[209herschel (Herschel)]

Hi everyone, I think I'm getting to know my 10" dob a little better. I just have a quick question about Collimating by viewing a star.

I pick a bright star, and defocus my 15mm ep. I get a good centre dark circle and am I right that this means the secondary mirror is correctly aligned under the focuser?

My issue is that I don't ever get really clear and spaced concentric rings outside the secondary. The rings I see are always hazy and shimmering, almost like very tiny sparkles. I've never seen clear bands of circles when I defocus. Is this because seeing conditions are bad? I'm trying to view stars close to the zenith to help out but I get a lot of planes flying overhead and there's a lot of light and the sky in my yard usually looks a dark grey. Is the only way to improve these results to go to a dark sky?

Thanks very much.