Collimination - SCT - question - seeing a diamond ring pattern when unfocused

[g__day (Matthew)]

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

[allan gould]

Looks like you need to pick a night of better seeing. It can be a pain to get right the first time.

[JohnG (John)]

Was just about to post the same thing Allan, I would say your in the area where just the slightest turn on a single collimating screw is necessary, you must have a night of just about perfect seeing to do that.

I am 300Kms away from you and last night was not one of those nights.

Cheers

[madtuna (Steve)]

Matthew I've seen the same diamond ring pattern when collimating my C11 so hopefully it is normal and not just your scope.
The Bobs knobs were a god send but yeah a good clear night sure helps

[g__day (Matthew)]

Thanks guys - so to double check - to get "airy patterns" do you need near perfect collimation and excellent seeing?

[JohnG (John)]

Put simply, yes.

You probably do this anyway but make sure your OTA has cooled properly before you attempt the final collimation.

Cheers

[g__day (Matthew)]

Its outside in a permanent astro-lab, so I'd guess its at ambient temperature by the time I start to image - maybe 8 or 9pm?

[g__day (Matthew)]

Thanks to a recommendation - CCD Inspector from CCDWare to the rescue - Collimation much improved, now I just need the clouds to depart!