Collimating question about centering the star imag

Everyone,
I am fairly new at collimating scopes, but have done about half a dozen with an artificial star before testing them outside. The hardest was a Bird-Jones, but my question is about centering the out-of-focus star in the FOV. I was out working last night with a scope and the out-of-focus star gave nice concentric rings, but out about 20% from the center of the FOV. When I focused on that star, at that 20% off center point, it was very good. I know that the goal is to get the out-of-focus star in the center of the FOV, but if you can get concentric rings NOT in the center of the FOV, does that mean it is collimated at that off center point? Is this totally wrong and "perfect" collimation can only be done at the center of the FOV? I have a copy of Suiter's book in the mail coming to me, and maybe my answer is in there...but I thought I would ask the group this question.
Thanks.

This scope is a "regular" Newtonian, NOT a Bird Jones, right?! If it's better at some off-center point than at center, and the opposite off-center point is even (4x) worse - than yes, that best point is the optical field center. If this is a Newtonian telescope (main mirror and a folding flat), you tilt the main mirror along that vector toward center until it is cured. BTW, the amount of tilt is given by: T = (0.14 x % x afov / mag), where % is how far out from center to edge best focused dot is found, afov is the eyepiece aparent full field of view (deg), mag is the telescope magnification using that eyepiece, and T is the needed tilt of the primary mirror (deg). In practice, you just fiddle with the knobs until it is cured. This assumes that it is indeed Only a collimation (alignment) problem. The Bird/Jones would be a different beast! Tom Dey

Collimation for relatively simple scopes (Newts, SCTs, etc) is all about getting the field point with "perfect images" moved to the center of the FOV. Once the collimation is sort of close (i.e. the location of perfect images in somewhere in the eyepiece), it's just a matter of moving those perfect images to the center of the eyepiece. In fact, you can usually tell what screws to adjust which way, if you find those perfect images somewhere in the eyepiece, and think what adjustment is needed to move that image to the field center.

There are several problems with having the "perfect images" too far off-axis. For example, if you switch to a very high power eyepiece, you may find those perfect images are now outside the (narrower) field of view. Also, most eyepieces will give their best images on-axis, so if the scope itself has the best images off-axis, there may be no location where the images are as good as they could be. For example, a Newtonian will have off-axis coma, and an eyepiece may have lateral color or spectrum-like color smears or astigmatism off-axis, so if it is mis-collimated there may be no location with ideal images. (On the other hand there are rare instances where eyepiece aberrations might cancel some other aberration in the scope - like a refractor with a wedged lens might be cancelled if the eyepiece has the opposite color aberration.)