Collimating a Cassegrain

• Coma on star images. This is a tail of any kind on a star images. On some scopes, it will be a fish-like shape, while on others the star will simply appear to have a slight fan on one side. If stars aren't pin-point sharp, suspect collimation.
• Out-of-focus star images show a secondary shadow that isn't at the center of the out-of-focus image.
• When you are close to focus, you see several offset images of a planet, instead of one. 

When do I collimate?

The best way to collimate is on a star, at night. You can create an artificial star to collimate during the day, but that is a whole subject on its own! To be effective, an artificial star has to be as close as possible to a point source, and be located at an appropriate distance from your telescope. Stars at night are ideal, but you will need reasonably steady seeing to do collimation. You use the diffraction rings around a slightly out-of-focus star to perform collimation, and if the air is turbulent, the diffraction rings will be so spread out that you won't be able to see them clearly. 

What tools do I need to perform a collimation?

• Two or three eyepieces that offer a range of magnification from about 200x to 600x.

• An Allen wrench or screwdriver appropriate to the screws that you will use to set collimation.

• A flashlight in case you need to look closely at the collimation screws.

• Patience!

Collimation guidelines:

• Always put the eyepiece directly into the visual back of the telescope. Never use a diagonal. You want the straightest possible light path for collimation.
• Make one adjustment at a time. Moving the collimation screws is like trying to tie your shoes by looking in a mirror. Everything is backwards and the familiar suddenly becomes unfamiliar.
• When you make an adjustment, you may need to tighten the screw securely. If the secondary is spring loaded, as on a Newtonian, you do not need to do this. However, on most Cassegrain-family scopes, loosening one collimation screw leaves the secondary dangling, and you will need to tighten the other two screws to see what you've actually got. This may seem like fussiness, but it will actually make the process go more quickly. Otherwise, you will think the scope is collimated, and when you lock down the screws, it will no longer be collimated.
• Always (repeat: always!) re-center the star after every adjustment.
• Be patient!

Setting up for collimation:

To start collimation, point your telescope at a moderately bright star. "Moderately bright" will vary based on seeing, the aperture of your scope, the eyepiece you are using, etc. So there is no hard and fast rule. The key, however, is to choose a star that is bright enough to give you diffraction rings just outside of focus, yet not so bright that the rings are thick or overly bright.

Place an eyepiece that gives you about 200x in the visual back, and center the star in the field of view. Defocus the image slightly. 

You will collimate by making adjustments until the diffraction rings of the out-of-focus image are as perfectly concentric as you can make them.

Collimation check #1, at low power.

You need a slightly out-of-focus star image to work with during collimation. The image at right shows what you don't want: an almost solid doughnut of light. This image is too far out of focus, and it will be hard to collimate with it. The problem is that the inner and outer edges of the doughnut are fairly far apart, and it will be difficult to judge when they are precisely concentric. On the other hand, if the doughnut is obviously not concentric, then you know you are very far out of collimation!

The image at right shows the proper amount of defocus you are looking for. These diffraction rings are concentric, and show the ideal state: a perfectly collimated telescope. This is where you will get to if you collimate carefully. Air turbulence will no doubt result in diffraction rings that move and shimmer, and perhaps even throw some spikes into the mix. Shimmering and spikes limit how perfectly you can collimate your telescope on any given night.

What does mis-collimation look like?

The image at right shows what a mis-collimated scope will look like. The diffraction rings are not concentric -- they are pinched or bunched up in one direction. You may also see some flaring or fuzziness on the side away from the pinching. This is the typical starting point when a scope is only somewhat out of collimation. For a very badly collimated scope, the diffraction rings may be very tightly bunched on one side, and very broad on the other. In such a case, you simply need to make larger initial adjustments. The diffraction rings may not be circular -- don't worry, that's normal. They will only start to look circular when collimation is getting closer.

Making adjustments: the big secret

There is a very simple rule you can follow that will make collimation a pleasure rather than a chore. I have watched people (including myself, once upon a time) begin collimation by making random changes to the collimation screws, and try to learn which screw controls which direction. Granted, after 20 minutes or so, you will be an expert on which screw moves collimation in which direction. But who wants to spend 20 minutes learning this? Especially when you will have to spend just as much time the next time you collimate!

At the start, the out-of-focus star image is in the center of the field. The goal is to move it some unknown distance, in a direction exactly away from the pinch, to achieve collimation. You don't want or need to do anything that doesn't accomplish this simple goal. 

So loosen one collimation screw a small amount. "Small amount" usually means about 1/8th of a turn, or something close to that. During this test phase, do not tighten the other two screws before checking your results. Observe whether or not the adjustment has moved the out-of-focus star image in a direction away from the pinch. If the answer is no, make a note of the direction of movement (on paper if necessary) and re-tighten the screw so that the out-of-focus star image is again centered. Adjust the pointing of your scope if the re-centering is not exact. Then try a different collimation screw. Repeat until you find one screw that moves the image as close as possible to the desired direction. Tighten the other two screws, and then re-center the out-of-focus image. You have now made your first collimation adjustment.

Evaluating the adjustment

Examine what has happened to the out-of-focus star image. You should see an improvement in collimation (unless you made too large of an adjustment). The area of the pinch may be less pinched, or the diffraction rings may be less oval. Flaring may be less, or may have disappeared. The exact nature of the improvement varies with the type of scope and the amount of mis-collimation you started with.

Note whether the pinching has changed direction. This may affect your choice of which screw to use for the next adjustment. If collimation looks perfect or very close to it, change to a higher power eyepiece and continue until perfection is achieved, or whatever the seeing will allow. It is only when you get to around a 600X eyepiece that you will get the kind of collimation that will knock your socks off while viewing planetary detail on a still night.

When you have gotten good collimation while slightly out of focus, you can improve it further by collimating in focus. It takes really steady seeing and a high-power eyepiece to collimate in focus. See for tips on in focus collimation.

Changing eyepieces

When you switch to a higher-power eyepiece, you should also decrease the amount by which you are out of focus. On a really still and transparent night, you can even tweak collimation by working with an in-focus star image. You can adjust the faint diffraction rings in the airy disk to make them as concentric and evenly bright as possible, and you can observe the bright point at the center of the airy disk to make sure that it is circular and as bright and sharp as possible. I have never been able to do this with less than a 600X eyepiece in the telescope, and only on extremely steady nights.