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
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Hi Jim, this is a very good point to bring up, and I have for years
also wondered about this?

This seems to be a bit of a "grey'area" in giving a clear cut
explanation, or what acessories are a "go", and what is a "no go".

I would assume the designers of said scopes would've taken into account
what "standard" acessories such as standard Visual Backs, the various
Camera Adapters, and the normal range of eyepieces to be used with such
an instrument.

I've always wondered about this with the C-14" OTA for instance, and
what detrimental effect, if any, would be caused by the implementation
of Secondary Crayford Focusers attached to the Rear Cell, and then the
addition of 2" Diagonals, etc.

I'm thinking with the additional length of these acessories, that now
the optimal FL of the scope has been compromised.

Perhaps others with a bit more knowledge of the subject than I have can
comment, and offer some guidlines, and what the detriment will be from
deviating from the designer's specs? Mark