I have being working on the collimation of my SCT using this as a guide
http://legault.club.fr/collim.html
Of the three step process described in this articles I'd guess I am somewhere in the middle of stage two. So stage one - de-focus a bright star at 200x magnification and I see a doughnut pattern with the central obstruction looking reasonably well centred either side of focus.
Then I chose a dimmer star, high up and upped the magnification to 400 - 450x, When the star was defocus a tad (just before you get a doughnut) - it looked more like an engagement ring shape - one part of the ring was say 15% - 25% brighter than the rest. Is this normal for a slightly out of coll. scope?
Lastly when I focused the scope as well as I could in this stage - I didn't see any "airy patterns" - rather the star looked slightly hoary rather than a uber tight pin prick. Again is this normal for just slightly off optics - or does limited seeing (full moon, some high clouds) and not finishing stage two of the collimation process stop me from seeing any differaction rings?
Many thanks folks,
Matthew