[Aavso-photometry] BVRI Photometry

[Radu Corlan]

On Wed, 27 Oct 2004, Keith Graham wrote:

> 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?


If you expand the logarithm, the formula can be written as:

m1 = -2.5log(F1) + 2.5 log(F2)) + m2


If we make

i1 = -2.5 log(F1) and i2 = -2.5 log(F2) the formula becomes:

m1 = i1 + (m2 - i2)

or

m1 = i1 + Z

i1 and i2 are clearly instrumental magnitudes, m2 is the standard 
magnitude, but what should we call m1? We cannot call it standard, because 
it hasn't been converted yet to standard magnitudes. So it is also called 
instrumental, which is where the confusion arises from. To avoid the 
confusion, some people call i1 and i2 "raw instrumental magnitudes", and 
m1 would be called just "instrumental".

Z is the frame zeropoint. If you determine your transformation
coefficients from the same field as yor target stars, or the night is 
photometric, the exact value of Z
doesn't really matter - it changes the intercept point of the
transformation coefficient fit. If you choose it to be m2-i2 for a
standard star, you will get a smallish intercept point.

If you use different fields in a non-photometric night, you can still 
calculate transformation coefficients, but the solution is more involved.


>  
> 
> 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?

This really depends on what colors you base your transformations. 

> 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?

Just imagine that all transformation coefficients are zero. Then the 
zeropoint is what you need to add to the raw instrumental magnitudes to 
get to standard magnitudes (like when you're doing single-band 
differential photometry). 

Radu

> 
>  
> 
>  
> 
> Thanks for any help you can offer.
> 
> Keith Graham
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> 

-- 

-------------
Radu Corlan       Snail Mail: Bucuresti sect. 1, 
rcorlan at pcnet.ro  str. Argentina nr. 28, Romania

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