NGC1647 Open cluster - Help in field testing a draft cluster magnitude chart

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. NGC1647
is approximately 45 arcmins in diameter at a distance of 540 pc near
Aldebaran at J044608.4 +190437.4.

At my observing location (Salt Lake City, Utah), an extreme wet
winter weather pattern has resulted in about ten clear nights since
October 31, 2003. NGC1647 is near its zenith position. It does not
look like this weather pattern is going to improve any time soon. I
would appreciate it if observers in other locations might take a few
minutes to field test this chart, time permitting within your
observing sessions.

The following NGC1647 charts were developed for self-education
purposes. Other beginning amateurs may find them useful for
developing their skills at estimating star magnitudes using large
binoculars and small telescopes. This project also was intended as a
personal vehicle to explore how to prepare magnitude charts in the
modern era predominated by internet disseminated photoelectric and
photographic photometry catalogues.

I. Online chart locations:

A series of NGC1647 charts and supporting tables can be found at:

http://members.csolutions.net/fisher...erved/NGC1647/

That directory contains several views of the same magnitude chart,
including:

Star numbering system used – not magnitudes (77kb)
http://members.csolutions.net/fisher..._num_chart.gif

Direct view magnitude chart – for use with binoculars (81kb)
http://members.csolutions.net/fisher...view_chart.gif

Even view magnitude chart – for use with Newtonian reflectors and
other scopes with an even number of reflections. (81kb)
http://members.csolutions.net/fisher...view_chart.gif
This chart is formatted by taking the direct view chart and reversing
it once left-to-right, and a second time up-to-down. That format is
similar to an AAVSO "b" scale chart.

Odd view magnitude chart – for use with SCTs, small reflectors and
other scopes with an odd number of reflections. (81kb)
http://members.csolutions.net/fisher...view_chart.gif
This chart is formatted by taking the direct view and reversing it
once left-to-right. That format is similar to an AAVSO "b-reversed"
chart.

NGC1647 area finder chart, including Aldebaran and nearby Cephid
variable SZ Tauri (12kb):
http://members.csolutions.net/fisher...rea_finder.gif
This chart is plotted from Cartes de Ciel.

Each NGC1647 magnitude chart comes with a corresponding supporting
catalogue:

Star numbering system supporting catalogue (22kb):
http://members.csolutions.net/fisher...ar_num_cat.htm
Including cross-referencing to HD and Tycho2 catalogues and spectral
types. To properly display this file in MS Internet Explorer, set the
"Text size" option to "smallest".

Direct view chart supporting table (29kb):
http://members.csolutions.net/fisher...view_table.htm

Even number of reflections view chart supporting table (29kb):
http://members.csolutions.net/fisher...view_table.htm

Odd number of reflections view chart supporting table (29kb):
http://members.csolutions.net/fisher...view_table.htm

Catalogue field explanations (7k):
http://members.csolutions.net/fisher...at_read_me.txt

Supplemental worksheet supporting charts, in Excel 2000 format
(763kb):
http://members.csolutions.net/fisher...Chart_Plot.xls
This Excel worksheet contains my astronomical functions in Visual
Basic for Applications (VBA). If you are concerned with macro viruses,
set security within Excel to "low" or "medium" before opening. That
disables any macros from running without user approval.

Homepage for this project:
http://members.csolutions.net/fisher...47_Project.htm

II. Printing and importing recommendations:

For MS-Windows OS users, the following techniques may be helpful for
printing the charts and importing catalogues into a spreadsheet.

a. Printing charts

Right-click on the applicable chart name and save the gif format file
on your harddrive. Use MS-PhotoEditor, a standard MS Windows
utility, to open and print the file. The Print dialogue within
MS-PhotoEditor contains an option to resize the chart to the maximum
paper size without distorting the chart scale.

b. Printing tables

Catalogue and supporting tables are in HTML format. MS-Windows OS and
Internet Explorer users, open the catalogue or table file in your
browser. Set the default font size to "smaller" or "smallest". Use
"PageSetup" to set the margins to their smallest settings.

To print the star number catalogue, also use "PageSetup" to set the
page orientation to landscape.

c. Importing tables to a spreadsheet

For MS-Windows OS and Excel spreadsheet users, the catalogues and
tables are in HTML format. Right-click on a file name and save it
your harddrive. Open the file with Excel. Excel will translate the
HTML table files into a spreadsheet.

