"Jim Hewitt" wrote in message
...
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
The design of an SCT, has the second mirror correcting for the spherical
aberration caused by the first. It only does this properly at one mirror
spacing. Now Meade have never quoted a distance for this, and in fact one
example a friend tested, seemed to have the best corrected point at the
rear port of the scope (silly!). However I'd hope this is the exception.
Celestron have at times quoted that they attempt to correct the scope for
the setup with their 'T' adapter, and a 35mm camera, which puts the best
corrected point about 4" behind the rear port, and I'd expect Meade to be
similar. You can test for this, using a Ronchi tester, and a set of
extension tubes. If you try the tester at different distances behind the
scope, focus, then defocus by the same amount in each position, the spot
where the lines are straightest is the best corrected spot. Small
differences have little effect, but when you see people with a foot of
accessories behind the scope, and especially using focal compressors as
well, the optical lengths involved can be huge, and take the performance
below that of a much 'worse' scope...
My own C11, had the best corrected point just under 5" behind the rear
port, which is a very good compromise position, since this is just
slightly less than the backfocus needed for the *.63 focal compressor if
mounted tight to the scope, and just slightly more than is needed for a
1.25" diagonal.
How are you doing the final collimation test on your scope?. The last
'tweak', really is tiny, and does make a suprising difference. Thierry
Legault's page about this, is the 'bible' abou this aspect of the SCT. :-)

Best Wishes