|
The limiting factor becomes the exit pupil not the eyepiece diameter or field of view.
The eye pupil entrance diameter of a young person can only dilate to about 7mm. This reduces with age. Older people tend to be the ones who can afford the giant telescopes.
Exit pupil(mm) = Mirror diameter(mm) ÷ magnification
or
Exit pupil(mm) = Eyepiece focal length(mm) ÷ f number
Regardless how big the diameter of a long FL eyepiece is the exit pupil becomes too big.
The 55mm eyepiece used in an f4 dob (of any mirror diameter) will have an exit pupil of 14mm. Your eye will only be able to accept 1/4 of the light exiting the telescope. Alex has previously made the point in the past that he likes being able to scan his eye around in the eyepiece of a scope with oversized exit pupil. His comments related to short FL refractors. I'm sure that even Alex wouldn't suggest using a 20-30 inch scope to scan the heavens.
Let's make the example specific
A 25inch f4 dob with a 55mm eyepiece will have a 2500mm FL 14mm exit pupil 46 power and about a 1 deg field of view assuming 50 deg eyepiece FOV.
But your eye will only be able to accept 25% of the light which is coming from a 12 inch diameter part of the primary aperture and any one time.
A 12inch f4 dob with a 28mm 82 degree eyepiece will have a 7mm exit pupil, 45x magnification, a nearly 2 degree field of view and a fully dilated eye can accept all the light.
Large telescopes excel at many things. Big wide views are not one of them. Smaller scopes are best for this.
Joe
|