Quote:
Originally Posted by Jason D
I am unsure if I understood the idea above. When star collimation is executed correctly then the defocused star rings will look concentric regardless of F-ratio. The key here is "executed correctly". Star collimation should be done at high magnification -- at least 25X the aperture in inches. So, for a 10" dob, a minimum of 250X magnification is needed. The star needs to be defocused by a small amount -- enough to show 3-5 rings. The secondary mirror offset will be invisible using this setup. When the star is defocused too much, then the results will be inaccurate.
Jason
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Hi Jason,
All I know is that when I collimated my C8 and my 114mm reflector, I had no problem getting concentric circles with pretty much any magnification.
With my 14.5" f/4.5 dob, I couldn't and still can't get concentric rings with normal magnifications - at the time my highest power eyepiece was a 6.4mm plossl which gave 259X. True, I didn't bother with a Barlow to try get 400X or 500X or higher.
Regards,
Renato