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Scooter's picture shows a slight miscollimation, but everyone is correct to note the outline of the reflection of the secondary will not appear concentric in the collimated scope. AND, the reflection of the spider vanes will not be coincident with the wires in a sight tube, either.
It's difficult to get a good photo through the focuser or peep hole of a tool. It's so easy to be ever so slightly off angle, which results in parallax and the appearance of miscollimation even if the scope is properly collimated.
The laser/barlowed laser technique earlier described works fine to collimate a scope, but it does not set the original position of the secondary in regards to rotation or position under the focuser. It may be hard to realize this, but the secondary can be successfully collimated even if it is not centered under the focuser and even if it is improperly rotated. All this results in is uneven illumination around the periphery of the field and a slight tilt to the field (which is often ignored).
If this were not so, lasers would be useless in collimation, because only the axial (center) is collimated by a single beam laser. Off axis rays are not.
Which is why a sight tube is useful to use even if you primarily collimate with a laser.
A sight tube allows you to make the outline of the secondary concentric with the focuser (for even field illumination) and to make it appear round (for proper tilt of the focal plane and even edge illumination). So it's quite useful to at least start out with a sight tube, though crosshairs aren't necessary for those two operations. Then, a laser or sight tube will perform the same function for correcting the tilt of the secondary. Actually, when we adjust the tilt of the secondary, we are aligning it with the axis of the focuser, which is why it doesn't matter if the focuser is perfectly perpendicular to the tube.
Then, the laser techniques describe (single beam out, barlowed return) will do the mirrors just fine.
For f/6 or longer. For f/5 and shorter, better alignment is still possible and tolerances for miscollimation grow less as the f/ratio shortens. And if you use a Paracorr, tolerances near zero below f/4.5.
For the shorter f/ratios, an autocollimator (like the Catseye Infinity XL) becomes nearly essential, and can collimate the telescope equal to or better than a star collimation (it's actually easier than a star collimation). I have been very cautious when using a sight tube and cheshire (same ideas as laser and barlowed laser) yet still found residual errors with an autocollimator. Indeed, the autocollimator is SO sensitive one can see collimation changes with tube contraction in falling temperatures, or changes in collimation with the altitude of pointing if the mechanicals aren't solid enough (that's an entire thread worth there).
Careful use of an autocollimator will show you the best star images your instrument is capable of. I've said that I was surprised at how much the seeing improved when I learned to use an autocollimator and I meant it.
The overall key to doing collimation is to repeat the process beack and forth between the tools a couple times. Each movement as seen in one tool has an effect on what's seen in the other. By repeating and going back and forth you arrive at a condition where the tools and procedures all agree.
Hope that helps a bit.
Don
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