Collimation Question - Using a defocused star

[209herschel]

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.

[astro744]

Use a higher power eyepiece as you will get better diffraction rings both in and out of focus. Try racking inward past focus and outward past focus.

See http://www.willbell.com/tm/tm5.htm for some example images of what you can expect to see. Note you are only collimating not evaluating the mirror surface quality so although steady air is best, you can do this in air that is shimmering and in light pollution.

Your aim is to have concentric rings at the highest power you can observe with as any error will be more visible at the higher power.

[209herschel]

Thanks very much. I was just googling some images of star tests and I found something that looks exactly like what I'm seeing. I've attached the image if you get a chance to look at it. It's the one on the top left. Thanks again for the advice - I'll try the 9mm tonight and I bought a barlow so I'll try that with the 9mm also and see how I go. Herschel.

[Renato1]

It can depend on what the atmosphere is doing where you live, and what people are doing around you (heaters/ airconditioners). The blurry images you attached look better than the images I had when I was trying to collimate my C8 years ago. It took about 10 nights of taking it out before I finally got the sharp image.

Still, despite the image not being sharp, you can see by the concentricity whether or not your telescope is close to collimation or way out of collimation.
Regards,
Renato