You'd better go back and check your work! You're overestimating! Remember
that the number density of photons from the probe that reach the Earth is
the total number of photons divider by the area of the sphere of radius D
(distance to HT Cas). Multiply this value by Arecibo's collecting area to
see how many photons you expect to be detected by the telescope.
Remember to use the correct units for the distance
to HT Cas (meters).
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