Affiliation
American Association of Variable Star Observers (AAVSO)
Tue, 01/07/2020 - 19:53
What is the relationship between the airmass and brightness of a star?
Hypothetical situation: A target star with a given exposure time has a peak brightness of 40,000 ADU when the air mass is 3 on a camera with a max linear value of 50,000 ADU? Would that same star have a peak brightness over 50,0000 ADU with the same exposure time when the air mass value drops to 1? What sort of margin would you suggest? This is for short periods of variability (exoplanet transits, W Ursa Majoris-type variables, etc).
The higher the airmass, the dimmer the star gets.
For a rough approximation:
perceived magnitude = magnitude in absence of atmosphere + airmass * extinction coefficient (first order)
Now, for an extinctio coefficient of 0.3 - good suburban sky? - the difference in airmass (3 - 1 = 2) would dim your star by 0.6 magnitudes. That's 1.7 times dimmer.
When I look up the magnitude of a star in VSX or the magnitude of a comp star, is the given magnitude the brightness in the absence of an atmsophere?
Normally this would not matter that much: both the comparison star and the variable would be close, so they would have the same airmass. They would (ideally) have also similar color index, so both would be affected by extinction in the same way. This would then cancel out when you do comparisons between the comparison star and the variable (differential photometry).
Now, if you have to work with stars placed at lower altitude (like Nova Sgr 2015 no. 2 with airmass around 5), then things get more complicated. A comparison star placed a couple of degrees lower in the sky will have a stronger extinction than the variable. You'd need to compensate for this.
Best would be to work with variables that are at least 20 degrees high over the horizon.