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mmmm Use a Defocused star for a start, you need to make sure that your Secondary is pointing directly at the middle of your focuser. without question. Then collimate the Primary back to the secondary. Don't use a really bright star as you can get reflections/distortions ect ect. go for a 3 mag star and use your camera to do the star test.
Center the defocused star in your FOV anywhere else is worthless for this task. now start tweeking move it till it is absolutely centered to the best of your eyes ability, being sure to adjust, then recenter, adjust, recenter.
then start bringing in the focus while the camera is taking shots. you will see the airy diffraction rings appear, and a decent exposure, eg 2-5 seconds should make it very apparent and easy to see.
Im not 100% positive, but make sure you are aware of Mirror flop, as SCT generally have 1 direction they are collimated to, eg, on the Infocus or Outfocus. I have collimated a RC and Cassigrain scope in this fashion and produced excellent results.
Hope that helps mike
Brendan
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