a try, at least. How do I do it? Read the manual?

When I tried to figure out the hows and whys of collimating, I had very little help of the manual for my 6-incher. I tried reading the sections on collimating in a few magazines and handbooks, without really understanding what should happen, and why. I kept on trying, and in due time I felt my efforts paid off. That is why I write this - I hope I can make it easier for you. But let me point this out: Much of what I write here is common knowledge, even if it is not easy to come by, but some is the result of my own studies and experiments - particularly the error analysis and some of the tools - and my recommendations here are very much my own (and very controversial in some places). I believe it is sound advice, but I may be wrong on some accounts - if you really find fault with what I say, don't hesitate to email me. Let it also be said that collimation is a subject of much heated discussion among us diehard Telescope Nuts, and I doubt that this will put an end to it.

I believe it will be easier for you to learn how, if you know why. By all means read your manual! Telescopes differ in design details, and your manual probably contains valuable information on how to adjust the screws and things on your particular instrument.

It's going to be a great night for observing, and I've got all the tools. I'm in a hurry, so skip the details - what do I do?

I've prepared a page that you can load and print out, but unless you have tried at least once, you should take the time you need to follow the detailed instructions and get truly familiar with the procedure.

What are the parts of a Newtonian, what do they do and what parts can or need I adjust?

This is basic stuff, and if you know it well already, just read quickly.

The optical parts are:

Date: 2015-12-11; view: 700