(PrisNo6) wrote in message . com...

You noted the following probable magnitude errors in the chart:

In a few cases, I was surprised that stars with fairly different listed
mags seemed about equally easy (or hard) to see. Notably, at 20X, #094
[9.7] seemed no brighter than #099 [10.0], and at 60X, #065 [9.6] seemed
little brighter than #066 [10.3].

Heavens, just because my crude guesses disagree with the charts, don't
assume that the charts are wrong! I'm no variable-star observer,
accustomed to estimating magnitudes with an accuracy of 0.05! I am
happy to chalk up the 0.3 mag difference between #094 and #099 to
my own incompetence, or to random variation. I was a little more
baffled by the 0.6 discrepancy between #65 and #66, and I am glad
to have it explained by color.

Whenever I have done limiting-magnitude experiments, I have *always*
had the experience of being able to see stars quite clearly that
were 0.2 or 0.3 magnitudes fainter than other stars which I cannot
see, no matter how hard I try. I have always suspected that star
color plays a big role in this, but I have never tried to analyze
it systematically. And aside from the question of varying sensitivity
at different wavelengths -- which also varies markedly from one
individual to another -- let me throw another monkey-wrench into
the works. In comparing urban light pollution against skyglow from
the Moon, I have noted that artificial light pollution is (not
surprisingly) much redder than scattered moonlight. Perhaps blue
stars are more visible in urban light pollution, and red stars
under moonlight, because of color contrast against the background?
Just a thought.

But the other huge bias, which is unavoidable when looking at
star clusters, is the effect of proximity to other stars. I
find that a bright star casts a surprisingly big "shadow" where
it is impossible to see fainter stars that would be readily
visible on their own. I'm sure that this effect is bigger for
me than for most people; I know that my eyes have very bad
internal glare. But I am also sure that it exists to some extent
for everyone. Conversely, two faint stars just below the threshhold
of vision may add up to a single faint fuzzy if they are near the
limit of split-ability.

So although clusters are good for limiting-magnitude studies
because of having lots of stars closely spaced in magnitude,
they are also bad because of the proximity effect. I suspect
that looking at NGC 1647 with hand-held binoculars, the
proximity effect would be dominant -- many or most of the
stars would be hard to split from each other. I'll let you
know if the weather ever clears up again during a moonless
time of night.

As for the charts, they are basically just fine. Do you suppose
it would be possible to print *both* the star numbers *and* the
magnitudes on a single chart? That would save a lot of work!

- Tony Flanders