"jerry warner" wrote in message
...
Frankly, this is what Ive heard also. Its not the optics per se in fact
our
lx200 has a nice primary and when you lock down the priamry carefully
so as to keep things oin axis, then collimate, the scope is quite
crisp....
but with poor contrast. Mechanically the scope is a dog.
jerry
The comment about chromatic aberration being 'all withing the Airy disk',
is however not actually true.
If you ray trace a typical SCT, using a BK7 corrector, with the optics
laid out in the current 'mass production' form, chromatic aberration, does
spread beyond the size of the Airy disk, but mainly at the extreme blue
end of the spectrum. If you try the experiment of ray tracing and setting
the focus on 'green' light, then add back other colours, leaving out just
red, and violet, the plotted spread remains tiny. Adding red, spreads it a
tiny amount (from 10.4um spot size to just over 15um). However add back
the violet, and the spot size jumps up to 34.8um. The corrector, only
introduces a tiny level of chromatic aberration, but because of where it
is, the error is amplified by the secondary, and even if focussed
perfectly for each colour, there is a slight spherochomaticism shown,
which comprises the largest part of the total aberration. The focus 'shift
as a result of chromatic aberration is almost nil, but the increase in
spot size is significant.
On most SCT's, this is one of the 'lesser' faults, but it is still there,
and detectable.
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').

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.