[Aavso-photometry] Extinction Coefficient Calculation

[Radu Corlan]

On Sun, 12 Dec 2004, Jean-Luc wrote:

> Hi all,
> 
> I need some advice regarding Extinction coefficient calculation. I did a 
> survey of XX CYG (11.2 - 12.2) in september and used both a comp star 
> and a check star in the same field (TYC 3948-2166-1, V=11.56). My 
> Airmass calculation was done with the usual formula for every measure 
> (Airmass ranging from 1.02 to 1.14 during the period).
> 
> In order to get the above atmosphere magnitude, I have to evaluate the 
> First Order Extinction coefficient for the V band : Kv.
> I read that in principle Kv should be obtainned as the ratio of  : Kv = 
> (Check star mag(t1) - check star mag (t0))  / (Airmass (t1) - Airmass (t0))
> My problem is that the period (3h) is not long enough to be significant.

A method that may work in this case is to determine the zeropoints of
individual frames (which are the average of the V(instr)-V(std) of as many
check stars as possible) and plot them against airmass. You should be able
to fit a line through the points, and that line's slope will be the
extinction coefficient. The zeropoints will spread less than individual
stars (because their errors are averaged out), and the linear fit will
help even more. At least you will be using all the data you have in a
near-optimum fashion. 

> 
> I tried the following :  knowing the real  V Mag of my check star (given 
> by Tycho-2 catalog) , I used it to adjust my local Kv coeficient :
> Kv(t) = -(Vmag(catalog) - Vmag (measured (t))) / Airmass(t)

Vmag(catalog) is a standard magnitude, but what is Vmag(measured)? If it's 
instrumental, you would have to comare it to the "instrumental mag ouside 
atmosphere" for that star, which is not a value anyone can measure, it 
can only be calculated.

Radu

> I got a Kv for every measure of the check star and did an average of the 
> values.
> This lead me to a value of Kv = 0.51 with a stddev of 0.03
> 
> 1/ Could anyone confirm this way is correct to get a Kv value ?
> 2/ I read somewhere that a Kv value should be around 0.20- Is my Kv 
> value of 0.51 out of range ?  - any error ?
> 
> Many thanks for your help,
> Best
> 
> Jean-Luc
> 
> 
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> 

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

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