wrote:

On 2011-07-03, jwarner1 wrote:
Simply put, does moving the eypiece position forward into the light cone

(toward the secondary) of a cassgrain system, have any effect in curing
or minimising
spherical aberration in cassegrain systems; the CAA 24" CAA Boller
Chivens is the
case in question.

This is the remedy Director of the CAA B&C, John Centala, has settled
on. John and
his colleagues John Leeson and Greg Frohner say this method was found in
some
mathematical treatise on cassegrain systems? (The exact reference
remains
'undisclosed'.).

Would moving the eyepiece forward into the light cone not be tantamount
to masking
the outer edge of the primary?

How would one determine which element in the system is causing the
'supposed'
spherical aberration?

(Note* After the telescope was assembled by those above I pointed out
that star
tests clearly indicated the secondary was being pinched. A year later,
the secondary
retainers were adjusted and stress relieved which resulted in improved
images but the
new claim of severe spherical aberration.)

Any comments appreciated.

The focal plane of the telescope and the focal plane of the eyepiece
must coincide for the telescope to be in focus - no ifs ands or buts.
You can't move the eyepiece "up into the light cone". The spacing between
the secondary and the primary on a classical cassegrainian telescope
has to be within a very tight range of only a few millimeters to avoid
spherical abberation. If fiddling with the secondary mount changed
the spacing between the hyperbolic secondary mirror and the parabolic
primary mirror too much the result would be spherical aberration.

Bud

Thank you Bud. This makes sense. I have passed this on.
Jerry.