[Aavso-photometry] Determination and use of BVRI TransformationCoefficients with Maxim DL

[Lionel Catalan]

Thanks Brad for the suggestion about including the integration time in the
calculation of instrumental magnitudes.

Lionel 


-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Brad Walter
Sent: Tuesday, January 15, 2008 5:48 AM
To: aavso-photometry at mira.aavso.org
Subject: Re: [Aavso-photometry] Determination and use of BVRI
TransformationCoefficients with Maxim DL

 Lionel, When computing your instrumental magnitudes I would suggest that
you use the form

minst (star) = -2.5 log (Int(star)/Tint)

Where Tint is the integration time of the image. Otherwise, you will not be
able to compare instrumental magnitudes obtained with different integration
times. 

Frequently the term differential magnitude is used to denote
Minst (star) -minst (ref) without adding mstd (ref) being added. See the CBA
submission format in the help document for CCD/PEP Batch Upload in WebObs,
http://www.aavso.org/bluegold/webobs.html 



Message: 2
Date: Sun, 13 Jan 2008 22:49:48 -0500
From: "Lionel Catalan" <lcatalan at lakeheadu.ca>
Subject: Re: [Aavso-photometry] Determination and use of BVRI
	transformation	coefficients with Maxim DL
To: aavso-photometry at mira.aavso.org
Message-ID:
	<58b78fe40801131949r4fa5458di30f0060e3d513be0 at mail.gmail.com>
Content-Type: text/plain; charset=WINDOWS-1252

Based on comments from Arne and Gord Sarty, I thought that the most
straightforward way of using Maxim photometry data to derive
transformation coefficients would be to convert Maxim's differential
magnitudes into instrumental magnitudes. This recognizes the fact that
transformation equations are written in terms of instrumental
magnitudes.

Because the photometry analysis tool in Maxim DL only calculates
differential magnitudes, a special procedure is required to derive
instrumental magnitudes. Differential magnitudes in Maxim DL are
calculated using a reference star with known standard magnitude as
follows:

mdiff (star) = minst (star) ? minst (ref) + mstd (ref)

where mdiff, minst and mstd refer to differential, instrumental and
standard magnitudes, respectively. The instrumental magnitude of the
reference star can be calculated as follows:

minst (ref) = -2.5 log (Int(ref))

where Int(ref) represents the intensity of the reference star
calculated as the sum of all pixel counts within aperture less
background. The value of Int(ref), which is simply called "Intensity"
in Maxim DL, can be read from the information window in aperture mode
by centering the aperture on the reference star while using the
photometry analysis tool. One must be careful to ensure that the
centroid of the reference star has the same coordinates when reading
its intensity and when calculating the differential maginutes of the
other stars.

Combining the two previous equations gives:

minst (star) = mdiff (star) -2.5 log (Int(ref)) - mstd (ref)

This equation is applied to magnitudes obtained with the B, V, R, and
I filters using Excel.

Lionel





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