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RE: Advice On Scope Purchase

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Posted by Goldfish on July 17, 2000 03:11:45 UTC

On extended objects, the smaller the f ratio, the
brighter the image. On point objects, the larger
the aperture, the brighter the image. So it looks like the 6" f/5 wins on both. It should
also be more compact (30" long) than the 5" f/8
(40" long). The disadvantage of the shorter
focal length is usually more aberration,
distortion, and curvature of the field (i.e.,
the focus point is different for stars near
the middle and stars near the edge). Also, you
need a sharter focal length eyepiece to get the
same magnification. The light gathering power
of the 6" should be significantly better than the 5" (in the ratio of 36 to 25). The resolution
is slightly better on the 6". If the 6" is well
made with a parabolic mirror, and the off-axis
distortions are not bad, that`s the one I`d pick.

P.S. I`m assuming the mounts for both scopes are
the same or equivalent. A bad mount can make
even the best optics worthless.

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