Collimating a Newtonian Reflector - A Guide for Novice Telescope Owners
by Joe Cali (OzEclipse)
Collimating a Newtonian reflector fills new telescope owners with fear yet it is a relatively simple process. With the telescope in a normal position, pointing about 20-30 degrees above horizontal, look into the eyepiece holder with the eyepiece removed.
If the instrument is collimated, everything should look concentric as in the bottom illustration. If it is not then:-
ADJUSTING THE SECONDARY MIRROR
1. First adjust the small screws on the back of the smaller diagonal mirror until you can see the whole of the big mirror centred in the diagonal mirror. You should also be able to see your eye centred in that small mirror.
ADJUSTING THE PRIMARY MIRROR
2. Adjust the screws on the back of the big rear mirror until the silhouette of the small mirror is centred over the big mirror. Some mirror cells have three screws with springs to keep the tension. Better designed cells, have three pairs of what are called push pull screws to move the mirror. In each pair, one screw pulls the mirror holder toward the rear plate, the other pushes against this. They are a little more time consuming to adjust. The pull screw needs to be loosened before the push screw can be adjusted. When the adjustment is complete, tighten the push screw carefully until they are tight. Push-pull mirror cells don't lose their collimation as often as spring loaded cells.
3. If the scope is way out of collimation, you may need to cycle through steps 1 & 2 more than once.
4. Check the diagonal again and make any minor adjustments to centre the image of the big mirror and your eye's reflection. This should only be necessary if the collimation was way out at the first step.
5. Make sure any lock nuts on adjusting threads are tightened to prevent slippage or loss of collimation.
6. At night with the telescope, there is one final step when you get a bit more experience. It's called a star test. You look at the diffraction rings around a very slightly defocussed and precisely centred bright star. The procedure and illustrations of how to interpret the diffraction pattern are described in detail in this article:
This procedure is more complicated and not recommended for complete novices. As you gain some experience, completing the collimation based on a star test will extract the very best performance from your Newtonian reflector.
The author Joe Cali built his first telescope, a 6" f7 in the 1970's as a teenager and has been a keen amateur astronomer since. He still owns and uses the 6" f7 and an 18" f5.5 dobsonian.