AST 3722C  Spring 2008
Homework #10 – Extra Credit
You don’t have to do this one
Due April 15
Instructions: Solve each problem below.
I’ll post the solutions online on April 15, so
there’s no possibility of turning it in later than that date.
These problems involve CCD
measurements and would help prepare you for the final.
•
1. Suppose you are told that the nightsky brightness atop Mauna Kea in Hawaii is
21.5 magnitudes per square arcsecond in Vband. You want to find how many photons per
second per square centimeter per square arcsecond this provides. Here are the steps involved
in getting there.
(a) Use handout #17 to convert the magnitude within 1 square arcsecond into a flux
density.
Assume that flux density is measured per wavelength interval.
Your answer will
effectively have units of erg/s/cm
2
/
˚
A/arcsec
2
.
(b) Calculate the power per collecting area provided by that 1 square arcsecond of sky.
Assume that when you look in Vband you’re looking through a filter that is a step function
centered on
λ
=
λ
0
= 0
.
55
μ
m and is Δ
λ
0
= 0
.
089
μ
m wide. Also assume that the sky’s flux
density is approximately constant at all wavelengths across the V bandpass.
This means
that to figure out power per collecting area within 1 square arcsecond, you simply need to
multiply by the bandwidth Δ
λ
0
. Express your answer in erg/s/cm
2
/arcsec
2
.
(c) Convert the power (i.e. photon energy per unit time) to a photon number per unit
time by assuming that all photons in V band have approximately the same energy, i.e. the
energy of a photon of wavelength
λ
0
. Express your answer in photons/s/cm
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 Spring '09
 Fernandez
 Astronomy, Photon, Light, Chargecoupled device, Mauna Kea

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