[vsnet-chat 4117] Photometry

[Seiichi Yoshida <comet@aerith.net>]

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.

  http://vsnet.aerith.net/misao/pixy/tutorial/photometry.html
  Photometry

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.
	http://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.

P.S.
The past MISAO project announce mails are available at:
  http://vsnet.aerith.net/misao/

--
Seiichi Yoshida
comet@aerith.net
http://vsnet.aerith.net/

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