Collimation Woes

[WillEyedOney]

I thought I would have a go at collimating my 8" Skywatcher Dob with my Cheshire eyepiece, however I am having a few problems that maybe some kind person can help me with.

I started off, as the instructions say, adjusting the secondary so I could see the clips on the edge of the primary, but no matter how I adjusted the three screws all I could see was maybe 2 clips at a time and things looked nothing like the illustrations and online guides.

I took a break and went back to have a good look without trying to adjust anything. After a while I realised that the secondary was off centre. Not too far in or too far out but up and down as I see it through the eyepiece. The lower side is below the bottom of the sight tube whereas the upper side has a large gap.

I am at a loss as to how to correct this. do I need to adjust the focuser or do I need to tweak the secondary vanes (though it would need a BIG tweak to align it correctly)

I bought this last year but have only just had a chance to start using it in earnest.

Any suggestions?

[NGC 1502]

Hi, It's hard to say from a distance what the problem is, but don't forget that the secondary can be rotated within the tube, that is, rotated as you look into the front of the tube. It could be why you can only see 2 mirror clips not 3. You could measure the length of the spider vanes with piece of folded paper. Measure from the metal cylinder that the secondary is attached to, across to the tube wall, they should all be the same. The secondary itself is offset on its mount, this is correct. You are starting out well, as the secondary needs sorting first, before collimating the primary. I use a simple collimating cap to get as close as I can, then a final tweek on the primary when viewing a defocused star. Go slowly, try to remember what you have already done, and you will get there in the end. Cheers, Ed.

[WillEyedOney]

Thanks for the reply. I had another go this morning when I was not half asleep and just slackened off the 3 bolts and give it a bit of manual persuasion, that's all it took, that and some adjusting of the centre screw, Saturn looks great.