Ever sinse I bought my 6" refractor, I've been forever trying to collimate it PERFECTLY...It's always been juussst a smidgin out.
The other night, I thought I'd try something different & try collimating with the diagonal in place.
This I did, & the results are great! Perfect collimation finally achieved!
I've also read that this is a 'no-no'...always collimate a refractor without the diagonal etc etc. Well I've been doing that for a few months now. I also 'squared' the focuser. I never take the diagonal off anyways so it's best to collimate with all optical train equipment in place IMO..