# Thread: Scope focal length vs Eyepiece focal length

1. ## Scope focal length vs Eyepiece focal length

On my daily walk with my daughter, my thoughts began to wander thinking
about building another scope, but the following riddle, so to speak, came
to mind. Given two hypothetical scopes, say one 8" diameter, 750mm Focal
length (scope A), and a 6" diameter 1500mm focal length (Scope B). If you
take an 4mm focal length eyepiece and place it on scope A, and focus in on
say Saturn, then an 8mm focal length eyepiece on scope B and focus in also
on Saturn. Given that both of these scopes have equal magnification, and
otherwise we will assume them to be of equal quality, and design, which
would be the better setup?

--
P Sorah A_T Mutual Data PERIOD COM

2. ## Scope focal length vs Eyepiece focal length

PS Surfer <psorah@voyager.net> wrote in
news:3f6f8698\$0\$51803\$9a6e19ea@news.newshosting.co m:

Scope A Focal Ratio = 750/203 = 3.7

Most eyepieces will not work well with such a fast scope. If the scope is
a Newtonian it will also have severe coma.

Scope B Focal Ratio = 1500/152 = 9.9

If you are talking Newtonian, then this scope will have negligable coma
and will work well with any eyepiece design.

Scope B. Your scope A is just too fast to work well with most eyepieces.

Llanzlan.

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