Collimation of scope

On commercial SCTs the only available collimation adjustment is the angular alignment of the secondary mirror via the three secondary adjustment screws. Unfortunately, in many SCTs there are greater or lesser misalignments in the non-adjustable elements, and if this is the case the telescope can never be perfectly collimated no matter how much the secondary is adjusted. This would be analogous to trying to collimate a Newtonian with a fixed primary mirror and focuser, and an adjustable secondary: if the primary and focuser are perfectly aligned it can be done, but if the primary or focuser axis is off you can never get perfect collimation.
The best that can be done is to minimize the aberations by adjusting the secondary to get the best star image with a star test, as the previous poster pointed out. I would add that this should be done without using a diagonal even though it may be a neck-breaker, because most star diagonals are not truly collimated and centered. Rod Molise says this is controversial because you will be using the diagonal for viewing anyway, but this is wrong because if you collimate with the diagonal in one position, and later rotate the diagonal to another orientation when you are viewing another part of the sky, you will change the collimation. The solution is to collimate straight through, then add the diagonal and adjust the mirror in the diagonal to reproduce the collimated star image.
If you have a good laser collimator (now why would I suggest that?), you only need to do the star test once. You can use the collimator to measure and record the secondary mirror setting, and return to that setting at any subsequent time. As soon as you are sure you have the best star test adjustment, remove the eyepiece from the back and insert the laser collimator. The beam will reflect from the secondary and return to the face of the collimator. Take careful note of the location of the beam impact on the face of the collimator, for this is a measurement of the setting of the secondary at that moment. In the future, if the scope goes out of collimation, you can restore the laboriously obtained star test adjustment with the laser collimator alone, by adjusting the secondary to return the reflected laser beam impact to the same spot on the collimator face.

Howie

How bad is it...I mean I've never seen a SCT that needed anything more than the standard collimation usually provided in the scope's manual. See for example:  http://skywatch.brainiac.com/collimation.pdf