[Aavso-photometry] BVRI Photometry

[Greg Crawford]

Keith,

You may not want to purchase more software, but one application I have found
extremely helpful in all sorts of transformation, and extinction calculations,
is MPO Canopus. The calculations are built into the software in a
sub-application called PhotoRed (which stands for photometric reductions). It is
the only Windows software I know of which handles the whole box-and-dice and a
whole lot more in terms of lightcurve work for asteroids and variable stars.

See here: http://www.minorplanetobserver.com/htms/mpocanopus.htm

And the disclaimer, I have no financial interest in the product; I'm just an
impressed customer.


Greg Crawford


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http://www.nelsonbay.com/~gc/observatory.htm
 

> -----Original Message-----
> From: aavso-photometry-bounces at mira.aavso.org [mailto:aavso-photometry-
> bounces at mira.aavso.org] On Behalf Of Keith Graham
> Sent: Thursday, 28 October 2004 5:37 AM
> To: aavso Photometry
> Cc: Aaron Price
> Subject: [Aavso-photometry] BVRI Photometry
> 
> Hi,
> 
> I have decided to take the next step up from V photometry and try BVRI. I
> have been reading the papers by Priscilla Bensen and Lou Cohen as well as
> other sources, and I have a few questions
> 
> 
> 1) In order to compute transformation coefficients, I first need to derive the
> instrumental magnitudes for b,v,r, and i  I use AIP4WIN, and it has the
> ability
> to derive instrumental magnitudes. However I need to put in the zero point.
> I am stuck on just how I can determine Z. One way I attempted it was to
> click on a star of known magnitude in the image. I then adjusted the zero
> point until the instrumental magnitude matched the actual magnitude. But
> this appears to me to "fudge" the actual instrumental magnitude to give the
> real magnitude of the star. So my question here is just how do I calculate
> the instrumental magnitude for the purpose deriving transformation
> coefficients. I did find a way to calculate instrumental magnitude w/o the
> need for a
> zero point using the formula:
> 
> m1 = -2.5log(F1/F2) +m2.
> 
>  But this formula requires knowing the magnitude of a second star.  Since m1
> is instrumental magnitude, I would assume that m2 would also be instrumental
> magnitude. Lou Cohen's paper says to select a star of known magnitude for
> m2, but then, again, this would appear to me to be the actual magnitude, not
> instrumental. So, is it correct that for this formula I can insert the actual
> magnitude of m2?
> 
> 
> 
> 2) I have developed a spreadsheet for BVRI photometry. It appears to me that
> ones needs instrumental values for v, r, and i (along with comps) in order to
> ultimately obtain trnasformed standard mags BVRI.  I found that if I removed
> any of the star - comp  v,r, or i values, all other values were affected for
> that given star-comp . This would stand to reason since the formulas for
> deriving those values all depend on one another. So, how would one derive
> transformed B,V, and R values without the I filter?
> 
> 
> 
> 3) Zero point really has me spinning. I still don't know what that is all
> about. I think it means a "common ground" factor that standardizes the
> instrumental mags, but just how to determine it is a mystery to me. Any clues?
> 
> 
> 
> 
> 
> Thanks for any help you can offer.
> 
> Keith Graham
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