III. Chart development method and comments

These NGC1647 charts do not plot Johnson V magnitudes; the plot an
adjusted visual magnitude that better mimic the response of the human
eye in the visual system. Stanton 1999. Once the visual magnitude
NGC1647 chart discussed here is field tested, an updated supplemental
chart showing the corresponding Johnson V magnitudes also will be
produced.

Modern star charts and planetarium programs used by amateur
astronomers plot visual magnitudes standardized to the Johnson ubv
filter system. Each CCD and major photometry catalogue has its own
passband filter, that can be translated into magnitudes in the
standard Johnson ubv system. Stanton 1999, Bessel 2000, Ochsenbein
1974.

It has long been known that Johnson V magnitudes do not accurately
represent the response of the human eye. Stanton 1999. The human eye
perceives fainter and redden stars to a fainter magnitude than the
corresponding magnitude in the standard Johnson ubv system.

Although it does not accurately reflect what the human eye sees
through binoculars or a telescope, the standard Johnson ubv system is
superior for plotting and reporting purposes. Johnson V magnitudes
are based on objective, machine reproducible measurements.
Photographic and photoelectric photometry have a much lower estimation
variance (0.001-0.03) than naked-eye visual estimates (0.10-0.50).

Because Johnson ubv system V magnitudes do not accurately reflect what
the human eye sees, especially for fainter magnitudes greater than
9.0v, observers can report a different ordering of the magnitude of
stars within a field of view. Both reports – one the Johnson V
magnitude on their reference chart and the second based on visual
observations – are "right" in the sense that they accurately report
what is seen by different measuring instruments ( photoelectric and
photographic vs. the human eye). However, the differences can result
in reporting differences. Those differences can confuse beginning
observers.

This effect can be seen in the chart discussed here for NGC1647.
Figure 1 – Johnson V vs. Stanton Adjusted m_v for NGC 1647 at:

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

There is good agreement between the standard Johnson V magnitude and
the adjusted visual magnitude between v6.0 and v12.2. Beyond v12.2,
more reddened faint stars appear in the cluster and the difference
between magnitudes seen by a mechanical photometer and the human
begins to diverge. Figure 2. Difference between Stanton and Johnson V
by Johnson V magnitude at:

http://members.csolutions.net/fisher...n_V_2_diff.gif

In order to study the technique of estimating visual magnitudes
through binoculars and small telescopes, magnitude charts and tables
were developed for NGC1647 between v6.0 and v13.4. Values plotted on
the chart were adjusted to a visual magnitude system from Johnson V
magnitudes per Stanton 1999. Supporting tables to the charts list
both the Johnson V and adjusted visual magnitude, so users can
explore any differences between the systems. It should be noted that
modern star charts are not usually plotted in this older visual
magnitude system; rather Johnson V magnitude is found and plotted
from current photometry catalogues.

Because open cluster NGC147 is 45 arcmins in diameter and has a good
selection of stars in a narrow range of color indices, it is a good
candidate for developing the skill of estimating magnitudes with
binoculars and small telescopes. See Webda online NGC1647 Color Index
Chart for color index range of stars in NGC1647
http://obswww.unige.ch/webda/cgi-bin...c1647+NGC+1647
between v6.0 and v14. Unlike its brighter and nearby cousin, the
Pleiades, NGC1647 does not have so many bright stars, such that
close, faint background stars can be overwhelmed, or a surrounding
gaseous nebulae, partially reflective. These factors can effect
magnitude estimation.

The NGC1647 charts were developed by finding low variance
photoelectric and photographic studies in astronomy journals. Turner
1992, Francic 1989. The data in those studies is compiled in the
Webda open cluster database. Mermilliod 2004. The star numbering
system in the Webda database is based on Cuffey 1937. Standard errors
for Johnson B and V magnitudes reported in Turner are v0.01 for
photoelectric photometry and v0.03 for photographic photometry. Where
necessary to fill-out chart magnitudes between v7.0 to v12.0,
low-standard error Tycho-2 photometry records were used, usually with
standard errors of 0.03 or less. The data gathering and selection
process is documented in the Star numbering system supporting
catalogue (22kb):

