[Aavso-photometry] formulae between FWHM and Airmass

[arne]

Laurent Corp wrote:
> Dear all,
> 
> Can you say me  if exist a formulae universal between FWHM and Airmass
>     FWHM = f(airmass)
> 
no such formula exists - you cannot predict seeing just by knowing
the airmass.  What you *can* do, however, is determine the change
in fwhm with increasing airmass, starting with knowledge of the
seeing at the zenith.  That theoretical formula gives

fwhm ~ fwhm_0 + fwhm_zenith / (cosZ)**0.60

See Coulman 1985 ARAA 23,19.  fwhm_0 is the amount of local seeing
(such as dome seeing), unrelated to the atmosphere.  The main point
to note is that seeing gets worse the closer you get to the horizon,
kinda intuitive if you realize that you are looking through more
turbulent air.

You can also see an empirical
comparison in Figure 2 of Harris and Vrba, "Seeing measurements and
observing statistics at the U.S. Naval Observatory, Flagstaff
Station," 1992, PASP 104, 140.
Arne