Check the finder and adjust if necessary

If you have tweaked the collimation, you may have shifted the optical axis slightly, and the finder is no longer accurately adjusted.

If your telescope is new, or if you have moved the spider, you can check that the spider vanes have not been tilted or twisted during collimation. Look down the tube and move your eye to see each vane look its thinnest, and check that you see it centered against the mirror (if the vane is centered - otherwise, check that you see it over the corresponding part of the mirror). If offsetting the secondary mirror makes the vanes slightly out of line, or out of perpendicular, this may make the diffraction spikes look less distinct or pretty, but it does not affect the sharpness or contrast of the image.

You can also check that the collimation holds at elevations from some 20 degrees to zenith (some main mirror mounts, where the mirror isn't glued or clamped, will not hold the mirror firmly enough to use all the way down to the horizon). If the collimation cannot be kept within tolerances, there is some weakness in the tube. It may be the main mirror support, but it may also be the spider or the secondary mirror mount that is not rigid enough.

Does this procedure give offset, or non-offset, collimation?

It seems that there are widespread misconceptions about secondary offset, or at least different ideas about what it should mean.

If you dot the geometric center of the secondary mirror face, and center this dot in the focuser (using a crosshairs sight tube or a laser collimator) and also center it inside the tube, you will get a truly non-offset secondary. This has been recommended, but it will give an error (usually acceptable) of type 2, and I don't see that it would be simpler, or offer any other practical advantage.

If you have optically centered the secondary in a sight tube as described above (step 3), it will be correctly offset along the tube. But what happens if your secondary is centered within the tube, without offset away from the focuser in step 2? This is sometimes called "non-offset collimation", or "neither centered, nor offset, but a compromise" (Menard/D'Auria: Perspectives on collimation).

Once the point is set where the focuser axis hits the secondary (be it at the optical or mechanical center, or any other point), the further collimation of the main mirror ensures that the combined optical axis will return to hit the secondary at that very point again. Thus, if the optical axis hits the optical center of the secondary, but it is the geometric center that is centered within the tube, it means that the optical axis will be offset from the tube axis - an error type 4 (it may also lead to some small error type 3). But since the offset is typically only 0.5-1.5% of the aperture, the error is of little significance. Thus, if the design of the spider is such that you cannot easily offset the secondary, you may leave it centered in the tube with no serious effects.

What tools can I use to make the collimation easy and accurate?

Here are some useful tools. You can make some (or all) of them yourself, see the DIY section (or follow the links) for details. Some of my designs there allow full control of tolerances - you can see immediately whether or not your collimation is within the accepted limits.

Tectron makes a set of 3 tools: crosshairs sight tube, Cheshire and autocollimator. Orion makes a combination tool (Cheshire + crosshairs sight tube). I have no personal experience with these. Several manufacturers make laser collimators. Whether you prefer a laser or a sight tube, a Cheshire is also helpful.

Date: 2015-12-11; view: 780