Naked-Eye Visual Magnitude Limit

Chris L Peterson posted:

Right. I think the central star of M57 is the classic example of this. There is
no doubt that in a smallish scope, say 12-14", the ability to pick it up depends
primarily on seeing.

It depends on seeing even if the scope is quite large. One of my professors
who was at Kitt Peak (Don Taylor) said that he never saw the central star in
M57 even in the 90 inch on the mountain, but it is visible in scopes as small
as 10 inches *if* the seeing is steady and if high enough power is used (over
350x). If the seeing isn't rock-stable, the star's Airy disk will simply blur
and blend into the glowing background nebulosity which exists in the middle of
the ring and will not be visible no matter how big the scope is. I have seen
it "blinking" on and off as the seeing got good and then bad (even in a 30
inch), so this behavior is important to understand when trying to see the
star. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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