William Hamblen wrote in message . ..

The lower limit in a newtonian or other telescope with a central
obstruction is when the size of the shadow of the secondary mirror
begins to approach the size of the entrance pupil of your eye. When
that happens you get an obvious and annoying black spot in your field
of view.

That's certainly right in theory, and it's a real practical problem
for daytime viewing or when looking at the Moon. But it's almost
impossible to go low enough in a normal scope for that to be an
issue for deep-sky observing, which is the only time you're likely
to want a super-wide FOV at night.

The extreme case is an F/4 Newt -- very fast -- with a 25% central
obstruction -- very large -- and a 40mm eyepiece. That still only
gives a 2.5mm dark spot, which is pretty much swallowed up in any
normal person's dark-adapted pupil.

- Tony Flanders