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

which includes cross-referencing to HD and Tycho2 catalogue star
identifications, and in the supplemental worksheet supporting charts,
in Excel 2000 format (763kb):

http://members.csolutions.net/fisher...Chart_Plot.xls

Stars with low variance photometry were then converted from the
Johnson V magnitudes to an estimated visual magnitude using Equation
14 from Stanton 1999:

m_v = V + 0.210(B-V)

Based on a review of the differences between the computed m_v and
Johnson V (listed in the NGC1647 catalogue
"NGC1647_star_num_cat.htm"), the difference in the computed visual
and Johnson V magnitude generally is not significant up to Johnson V
12.2.

Approximately 70 stars between v6.0 and v13.4 with low variance
photometry. The computed m_v was then plotted on each NGC1647 chart
using a closed circle and was labeled with a magnitude.

Not all stars are within NGC1647 are plotted or labeled with a
magnitude. NGC1647 consists of approximately 350 stars. Other stars
with a Johnson V lower than 13.4 are also plotted, but these stars
have less-reliable and higher-standard-error photometry. The
uncertainty associated with their photometry measurement precludes
using these stars for magnitude plotting and estimation. But in order
to provide a better spatial visual reference for the observer, these
stars are plotted using their raw Johnson V magnitude, but with an
open circle that is not labeled with a magnitude.

The next step in this chart development process is to field test the
NGC1647 chart, primarily to see if order of magnitudes for
low-variance photometry stars accurately reflects the magnitude order
of stars as seen through the eyepiece.

The supporting star tables also report an uncertainty variance for
each magnitude in the visual system. For discussion purposes, this is
reported as a two-decimal number. As Stanton 1991 notes, the ability
of human observers to estimate visual magnitudes has a error between
0.1 magnitudes for experienced observers and between 0.1 to 0.3
magnitudes for the "average observer".

An inherent 0.2 magnitude uncertainty is adopted on the NGC1647 chart
when the Johnson V magnitude is transformed from to the visual
magnitude system. The second precision digit (e.g. 0.24) generally
reflects uncertainty contributed by the error in the low variance
photoelectric or photographic estimation of Johnson V.

IV. Prior charts for NGC1647

The Webda online database contains two magnitude and chart numbering
charts for NGC1647 prepared in 1900 and the 1970s. Stars are labeled
with star numbering system ids, but not magnitudes. NGC1647 appears
in the corner of AAVSO chart V Tauri 0446+17 (b), but since there is
no prominent variable within the cluster, magnitudes are not
estimated. http://www.aavso.org/ NGC1647 is located within
International Meteor Organization visual limiting magnitude estimation
area 8.
http://www.imo.net/visual/lm.html

V. Historical investigations of NGC1647

In addition to the modern era investigations of NGC1647 (Turner 1992;
Francic 1989), Hertzsprung used the 60-inch Mt. Wilson reflector
stopped down to 40 inches with a diffraction grating made from rubber
cords to examine NGC 1647 as part of his early studies of the color
index. Hertzsprung 1915; Seares 1915. Although he was using the
most advanced telescope of his day, technology was such that
Hertzsprung could only estimate magnitudes with an accuracy of v0.10.

More recent studies have focused on whether nearby variable SZ Tauri,
a classical Cephid located about 2 degrees east of NGC1647, is a
member of cluster NGC1647. See the NGC1647 area finder chart. Turner
1992 concluded SZ Tauri is a member of NGC1647 based on photometry.
Geffert 1996 concluded SZ Tauri is not a member of NGC1647 based on
astrometry (proper motion).

Cantrell presented a new photometry study for NGC1647 at the January
2004 meeting of the American Astronomical Society, but the study has
not yet been published in available journals.

Again, I would appreciate it if observers in other locations might
take a few minutes to field test this chart while it is still near the
zenith, time permitting within your observing sessions.

This is an amateur observer effort. All criticisms and correction of
any errors is solicited and welcomed.

