Collimation: Aligning a Reflector's Mirrors

Collimation means aligning a reflector's two mirrors so they point at each other correctly. It is the only routine maintenance a Newtonian has, it takes a few minutes, and it is the single most over-feared aspect of owning one.

A Newtonian has two separate mirrors in a tube, each held in an adjustable cell. The large primary at the back gathers the light and the small secondary near the front turns it out to the eyepiece. Both are adjustable because both need to be aimed precisely, and anything adjustable can drift.

A refractor's lens is fixed permanently in its cell at the factory and there is nothing to adjust. That is the maintenance advantage refractors are sold on, and it is genuine.

The star test is the standard diagnostic and it needs no tools at all.

1. 1Point at a moderately bright star at medium to high magnification, on a night when the air is reasonably steady.
2. 2Defocus slightly, until the star becomes a small disc rather than a point.
3. 3Look at the disc. In a well-collimated Newtonian it is a round disc with the secondary mirror's shadow centered inside it, like a doughnut.
4. 4If the shadow is off to one side, the telescope needs collimating.
5. 5If the disc boils and churns rather than sitting still, that is atmospheric seeing or a mirror still cooling, not collimation. Try again later.

There are two adjustments and they are done in this order.

1. 1The secondary mirror, near the front of the tube. It is adjusted so that looking down the empty focuser you see the whole primary mirror centered in it. This one rarely needs touching once set.
2. 2The primary mirror, at the back, adjusted by three screws on the rear cell. This tilts the main mirror so its reflection is centered. This is the adjustment that drifts and the one you will actually make.

The usual tool is a collimation cap or a laser collimator. A collimation cap is an inexpensive plastic disc with a small central hole that makes it obvious when the reflections are concentric. A laser is faster and needs to be collimated itself to be trusted.

• It is not a repair. It is an adjustment, and it is expected.
• It is not the cause of most blurry images. Focus, magnification, cool-down and the atmosphere are all more likely. Focus troubleshooting.
• It is not needed on a refractor, a smart telescope, or most Schmidt-Cassegrains in normal use.
• It is not a reason to avoid buying a Dobsonian. Most owners find it takes less time than setting up a tripod.

If a Newtonian is soft in every eyepiece at every magnification, on a steady night, after cooling, then collimation is the answer. If it is soft only at high power, that is a different problem.