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ok thanks
I should update the title to read airy rings rather than the disc.
so viewing it through a ccd at prime focus at f4 I guess is out of the question, even with a 2x barlow.
I do have a 15mm EP, I might try that.
so do you first focus, then defocus just a bit? or does it have to be focused to see the rings. do you use a fairly bright star?
getting it on the ccd would've been great as i can then see if there is any astigmatism or pinch in the optics.
My biggest problem is centering the secondary.
I won't go into length on the offset for the secondary, but the problem is that for my optics, 10inch F4 and an 88mm secondary, the offset is around 5.5mm, and that means when I defocus on a star to see if the secondary is centered, I have to account for that 5mm offset and get it off centre.
I might instead measure the distance from the top end of the secondary to ota surface and distance from the other end of the secondary to the other side of the OTA and make sure one side is 5mm less than the other.
I do have the hubble artificial star and might try viewing it with an EP at a distance.
Alistair
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