To collimate a telescope means to align all the optical elements it contains so the image produced is optimal. Any mirrors (primary/secondary) or lenses must be centered relative to each other, and the respective optical axes must become parallel. A refractor is typically collimated by the factory, while a Newtonian reflector or a Schmidt Cassegrain Telescope (SCT) must be collimated often by the user to provide optimal image quality. Bad collimation is perhaps the most common cause of poor image quality in amateur telescopes.

The images above are real webcam images from my C8 collimation check April 5, 2002. The images show out-of-focus star images, an extremely sensitive method for verifying collimation. Notice the nice symmetric shapes, the diffraction rings, and the bright center spot at right (closer to focus). This scope is in nice collimation, but the final test is in close-to-focus images (not shown) possible under the best seeing conditions.

Effects of poor vs. good collimation

The effects of collimation can be dramatic. Below are two images of Saturn shot with the same scope (C8 + 3x barlow) and camera (Vesta 675K webcam), slightly different adapters. The first image is from September 23, 2001 when I had not touched the factory collimation. The second image (November 19, 2001) was taken after I had collimated the scope for the first time. Judge for yourself.

General collimation procedure

The best online collimation instruction can be found at Thierry Legault's home page. Another great resource is the book Star Testing Astronomical Telescopes by Harold Richard Suiter.

A very detailed and illustrated procedure is also provided by Sylvain Weiller at It discusses collimation using a webcam, and it is an excellent resource.

Collimating a Celestron C8

I have found it very effective and quite easy to collimate my Celestron C8 SCT. The only possible user-adjustment (normally) with an SCT, is to control the tilt of the secondary mirror, using the 3 collimation screws in the front of the secondary housing, in the center of the Schmidt corrector plate.


My C8 came with 3 Phillips collimation screws that required use of a screwdriver when collimating. Using a screwdriver is very inconvenient in the dark and cold. You also run the risk of hitting the corrector plate with the screwdriver. Not good! Luckily, there is an easy and cheap fix. I bought Bob's Knobs which replaces the Phillips screws with knobs that require no tools to be used. This may sound trivial, but it isn't. It makes collimation a joy, and it is strongly recommended. The image below shows my C8 with Bob's Knobs installed:


Daylight collimation

Collimating at night using a real star is the recommended way. It will give the best results. But sometimes, seeing and other practical problems may prevent effective night-time collimation. Then you can use an artificial star during the day:


This simple tool consists of a Christmas ornament reflecting the light of the Sun. Place it as far away as practically possible, preferably 100m or more, but a few meters will also do if required. Make sure the sun is behind you and focus on the reflection in the ornament. The images should be similar to the images on the top of this page, but if the ornament is too close, you will not get perfect diffraction rings. Then do the best you can, and check with a real star later. You should achieve good results.