"DWilson" wrote in message
...
Hi Rod -
Thanks for the reply. From what you are saying, it sounds like I would
not have this problem using a 2" eyepiece without the 1.25" adapter. Am I
correct?
The 'key', is in a lot of old threads about focal reducers. The f*.63
reducer, is a lens, with a focal length of about 240mm. The reduction this
gives, is dependant on the spacing between it and the focal plane of the
eyepiece/film. The reducer gives it's 'design' reduction, when spaced with
about 90mm from the front of the threads, to the focal plane. Spaced like
this, the backfocus required, becomes 90/0.63 + 12mm (12mm, is about the
length of the reducer internally). Hence around 155mm. However if you
increase the reducer spacing, even a little, things start to 'shoot up'. The
problem is that the increased spacing, gives more compression, and the
backfocus rises massively. So (for instance), if the spacing goes up to
110mm, the new compression ratio becomes (240-110)/240 = 0.54*, and the
backfocus becomes 110/0.54 + 12 = 216mm.
You can see that a tiny increase in spacing (20mm), has had a massive effect
on the scope backfocus required (61mm).
Now the use of a 2" eyepiece, or a very 'slim' 2" to 1.25" adapter, will
bring the focal plane, close to the top of the the eyepiece hole in the
larger diagonal, and have a very significant effect, and should reduce the
backfocus to the point where your system will work. Try a very crude
experiment. Focus your scope as far as it will go, and hold a low
magnification eyepiece in the 2" opening. Move the eyepiece up and down till
the image is close to focus. If the top to the eyepiece 1.25" tube is above
the rim of the diagonal tube, then a 2" eyepiece should work fine, and if
there is a reasonable gap, a 'low profile' adapter should also work.

Best Wishes

"Rod Mollise" wrote in message
...
Has anyone run into this or have any solution? I would really like
to be able to use the William Optics diagonal all the time. Thanks for
any
advice.

Hi:

The problem is the 1.25" adaptor. The solution? A couple of outfits have
sold
"low profile" 1.25"/2" adaptors in the past, notably AstroSystems...that
may
give you enough infocus to cure your problem.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
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