<PRE>
The photon statistics callibration was done by taking a set of
9 images and comparing the a single image a with the average of
the the nine <a>. Then if photons = C*ADU we have
C0 = <(a-<a>)^2>/<a>^2.
This has to be corrected for the fact that <a> is not a perfect average so
that if N is the number of images in the average
C1 = C0/(1+1/sqrt(N))^2
The RMS dark noise also has to be subtracted, so that if we have M dark images
C2 = C1 - (<(d-<d>>^2/<d>^2)*(1+1/sqrt(N))/(1+1/sqrt(M))
The callibration given was for N=9 and M=3 which is far from optimal and should
probably be done again with more statistics.
Finally, this calculation will not be correct if there is a spread of the
incident photons. For a photon spread of 200 um and a 2.4 to 1 taper
each photon spread over around a 3 pixel square. To correct for this the
images were binned to 1/4 their original size before calculating the RMS
noise.
Note that ADU/Photons = 1/C
Larry Lurio April, 13, 2000 |