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Re: more details of signal-to-noise calculations for Mark IV



> I'm still reading Michael's message but I don't see where he
> assumes we are pointing.  Is it to zenith (airmass = 1.0)
> or down at the equator?
> Is atmospheric extinction built into the "photons from a mag
> zero star" constant?

  Good point.  The calculation assumes that one is looking at a 
star _above_ the atmosphere, at an airmass of zero.  The extinction
due to ordinary air is

        20%  in the V-band at zenith      30% in V-band at 45 degrees over
         8%  in the I-band                11% in I-band at 45 degrees over

  So the numbers in my previous tables are slight overestimates of
the expected signal-to-noise.


-- 
-----                                      
Michael Richmond                   "This is the heart that broke my finger."
mwrsps@rit.edu                     http://a188-L009.rit.edu/richmond/