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

Tony -

Instead, I viewed this lovely cluster through my 70mm refractor looking out
my dining-room window. Thanks for a very pleasant half hour!

Thanks again for taking the time to review the NGC1647 magnitude
chart. At 41 deg N, I'm still socked in under the jet stream and
cloud covered skies. I'm glad that you found the chart enchanced your
enjoyment of a half-hour of evening viewing. That is part of the
intent of the chart. For the role of this cluster in astronomical
history, you might want to take a look at Hertzsprung (1915).
Hertzsprung, E. 1915. Effective wavelengths of 184 stars in the
cluster N.G.C. 1647. ApJ 42:92H

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].

In response to your notes I adopted the following correction procedure
before adopting a manual adjustment to the magnitude charts.

1) Is the difference outside the expected range of observing error?
2) Check the plotted data.
2) Recheck the observed magnitudes in less light polluted skies.

The magnitude differences for these stars are outside the expected
range of observing error, generally +- 0.2 mags for an "average
observer" and +-0.1 mags for an experienced observer such as yourself.

I checked my photometry data for these four stars. That data is
assembled as an appendix at the end of this note. Turner's
photoelectric data are used for stars 94 and 99; Tycho 2 data for 65
and 66. I rechecked the V and B-V data against the source Turner
article and the Tycho-2 catalogue.

For Star 99, I made a transcription error for the B-V data for star
99. It's recomputed magnitdue should be 10.1, and not 10.0. This only
exacerbates the discrepancy between the plotted and observed
magnitudes that you identified.

With respect to Stars 65 and 66, Star 65 is a spectral class G0 star
that I added back in in order to have more complete sequence of stars
in tenths of a magnitude. The Tycho-2 color index for this star is
high - 0.6 (10.249 B_tycho - 9.636 V_tycho = 0.6 CI_tycho). It looks
like I should pull this one from the chart because its reddening makes
a discrepancy from between the Johnson V plotted value and what is
seen by the human eye. Or maybe I'll leave it in and just note it as
an example of how star reddening causes a difference between Johnson V
magnitudes and those seen by the human eye.

These checks did not fully resolve the problems you identified.

You noted that:

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.

and that:

The faintest stars seen at various powers we
snip 20X - #099 [10.0] 60X - #031 [10.6], maybe #048 [10.7],
but hard to split from #049 [10.3]

Your observing was done in poor urban skies at 17.0 mags per square
arcsecond. From our usenet conversations during the fall of 2003, I
believe that translates into an NELM of about 3.5 to 4.0 mags - a
typical urban light polluted sky. The four star discrepancies
identified occurred when you are viewing within 1.0 mags of the
magnitude observing limit of your 70mm refractor at 20x (v10.0 for
stars 64 and 65) and at 60x (v10.3-10.7 for stars 94 and 99).

Since the discrepancy stars are near or at the NELM limit of your
light polluted session, my instinct is to wait and recheck their
observed magnitudes in ZLM or NELM v5.5-6.0 skies. I'm thinking that
there might be some kind of background sky contrast problem that
effects visual perception. I would like to rule that out before
making a manual adjustment to the chart.

60X - #031 (10.6), maybe #048 (10.7), but hard to split from #049 (10.3)

If stars 48 and 49 cannot be split at 60x, looks like those two should
be removed from the chart as unsuitable for binoculars and small
scopes. That will result in gaps in the magnitude range by tenths of
a magnitude.

Any recommendations?

Correlating the star numbers on the correct-image chart with the
magnitudes on the mirror-reversed chart was a minor nuisance.

Okay, I admit it - -- I crapped out on doing the star numbering
system charts in even and odd reversals because the labels have to be
hand-placed.

Which do you think would be the most useful to the most number of
amateur observers - the even (mirror) or odd number of reflections?

Any ideas on software that would plot these kinds of charts
automatically? Am I doing this the hard way?

The chart would be easier to use if the dot sizes were
significantly smaller.

I will update the web posted charts reducing the star sizes to about
40% of their current area, sometime in the next week.

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.

In general, it sounds like for the your observed range of v6.5 to
about 9.0, the NGC1647 chart accurately reflects the order of the
magnitudes as seen by the human eye through a telescope.

That leaves about v9.0 to v13.0 still to check. Again, if other
lurkers are interested, any help you can provide would be appreciated.

Shortly, I will update the project site with a similar NELM estimating
chart for v4.8 to 7.0 , using low-variance photometry stars in the B,A
and F spectral classes, for the area around NGC1647 and the Haydes
stream. Preliminary charts are available now. That chart supplements
the existing International Meteor Organization Limiting Magnitude
Chart Area No. 8 which already includes NGC1647.

My observing notes page for this activity is at:

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

Thanks again for taking the time to look at this. - Kurt

=================================================
Photometry Appendix for discrepancy stars 99, 94, 65 and 66
=================================================
Webda_id 0099* 0094 0066 0065
X 1.05 1.96 -13.93 -13.05
Y 7.57 4.66 -3.77 -2.45
RA_J2000 4 46 16.1 4 46 22.3 4 44 30.9 4 44 37.2
Dec_J2000 +19 17 06.8 +19 12 18.4 +18 58 26.1 +19 00 34.6
HD_id HD 284840 HD 284839 HD 286010 HD 286009
Tycho_id 1275-1826-1 1275-1672-1 1275-765-1 1275-1183-1
Spec B9 B7 B8 G0
V_xy 10.16 9.68 10.4 9.83
B-V_xy
HIP_id HIP 22185
V_Tycho 10.194 9.708 10.296 9.636
V_Tycho_se 0.041 0.029 0.051 0.031
B_Tycho 10.55 9.936 10.77 10.249
B_Tycho_se 0.041 0.028 0.054 0.038
V_TurnerPE 10.09 9.69
V_TurnerPE_se 0.01 0.01
B-V_TurnerPE 0.41 0.23
B_TurnerPE_cp 10.5 9.92
B_TurnerPE_se 0.02 0.02
V_TurnerPH
V_TurnerPH_se
B-V_TurnerPH
B_TurnerPH_cp
B_TurnerPH_se 10.29
Photomty_type Turner PE Turner PE Tycho2 Tycho2
Johnson_V 10.09 9.69 10.29 9.63
m_V_computed 10.17 9.73 10.3 9.6
m_V_se 0.24 0.24 0.3 0.2
B-V_used 0.41 0.23 0.47 0.61
m_V_Diff 0.08 0.04 0.01 -0.03

* - Star 99's estimated magnitude is corrected here for a
transcription error in the B-V value. The B-V_TurnerPE value of 0.41
was transcribed originally at an incorrect value of -0.41.