Now consider a real 2.5m telescope. Assume its
CCD camera has both a read noise of 10 counts and a dark current of 10
counts/second for every pixel.
The star is observed for 20 seconds and a total
of 20,400 counts are recorded from the image and this image is spread over 40
pixels in the CCD camera. What is the S/N of the detection of this star?
What is the S/N if the telescope detects a
background nebula that contributes a uniform background of 99 counts in each of
the 500 pixels that surround the star image?