8" SCT focal ratio changes and magnification


IT IS SAID that when one focuses in the SCT or Mak. The focal ratio of
the scope changes. So how do you tell the true magnification of a
certain
eyepiece beyond that convensionally calculated.

For example. An 8" SCT F10 producing approx ~2000 focal length.
When you use a 15mm Plossl Eyepiece. The standard calculation
is 200mm (aperture) x 10 (focal ratio) = 2000mm/15mm (eyepiece) =
133.33X magnification. But when you adjust the focuser, the focal
ratio
is supposed to change so instead its 200mm(aperture) x new focal
ratio = magnification. Therefore how can you tell the real
magnification
using difference eyepieces focal length??

Teni

.

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