(PrisNo6) wrote in message . com...

This post is to request the group's assistance in field testing a
visual magnitude chart for NGC1647, an open cluster in the Taurus
constellation. The purpose of the NGC1647 chart is to provide a study
example for estimating limiting magnitudes suitable for large
binocular and small telescope users between m_v 6.0 and 13.4.

It is rather late, but I finally got around to field testing your chart
the night before last. It was just the thing for a busy night with nearly
full Moon and obviously poor seeing, when a full-fledged observing session
would have been more trouble than it was worth. Instead, I viewed this
lovely cluster through my 70mm refractor looking out my dining-room
window. Thanks for a very pleasant half hour!

The results have few surprises; broadly, they confirm the figures that
I have already posted at http://mysite.verizon.net/vze55p46/id9.html.
The transparency was good but not great, it was early in the evening
when the light pollution is worse, and the 4-day-before-full Moon
was nearly at the zenith. The cluster was about 45 degrees above the
horizon. I estimate the sky brightness at the cluster as mag 17.0
per square acrsecond.

At my lowest power of 16X, the cluster was just a tantalizing ghost;
five stars clearly visible and hints of many more. Raising the power
to 60X brought large numbers of stars into visibility. The faintest
stars seen at various powers we

16X - #102 [mag 9.4]
20X - #099 [10.0]
60X - #031 [10.6], maybe #048 [10.7], but hard to split from #049 [10.3]

Visibility tended to follow the listed magnitudes except for a clear
bias effect that stars are harder to see in crowded sections of the
cluster than in isolation. Thus, at 16X, the most obvious stars were
#022 [9.1] and #084 [9.2] on the edge of the cluster, more obvious
than mag 8.7 stars in the heart of the cluster.

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].

The chart would be easier to use if the dot sizes were significantly
smaller. Correlating the star numbers on the correct-image chart with
the magnitudes on the mirror-reversed chart was a minor nuisance.

- Tony Flanders