"Chris L Peterson" wrote in message
...

My reference to scintillation with respect to visual limiting magnitude
was
based on the [possible faulty] assumption that scintillation is the
primary
seeing effect at visual resolutions. My own experience is that stars at my
borderline detection level (about mag 6.7) are seen when the sky is
steady, but
not when there is high scintillation.

I concur.

Instability in the atmosphere seems almost the equal to some upper threshold
on degraded transparency, given a "cloudless" night of good seeing.

I wish I could quanitify that a little better, but I can't. My experience
simply being that as the seeing degrades, my mag 5.7 NELM "detection
threshold" star between eta and zeta UMi becomes only momentarily
detectable, in a similar fashion as when the transparency degrades. At some
point, either degraded seeing or degraded transparency exceed a threshold
and the star becomes invisible, but long before the skies are what I would
consider poor for astronomy.

Conversely, if that star is visible directly, then the skies are excellent
(or very close to "really good"), which I think is the significance of the
exercise in detecting stars at the limit. The ultimate limit being defined
by the dimmest star an individual can see under the best conditions the
local atmosphere can offer.

The fact that someone half my age could see to magnitude 6.2 in UMi under
the same conditions, is meaningless. It only defines _their_ indicator of
good sky conditions. Mine is simply a half magnitude less because of
physiology. Thankfully, binoculars and telescopes allow us to see much
deeper than our eyes can without aid. :-)

-Steve Paul