Please provide any comments via usenet or via email to:

Kurt Fisher 2/2004

Acknowledgements:

This note makes use of data from:

Webda Online Database of Open Clusters, a project of Jean-Claude
Mermilliod

Simbad Online Database, a project of the Centre de Données
astronomiques de
Strasbourg de l'Université Louis Pasteur et du CNRS.
http://simbad.u-strasbg.fr/sim-fid.pl

References:

Bessell, M. July, 2000. The Hipparcos and Tycho Photometric System
Passbands. PASP 112:961
http://adsabs.harvard.edu/cgi-bin/np...ASP..112..961B

Cantrell, K.A. et al. January 2004. Study of NGC 1647 in the
u'g'r'i'z' filter system. Session 14.09, AAS 203rd meeting (abstract)
http://adsabs.harvard.edu/cgi-bin/np...AS...203.1409C

Cuffey, J. 1937. Red indices in galactic clusters. Harvard College
Obs. Tercentenary Papers, No. 21. ( NASA ADS Bib. 1937AnHer.105..403C
)
http://adsabs.harvard.edu/cgi-bin/np...nHar.105..403C

Francic, Stephen P. 1989. Mass functions for eight nearby galactic
clusters. Astron.J. 98(3):888 (photographic in Johnson ubv)
http://adsabs.harvard.edu/cgi-bin/np...J.....98..888F

Geffert, M. et al. 1996. The astrometric accuracy of "Carte du Ciel"
plates and proper motions in the field of the open cluster NGC1647.
Astron. Ap. Suppl. Serv. 118:277
http://adsabs.harvard.edu/cgi-bin/np...6AS..118..277G

Hertzsprung, E. 1915. Effective wavelengths of 184 stars in the
cluster N.G.C. 1647. ApJ 42:92H
http://adsabs.harvard.edu/cgi-bin/np...pJ....42...92H

Johnson, H.L. and Morgan, W.W. 1953. Astrophysics.J. 117:313 (Johnson
ubv system seminal paper
http://adsabs.harvard.edu/cgi-bin/np...pJ...117..313J

Mermilliod, Jean-Claude. 2004. Webda: A Site Devoted to Star
Clusters. Online database. http://obswww.unige.ch/webda/ accessed
2/2004
See Webda online NGC1647 Color Index Chart
http://obswww.unige.ch/webda/cgi-bin...c1647+NGC+1647

Ochsenbein, F. 1974. On the relationship between the apparent
magnitudes given in several catalogues and the ubv system. Astron.
Astophys. Suppl. 15:215
http://adsabs.harvard.edu/cgi-bin/np...6AS...15..215O

Seares, F. H. 1915. Color-indices in the cluster N.G.C. 1647. ApJ
42:120S.
http://adsabs.harvard.edu/cgi-bin/np...pJ....42..120S

Stanton, Richard H. 1999. Visual magnitudes and the "average
observer": The SS Cygni field experiment. JAAVSO 27:97
http://adsabs.harvard.edu/cgi-bin/np...AVSO..27...97S

Turner, David G. 1992. Galactic clusters with associated Cepheid
variables III. NGC 1647 and SZ Tauri. Astron.J. 104(5):1865
(photoelectric and photographic in Johnson ubv with average se of 0.03
for photoelectric and 0.01 for photographic)
http://adsabs.harvard.edu/cgi-bin/np...J....104.1865T

UNSO. 2004. Catalogue Information and Recommendations. Web page.
http://ad.usno.navy.mil/star/star_cats_rec.shtml

Usenet sci.astro.amateur discussions:

Brian Skiff. 4/2/1996. Star magnitudes, catalogues, etc.

Bill Ferris. 2/15/2002. 18:34:55 PST. UNSO-A2.0 vs. GSC

(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. snip NGC1647 is approximately 45 arcmins in
diameter at a distance of 540 pc near Aldebaran at J044608.4 +190437.4.

After dusting off some forgotten HTML and CSS, I prepared some web
pages that self-scale the charts for printing by your browser. By
using these self-scaling pages, chart printing should be easier:

Star numbering system used - not magnitudes (77kb)
http://members.csolutions.net/fisher..._num_chart.htm

Direct view magnitude chart - for use with binoculars (81kb)
http://members.csolutions.net/fisher...view_chart.htm

Even view magnitude chart - for use with Newtonian reflectors and
other scopes with an even number of reflections. (81kb)
http://members.csolutions.net/fisher...view_chart.htm

Odd view magnitude chart - for use with SCTs, small reflectors and
other scopes with an odd number of reflections. (81kb)
http://members.csolutions.net/fisher...view_chart.htm

NGC1647 area finder chart, including Aldebaran and nearby Cephid
variable SZ Tauri (12kb):
http://members.csolutions.net/fisher...rea_finder.htm

The project homepage, which contains links to all of the above is:
http://members.csolutions.net/fisher...47_Project.htm

Regard, Kurt

(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. snip NGC1647 is approximately 45 arcmins in
diameter at a distance of 540 pc near Aldebaran at J044608.4 +190437.4.

