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

[Jim Jones]

Lionel

I have been following this thread and am having some problem 
understanding exactly what the problem is.  I am missing Gord Sarty's 
comments so suspect that they were not sent to the list.  Perhaps you 
could re-post them to the list.

When you select an object star (MaxIm's terminology)  and a reference 
star(s), MaxIm is obviously making a differential measurement.  However, 
MaxIm reports the instrumental magnitude of the object star if you have 
entered the magnitude of the reference star into the "Ref Mag" window.

Beyond that, I don't believe that the extinction and zero point ( Qv, 
Qvb,....) are normally needed when doing differential photometry so long 
as your FOV is not gigantic.  In Arne's book he mentions that "if the 
variable and comparison are separated by more than a degree it may be 
wise to apply an extinction correction".  I love quoting Arne ;>}  So I 
assume for reasonable CCD fields it can normally be ignored.

Maybe I'm missing something here.

BTW, I agree with Brad Walter's observations about the short comings of 
MaxIm.  I think that if we could ever produce a consolidated list of the 
data that we needed, Doug George would provide it.  I think if we 
approach him individually and present him with a moving target, he will 
pretty much ignore us.

Jim Jones

Lionel Catalan wrote:
> 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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