[Aavso-photometry] Airmass Statement in Alert Notice 370

[arne]

Brad Walter wrote:
> Alert Notice 370 contains the statement 
>  
> "The GJ 436 M3V parent star is very red. As a result, it will be very
> important to submit your airmass values."
>  
> Why is airmass more important for a very red star than, say, a very blue
> one. Everything I have seen shows that transmittance at the blue end of the
> spectrum is affected more by airmass than at the red end. Is it reason
> because there is a large color difference between GJ 436 and the comp stars?
> _______________________________________________

Yes - the reason is the large color difference between GJ 436 and
the comp stars.  You will get differential extinction, even in
a small field of view, and with a 6-7mmag transit, this becomes
important. It is also because we are asking for B and V measures,
so it is the bandpass of measurement that determines the extinction.
Arne