NGC1647 and Haydes Area-wide Magnitude Charts for Large Binoculars and Small Scopes

NELM v4.8 to 7.0 Chart

K. Fisher fisherka@csolutions.net 3/2004

Introduction

An Naked-Eye Limiting Magnitude (NELM) chart was developed for a 20 degree radius around NGC1647, an open cluster in the Taurus constellation. Another chart plots magnitudes within a narrow field of field around 1 degree of NGC1647. The purpose of this NGC1647 chart series is to provide a study example for estimating limiting magnitudes suitable for large binocular and small telescope users between m_v 4.8 and 13.4. NGC1647 is approximately 45 arcmins in diameter at a distance of 540 pc near Aldebaran at J044608.4 +190437.4. The area-wide NELM chart also is applicable to the Haydes moving group.

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.

This chart has not been significantly field tested. Because NGC1647 has fallen off zenith (in March 2004), formal star testing of this NELM chart is deferred until September 2004. Some preliminary field testing notes have been made.

I. Online chart and resource locations:

Star name cross-referencing chart - Henry Draper, Tycho and Flamsteed catalogue identification numbers:

Raw jpg file (151kb): http://members.csolutions.net/fisherka/astronote/observed/NGC1647/NGC1647_v48_70_chart_names.jpg

Web browser self-scaling for printing (152kb): http://members.csolutions.net/fisherka/astronote/observed/NGC1647/NGC1647_NELM_v48_70_chart_names.htm

Supporting table of stars by right ascension (13kb): http://members.csolutions.net/fisherka/astronote/observed/NGC1647/NGC1647_v48_70_star_names_table.htm

Catalogue field explanations (7k):

http://members.csolutions.net/fisherka/astronote/observed/NGC1647/NGC1647_v48_70_cat_read_me.txt

Cartes du Ciel "Size" type catalogue files:

Zip file (8kb) containing Ciel *.cds and *.dat files: http://members.csolutions.net/fisherka/astronote/observed/NGC1647/NGC1647_v48_70_Ciel_cat.zip

Supplemental worksheet supporting charts, in Excel 2000 format (1.3M):

http://members.csolutions.net/fisherka/astronote/observed/NGC1647/NGC1647_v48_70_stars.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/fisherka/astronote/observed/NGC1647/NGC1647_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

If you are using the web-browser self-scaling charts, discussed above, use "Print Preview" to check chart scaling before printing. Charts can be slighty enlarged by setting your browser printing margins to their smallest setting. The chart will automatically resize.

If you want to print a raw gif file, 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. For 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

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.

In order to reduce variance in making naked eye magnitude estimations (due to star reddening, variable stars and/or stars in binary or multiple component systems), the Yale Bright Star Catalogue, 5th ed, and the Simbad Online Database, were filtered to identify 668 stars between Johnson V 4.0 and Johnson V 7.1 within 20 degrees of NGC1647. Stars were excluded from the preliminary list that were reddening (B-V >0.4), were variable stars and/or that were in binary or multiple component systems. The preliminary list of single stars, generally of spectral class A, B or F, was further narrowed to those within 20 degrees of NGC1647 and those stars that had low-standard error photometry in the Tycho-2 catalogue. This provided a list of 53 stars with low variance photometry and that did not have reddened spectrums. The Johnson V values of this set of photometrically measured stars should have the same magnitude when viewed by the human eye.

A NELM estimation chart already exists that covers the area around the NGC1647 open cluster. The International Meteor Observers organization (IMO) publishes a naked eye limiting magnitude chart for the area bounded by Aldebaran (alf Taurus), zeta Taurus, and beta Auriga, titled IMO Limiting Magnitude Area 8, with an accuracy of 10ths of a magnitude. IMO NELM Area 8 is located at the center of the NELM chart presented here. I was unable to verify the identity of the specific stars used in the IMO chart, and for my application, I needed some statement of the uncertainty of the photometric or photoelectric measurements of each star's Johnson V and B magnitudes. Additionally, a Simbad search of stars in IMO NELM Area 8 returned many variables and stars in multiple systems. These stars could not be separated from the IMO star list.

Therefore, this supplemental NELM chart with known photometric uncertainties from the Tycho-2 catalogue was developed.

Selected stars from the Tycho 2 catalogue with low variance photometry were then converted from the Johnson V magnitudes to an estimated visual magnitude using the equation in Note 7 of the Tycho 2 catalogue (Hog 2000) and Bessell 2000:

Johnson V = VT - 0.09 * (BT - VT)

A further comparison of the Johnson V was done to determine if stellar reddening would result in the human eye seeing a star at a different magnitude in the visual system than its Johnson V value, following Stanton 1999. Because all the stars selected are of a bright magnitude (<7.1) and are of spectral classes A through F, no difference was found between the Johnson V and the computed magnitude of the star in the naked-eye visual magnitude system.

As of the date of their initial publication, the following charts have not been field tested. After use of these charts, please direct emails with any corrections or suggestions to Kurt Fisher fisherka@csolutions.net

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 1999 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 NELM wide-field chart. When the Tycho 2 magnitude is transformed to the Johnson V system the resulting uncertainty is attributed to the second precision digit (e.g. 0.24).

IMO NELM Area 8 is available to cross-check limiting magnitudes measured in the chart presented here. The IMO chart has the strength that 100% of all plotted stars are within 10 degrees of NGC1647, but may include reddened stars that deviate from the Johnson V magnitude when measured with the human eye. The chart presented here has the strength of only using B,A and F spectral class, single stars, but has the weakness that the measuring stars are spread across a 45 x 30 degree rectangle of the celestial sphere. The median angular distance of the stars to NGC1647 on the NELM chart presented here is 12 degrees; the maximum is 20 degrees - much higher than that in IMO NELM Area 8.

Closing

The next step in this chart development process is to field test the NELM chart for the area surrounding NGC1647, primarily to see if order of magnitudes for low-variance photometry stars accurately reflects the magnitude order of stars seen by the naked eye.

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 are solicited and welcomed.

Please provide any comments via email to:

Kurt Fisher fisherka@csolutions.net Last updated: 3/4/2004

Acknowledgements:

This note makes use of data from:

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

The Tycho 2 catalogue: Hog E., Fabricius C., Makarov V.V., Urban S., Corbin T., Wycoff G., Bastian U., Schwekendiek P., Wicenec A. 2000. The Tycho-2 Catalogue of the 2.5 Million Brightest Stars. Astron. Astrophys. 355, L27 (2000) VizierR query form

References:

Bessell, M. July, 2000. The Hipparcos and Tycho Photometric System Passbands. PASP 112:961 NASA ADS

Johnson, H.L. and Morgan, W.W. 1953. Astrophysics.J. 117:313 (Johnson ubv system seminal paper) NASA ADS

Stanton, Richard H. 1999. Visual magnitudes and the "average observer": The SS Cygni field experiment. JAAVSO 27:97 NASA ADS

Usenet sci.astro.amateur discussions:

None.