Back to something that Roger wrote [and I missed the first time around]:

The commonest thing degrading images, is collimation (90% of SCT's, only
have 'adequate' collimation, rather than really good levels), followed
by
degradation caused by focussing significantlty away from the scope's
'design' position (this makes a big difference, with it often being
suprising how good images become when an attempt is made to get these
two
factors close to 'right').

I have a 5" CAT. I thougth I had good images with good collimation until I
compared my views with another 5" CAT. same OTA, just different mount and
vintage. The other definitely has superior images.

What is this about focussing near the scope's 'design' position? How does
one determine this? In my case, since they are the same OTA, likely this
focus issue is not the reason for the image quality difference.

Comments?

Thanks.

Jim


Best Wishes

Mark D wrote:

THe corrector plate (the only refractive part of the scope) will do
some
color aberration but that amount won't be visible in the image as it
is
all within the Airy disc for the error.
More important is the quality of the reflective surfaces as well as
the
corrector putting the image to a poorer quality than that of a
newtonian
reflector. While the Coulter scopes were often of poor quality, you
can
get a really good one on occasion. Same thing with the SCT design
although Meade did tend to do the 16" a lot better than their smaller
scopes. You'll probably get a 1/8 wave accurate scope in the 16" size
which is good enough that you'll never see any real problems.
======================================

Bob, I've heard the exact opposite about Meade's 16" SCT's, in that
they are all mostly real dogs, particularly in comparison to thier
smaller SCT's. Mark D.