Heres a small cut and paste of the similar discussion about the Allira and the spot diagram on the homepage and the RC i was involved in many many months ago on cloudy nights.
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I do think their 1st spot diagram for the RC is suspect because it doesn't look like a spot digram for any scope I have ever seen, not to mention an RC and I think the corner image from my RC indicates the distortion isn't anywhere near what the spot diagram would indicate.
Dean,
I assure you that those spot diagrams are 100% accurate. What you may not notice immediately that RC diagram quoted on Anssen site is focused for an on-axis image, so astigmatism and field curvature become rather severe off axis. If you refocus for best overal images (approx 30% from centre) things will look considerably better (but still INFERIOR to Allira's).
RC's own spot diagrams are in fact misleading too - triangular shape you noticed is from vignetting. The outermost parts of the image don't make it so astigmatic shape is truncated, hence non-symmetrical appearance. Even then, spot images are rather poor (notice scale of 200 microns !). One can argue that RCOS's diagrams are better represenatation of real world images, but nevertheless.
Allira seems to be a variant of Dall Kirkham + dedicated Ross corrector. These systems are capable of exceptionally good images across a very flat field. Definitely an interesting design that has been curiously neglected by ATMs.
All said, the quality of the REAL image in the end depends on many factors that are well outside the domain of Zemax.
How well the particular specimen of the scope is made, how well it is focused, how well it is collimated, how well it KEEPS collimation and focus, how well the mount tracks, seeing conditions, you name it. Had you used a well made and collimated D-K+Ross (I can see why they try to distance from this by choosing a fancy name :-) on the same mount and guided by the same gear, I am sure you would easily come up with superior images. In fact your own analysis shows an out of square being the most significant aberration on Allira's image (most likely from focuser sag), while your RC was dead-on. Large FWHM was likely from poor tracking/guiding/seeing. Yes, I can hear "excuses!" but these are merely facts. The DESIGN is indeed capable. The optics/mechanics/mount/seeing/astrophotographer in question might not be.
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