Yeah, read on:
http://w1.411.telia.com/~u41105032/myths/myths.htm
"myth: You have to square the focuser very accurately
I'm not quite sure of even what "square" is supposed to mean - likely it means set perpendicular to the tube, or possibly to the optical axis - or both, always assuming you have made them coincide. There is nothing wrong with doing it, of course, but the secondary is optically flat, and the angle of reflection is not critical. Most secondaries are made to look circular when tilted 45 degrees (to reflect 90 degrees), but if the angle deviates from this by a few degrees, the only consequence is that the secondary will appear slightly elliptic - it won't affect the image.
The important thing is that the focuser and the secondary are lined up, as seen from the focus. If not, you should adjust either, as appropriate - if the secondary seems to be off in a direction from or towards the primary, you can usually move the bolt that holds it in the spider. If the error is "sideways", and the secondary is indeed well centered in the optical tube, it must be the focuser that is off to one side and needs shimming. If this error is left alone, the penalty is that the fully illuminated field at focus won't be centered - a small error here is no disaster, but the center of the field should always cover the center of the field of view, and best with a small margin to avoid the effects of a possible narrow turned-down edge on the secondary. Check with a sight tube in daylight when the telescope is collimated - when the peephole of the sight tube is close to the focal plane (top of the drawtube at a "normal" position), the whole edge of the primary should be visible in the secondary. If the focuser is tilted a little "down" or "up" the tube, this will be compensated for when adjusting the tilt of the secondary - the only consequence is that the fully illuminated field will be slightly elliptic - a thing you will never notice."
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myth: You should always finish collimation by star collimation
At the focal plane of a paraboloid mirror, there is one point where coma, the one-sided aberration, has a minimum. This point is, by definition, the focal point. Locating it can really only be done by star collimation - using a real star, or a sufficiently distant artificial one, collimating the primary mirror for a symmetric star image (high magnification, and image close to focus for best sensitivity) at the center of the field of view. If you put a small peephole at the true focal point and look, you will see a distant reflection of the peephole itself (if its inside is illuminated), as if far behind the mirror surface. The spot on the mirror that the peephole seems to lie behind is - again by definition - the optical center of the main mirror.
The common thing to do is to place a spot at the geometric center of the primary mirror and use it with a Cheshire eyepiece, centering the reflection of the Cheshire behind the spot. Normally this is close enough (I am still waiting to hear about anyone with a mirror where the optical center is significantly offset from a well centered spot!). Using the Cheshire is a lot easier, quicker and better reproducible than star collimation is - particularly with less than excellent seeing. However, if you find the one-sided asymmetry of coma on a star at the center of the field, it can only mean one thing - the center spot isnīt at the true optical center. The best thing is to move it or put a new one, centered in the reflection of the Cheshire (the next best is to make a note of the direction and distance of the offset), for later collimation. Now you know you have the true optical center marked.
Thus, you should take the trouble and do a careful star collimation under good seeing to ensure that the primary is accurately marked - but when this is done once, collimating with a Cheshire or Barlowed laser is easier and more reproducible. There is no point in star collimating every time."