(Tony Flanders) wrote in message m...
(PrisNo6) wrote in message . com...
snip

Thanks for your continuing interest.

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!

Will do. Give me a couple of days to dig-up my HTML frames code.
Initially, I was trying to keep the download times for low-band width
users for each item - hence splitting the tables and chart images into
two files.

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.

Your observation encouraged me to improve my understanding of the
color index problem. I beefed up the NGC1647 v8.0-13.0 chart homepage
(in the "Chart Development" section) with a discussion and some
figures on the distribution of Johnson color indices in the underlying
catalogue and magnitude charts:

http://members.csolutions.net/fisher...ect_v90_13.htm

There are some high CI outliers, which I had intended to exclude in
order to reduce chart variance.

http://members.csolutions.net/fisher...onCI_chart.gif

Some where added in to give a complete gradient to the chart's plotted
magnitudes. There is an inevitable trade-off between getting a full
gradient of magnitudes and having a chart catalogue consisting only of
low-photometry variance and low-color index stars.

[L]et 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.

Good point. I had always assumed that light pollution was bluer. I've
never heard of any studies on whether urban light pollution scatters a
reddened or bluer light. If it is reddened, then that would tend to
wash-out the increased ability of the human eye to see fainter
reddened stars, as compared to Johnson V magnitudes.

While I'm working my charting down (or is it up) to the urban
magnitude scale of v2.0, my current chart testing emphasis is in
non-urban, good skies.

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.

This is in part why I chose NGC1647 over the well-magnitude charted,
but nearby, Pleaides moving group. NGC1647 stars are generally
fainter than the Pleaides, but still have a good enough spread to be
distinguished in one eyepiece view in 15x-20x binos and small
telescopes at low magnification. In NGC1647, for example, at 20x, I
can split the triangular four star asterism at center of the field,
without a proximity effect. Star Nos. 040 (surrounded by), 041, 037,
039.

What you can split depends on aperature, magnification and _the
relative magnitude of the bright and dim star._ I have a citation I'll
dig up on this. The relative magnitude of the stars makes for a really
pronounced effect, angular separation being equal.

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.

My main personal use and target audience for the charts are small
refractors, small reflectors and tripod mounted binos in the 15x to
20x class. My own experience is that the limiting magnitude
estimation with a hand-held binocular, including a small 8x40 bino, is
pretty much impossible and leads to collection of measurements that
that such a large variance that they are not useable.

Heavens, just because my crude guesses disagree with the charts, don't
assume that the charts are wrong!

Gotcha. I'm just keeping notes trying to weed out potential errors in
the order of magnitudes.

Thanks again, Kurt

My main NGC1647 notes homepage is at:

http://members.csolutions.net/fisher...47_Project.htm