After dusting off some forgotten HTML and CSS, I prepared some web
pages that self-scale the charts for printing by your browser. By
using these self-scaling pages, chart printing should be easier:

Star numbering system used - not magnitudes (77kb)
http://members.csolutions.net/fisher..._num_chart.htm

Direct view magnitude chart - for use with binoculars (81kb)
http://members.csolutions.net/fisher...view_chart.htm

Even view magnitude chart - for use with Newtonian reflectors and
other scopes with an even number of reflections. (81kb)
http://members.csolutions.net/fisher...view_chart.htm

Odd view magnitude chart - for use with SCTs, small reflectors and
other scopes with an odd number of reflections. (81kb)
http://members.csolutions.net/fisher...view_chart.htm

NGC1647 area finder chart, including Aldebaran and nearby Cephid
variable SZ Tauri (12kb):
http://members.csolutions.net/fisher...rea_finder.htm

The project homepage, which contains links to all of the above is:
http://members.csolutions.net/fisher...47_Project.htm

Regard, Kurt

(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

(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.

(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

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

snip

Tony -

Just to follow-up; no response necessary.

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

All charts have been updated with smaller dots, e.g. -

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

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

An odd view (mirror) star number chart was added.

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

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 NGC1647 v8.0-13.0 chart homepage was beefed up to address the
distribution of Johnson color indices in the underlying tables:

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

Thanks again, Kurt

My main NGC1647 notes homepage is at:

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

(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

Kurt wrote:
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.

Urban light sources are almost certainly redder to begin with. The Sun
has an effective temperature around 5800 K, whereas I'd guess that most
man-made lights are around 2000 to 3000 K, something in that neighborhood.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Ï "Brian Tung" Ýãñáøå óôï ìÞíõìá
...
[snip]

Urban light sources are almost certainly redder to begin with. The Sun
has an effective temperature around 5800 K, whereas I'd guess that most
man-made lights are around 2000 to 3000 K, something in that neighborhood.

Depends on the lightbulb :*)

The distribution of lighting patterns follows developmental plans closely.
Urban (far from cities, if I recall the term right) areas, tend to be
populated with Mercury lights, contrary to rural areas, which tend to be
populated by Sodium lights.

Reason being that the need for efficiency in urban areas is not so
stringent. From what I recall during my days in the States, many large urban
driveways around Chicago and many Chicago suburbs in 1990 still used clear
High Pressure Mecury lights.

In Europe Mercury lights are slowly becoming obsolete. One can roughly judge
the distance of a particular location to the center of a city, by counting
Mercury lights. Mercury lights have much longer lifespans, so any leftovers
are not replaced readily here. However, even poorer areas, would be
populated by incadescent and CFL's.

So the pattern here is (in Europe), poorly populated areas, incadescent and
CFL, so around 2,300-2,700 K.
Medium sized urban areas: High Pressure Mercury (fluorescent) lights (with
some High Pressure Sodium), so around 4,500 K.
Rural areas: High Pressure Sodium lights and Metal Halide lights, so around
2,200 K, with a tendency to up this to 2,700 (halides).

The pattern may be slightly different in the States, since I had not
observed any CFL or incadescent lighting in poorly populated areas. (Note
that CLEAR High Pressure Mercury lights, go as high as 7,000 K, which indeed
turn the sky bluish).

The sky in Athens glows bright orange, which comes from the predominant
resonances of Sodium, around D1/D2. Unfortunately High Pressure Sodium
lights are not monochromatic, so there is a quite wide band of wavelengths
around D1/D2 which get emitted and polute the sky.


Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

--
Ioannis Galidakis
http://users.forthnet.gr/ath/jgal/
------------------------------------------
Eventually, _everything_ is understandable