Japanese version Home Page Updated on March 25, 2001
February 28, 2001
MISAO Project Announce Mail (February 28, 2001)
Hello. I am Seiichi Yoshida working on the MISAO project.
Maybe someone have wondered observing a star, how to determine the magnitude of the star and what catalog to use as reference.
Now the web page on the guideline about "How to determine the magnitude of a star" is available at the MISAO Project Home Page. The contents of the page are the summary of instructions by Taichi Kato and Seiichiro Kiyota in the VSOLJ(Variable Star Observers' League in Japan) mailing list. And the PIXY System 2 supports the three types of photometry mentioned in this document. The web page also introduces about the photometry using the PIXY System 2.
1. Guideline of photometry
Depends on the filters, CCD chips and the reference catalogs, there three types of photometry.
(1) Standard system
When you observe a star using some standard filter, such as V, B, Rc, etc., you have to use catalogs which contains the magnitude in standard system, the same system as the filter you use, as reference. For example, when you observe a star using a standard V filter, you have to refer to a catalog which contains standard V magnitude (Tycho Catalogue, for instance).
When the standard V magnitude of a star is recorded in the catalog as 10.0 mag, the magnitude of another star which is 2.5 times brighter than it will be 9.0 mag.
In principle, standard filters are specific to the CCD chips. You have to use the proper filter for your CCD chip. Instead, the magnitude in the standard system measured as mentioned above is consistent with other observers' data in the same system.
Sometimes the magnitude in some standard system are not recorded in the catalog. Then you can convert from magnitude in other standard systems. For example, when you observe a star using the standard Rc filter, there is no Rc data in the Tycho Catalogue. Then you can convert from V magnitude and B-V value recorded in the catalog into the approximate Rc magnitude following the formula:
Rc = V - 0.5 * (B-V)
and determine the magnitude using this approximate Rc magnitude.
(2) Non-standard system
In case of unfiltered CCD images, or images using non-standard filters (IR-blocking filters, for instance), you have to estimate the feature of your own system at first, then convert from the standard magnitude recorded in the catalog into the specified magnitude for your own system.
In general, the feature of a system is resembled by the following linear equation of standard V magnitude and B-V value:
Instrumental mag = V + k * (B-V)
For some typical CCD chips such as KAF, TC-241, etc., the coefficient of (B-V) is obtained in Arne A. Henden's research:
Henden, A. A., "The M67 Unfiltered Photometry Experiment", 2000, Journal AAVSO vol. 29, page 35. ftp://ftp.nofs.navy.mil/pub/outgoing/aah/m67/paper/
When you observe a star using other CCD chips, or using non-standard filters, you have to calculate the formula for your own system based on the images at first.
When the formula is obtained, you can convert from the standard V magnitude and B-V value in some catalog (Tycho Catalogue for instance) into the magnitude specified for your own system by the formula, then determine the magnitude based on the converted magnitude.
For example, the standard magnitude of a star are recorded in the catalog as:
V mag = 10.0 mag B-V = 0.8 mag
If the image is an unfiltered CCD image of KAF chip, the feature of your own system is:
Instrumental mag = V - 0.5174 * (B-V)
by Henden's research. So the magnitude of this star specified for your system will be:
Instrumental mag = 10.0 - 0.5174 * 0.8 = 10.0 - 0.41392 = 9.6
The magnitude of another star which is 2.5 times brighter than it will be 8.6 mag.
The magnitude measured as mentioned above is specified for your CCD chip and filter. It is consistent with other observers' data using the same type of CCD chip and filter. However it is not consistent with other observers' data using difference type of CCD chip or filter.
(3) Simple magnitude comparison
When the feature of your CCD chip and filter is uncertain, or there are no stars in the image field whose standard magnitude are recorded in the catalog, it is impossible to determine the magnitude in the regular style.
Especially in case of narrow images, only such catalogs as the GSC or the USNO-A2.0 may be available to refer the magnitude.
In those cases, only you can do is to determine the magnitude of a star simply comparing to the magnitude recorded in the catalogs.
For example, the magnitude of a star is recorded in the USNO-A2.0 catalog as:
R mag = 10.0 mag B mag = 14.0 mag
Then the magnitude of another star which is 2.5 times brighter than it will be: 9.0 mag using the R magnitude, or 13.0 mag using the B magnitude.
The magnitude measured as mentioned above can have a large shift from the true value. Because the base point is not fixed, the magnitude of a star measured from several images shows large scatter. For example, the magnitude of a star can be 10.0 mag in one image, and 11.0 mag in another image.
2. Photometry using the PIXY System 2
The latest PIXY System 2 has a user interface of the three types of photometry mentioned above. It also calculates the feature formula of your own system in case of non-standard system automatically based on the image.
The steps of photometry using the PIXY System 2 are as follows.
First of all, examine your image as introduced in the tutorial page available at the MISAO Project Home Page. In this process, such catalogs as the USNO-A2.0, GSC, etc., are used in order to determine the R.A. and Decl., and preliminary magnitude of all detected stars.
The data of the USNO-A2.0 or GSC are recorded in the examination result. However, these catalogs do not contain magnitude in the standard systems. So when you would like to determine the magnitude in the regular style, you have to identify with the Tycho Catalogue or so as introduced in the tutorial page.
When you select the "Photometry" menu in the PIXY System 2 Desktop, the photometry setting dialog appears, where you can select the catalog, CCD chip, etc., and set check box when to calculate the feature formula for your own system.
Clicking the OK button, a table appears which contains the magnitude in the catalog, other data in the catalog, measured magnitude and the residuals. You can reject some data on the table. Finally, click the Apply button and the magnitude of all detected stars are determined.
Then you can see the magnitude by clicking a star on the chart.
For more details, please visit the tutorial page at the MISAO Project Home Page.
-- Seiichi Yoshida email@example.com http://www.aerith.net/