In these scopes the spherical correction varies with
color, so it is useful to isolate one color. In red light
I would say the spherical correction looks quite good.
I have sometimes wondered if Celestron is testing these
in red light with a He-Ne laser.
There maybe a trace of astigmatism. The left pattern
looks to my eye to be very slightly elongated along the
5 to 11 o'clock axis. The right pattern appears
elongated along the 2 to 8 o'clock axis. A better
test would be if you see astigmatism at best focus.
You can compare the in-focus Airy pattern against
illustrations in H. R. Suiter's book, or images
generated with the Aberrator program
http://aberrator.astronomy.net/
That can give you some quantitative answer.
There are some faint bright and dark spokes in the
pattern. My suspicion is that these are in the
corrector plate. I think they use only cheap float glass
in these which will have stripes and streaks
in it (though the XLT version uses better BK7 glass).
The spokes could also arise in the primary mirror
due to print-through or machine generated curves.
My impression is that this error is minor, but again
the question is whether the best focus star image
shows the spokes. Thermal effects and air currents
could also come into play if the scope is still cooling
down.
Collimation is a little off as both patterns are
a little brighter towards the lower right (effectively
showing coma aberration).
Overall I would say it is a nice scope.
If one wants to spend significant effort on the
evaluation, these images could be processed
through the Roddier software to generate a
quantitative evaluation of the errors. There is some
information on this at
http://tech.groups.yahoo.com/group/roddier
But there is some learning curve and work involved.
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