[Aavso-photometry] Extinction Coefficient Calculation

[Jean-Luc]

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.

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)
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