You look at Vega, a star of apparent magnitude m=0,
with a telescope of diameter d = 6 inches = 15 cm.
How many photons enter the telescope each second?
Each second, one million photons per second
from Vega pass through an area of 1 sq. cm.
We need to find the area of this telescope:
diameter = 15 cm
radius = 7.5 cm
area = pi*radius^2 = 177 sq.cm.
Therefore,
flux = (1,000,000 photons/sec/sq.cm) * (177 sq.cm.)
177,000,000 photons/sec into telescope
That's a lot of photons!