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Old November 21st 03, 04:20 PM
Jon Isaacs
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Default Is 1/4 wave good enough for a rich field?


I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.


I wouldn't worry to much at low powers because Coma and eye aberations from the
large exit pupil will probably be the big factors in determining the quality of
the views.

The problem with F4 scopes is that you run out of exit pupil quite quickly.
Given an maximum exit pupil of 7 mm, this means that an eyepiece over 28 mm
will be wasting aperture.

Now there maybe nothing wrong with wasting aperture but at F4 your are dealing
with heavy duty coma and require some pretty good eyepieces. A coma corrector
will help but now you have are operating at F4.65 and it is costing a few
hundred bucks if you don't aleady have one.

Those big views are not so impressive if the FOV is filled with Coma both from
the mirror and from the eyepieces.

From my point of view, a 6 inch F5 mirror or a 5 inch F5 scope is a better
choice.

At low powers the 5 inch scope will provide equivelent brightness for a given
eyepiece, and a 6 inch scope can be used with longer focal length eyepieces to
achieve the same field of view without exceeding the exit pupil considerations.

For visual use, i think the only justification for a F4 scope is to make it
more compact, and in this case, the trade off is just not worth it.

jon who owns a 5 inch F5 scope and thinks it makes a nice widefield scope
with a 32 mm WF eyepiece.