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