Hello Paul
Here is an interesting image for you.

http://www.martinpughastrophotography.id.au/Defocussed.jpg

A few days ago I collimated the primary baffle and did a minor tweak on the secondary mirror.

The first set of 10 x 25 second images (unbinned) gave collimation at 0.0", curvature 8.5% aspect ranged from 13-20 (350 or so stars per image) The second set of 10 x 25 second images gave collimation 0.6" , curvature 9.7%, aspect ranged from 17-23(around 1200 stars per image)

And yet look at this JPG..defocussed shot...on axis lovely, but its no wonder that star distortion occurs at the edges...even with or without the AOL....I am grateful that the coma (?)is apparent on both sides. This is a 45 second image, unbinned, PEC enabled, AOL off.

But it also illustrates perfectly what you said about how aberrations would occur most from left to right than top to botom because of the size of the chip.

What do you think?

Cheers
Martin

Hi Martin,

When the system is this far out of focus, you can't expect the star shapes to be perfectly round away from the optical axis, even with a FF corrector. The optics are designed and optimized for best on-focus behavior, which, I'm guessing, is the way you'd want it

RC's are generally coma-free, but field curvature is not totally eliminated. A field flattener corrector helps here, but a defocused light will significantly exaggerate the remaining curvature, which is what I'm seeing in your test shot.

Regards,

-Paul

Thanks Paul

You always offer some great explanations...I really appreciated your diagnostic on my earlier image. You highlighted things I never even thought of.
thanks again

Martin

The other thing, is just how large the image really is. I'd guess from the image posted, that this is a 35mm 'full frame' camera, perhaps one of the STL units. As such, <10% curvature across a field this big, is really good. If you used a ST10 on the same scope setup, you'd probably be well under 2%.
This is why I asked some time ago, if CCDInspector, could have an option to either 'normalise' the curvature figure, or give the curvature across a standard smaller frame, for 'comparison' purposes.

Best Wishes

Hi Guys,
I have just aquired a 10" RCOS f9 tube and was wondering if there are any 3rd party field correctors I could use on it.
RCOS dont make this scope any more but do make a 10" f7 Astrograph and have a dedicated Field corrector for it but they said it NG for mine. They never made one for mine but suggest using one from a 12.5" RCOS and averaging the focus between the centre and edge. I am not too keen on this and so looking for an aftermarket item that may/will suit.
regards
Nick

Hi Nick,

Generally, a FF custom designed for a specific telescope will work best. If that's not available, the next best thing is to find a FF that is designed for the same type of optic as your telescope (R-C) and as close as possible to the the same focal length. If same focal length is not available, a FF designed for a slightly longer FL will work better, as it will just undercorrect the image, while a FF desgined for a shorter focal length will overcorrect.

I would suspect that a FF for a 12.5" F/9 RC will also not be a very good fit for your scope. It will undercorrect your field of view, which is an improvement compared to overcorrecting, but is still not ideal. For example, if such a thing exists, a FF for a 12.5" F/7 RC would work much better for your scope than the other two.

Regards,

-Paul