# Homework Help: Signal to Noise ratio in CCD

1. Apr 26, 2012

### BOYLANATOR

1. The problem statement, all variables and given/known data
The 2-D image of a faint galaxy observed by a CCD covers 50 pixels. For an
exposure of 5 seconds a total of 10^4 photo-electrons are recorded by the CCD
from these pixels. An adjacent section of the CCD, covering 2500 pixels, records
the background sky count. During the same exposure time a total of 10^5 photoelectrons
are recorded from the adjacent section. Show that, after subtracting
the background sky count, the signal-to-noise ratio for the detection of the
galaxy is estimated to be 73.

2. Relevant equations
Average count rate = $\hat{R}$=Nobs/t
σpoisson=√(Nobs)

3. The attempt at a solution

Number of photons per pixel per second due to galaxy + background + noise = 40
Number of photons per pixel per second due to background + noise = 8
So there 32 "signal" photons per second per pixel.

I am unsure how to calculate the average noise per pixel.
Any help is appreciated.