From analysis of photon noise statistics, the following conversion factors were found:
Gain measured from Poisson noise statistics shows a strong dependency on the exposure level on some CUO cameras. For the W11-4, the relation found is plotted in figure 2, indicating a deviation from linearity of about 3%.
A more precise measurement of linearity is reached by measuring the ADU level versus exposure time, using a stable light source. Measurements made in this way are plotted in figure 3. By dividing the counts with the exposure time, corrected for a shutter delay of 0.12 second, a significant linearity deviation is found. The amplitude is 0.7% over the entire high-gain dynamic range. Almost the same shape and amplitude of the deviation is found for both amplifiers, in both high and low gain mode. For high-precision photometry, this must be corrected for. Apparently, a simple correction as suggested below will suffice.
Where c is:
The Poisson noise analysis clearly goes wrong: it finds a deviation of the wrong amplitude and even of the wrong sign. A noise modulation/source must be causing this, but the origin is currently unknown.
Figure:
Gain versus exposure level measured from noise statistics for
amplifier A in high-gain. The gain apparently changes by 3% over the
dynamic range, but change is shown to be smaller by the more
precise measurement plotted in figure 3.
Figure 3:
A plot of ADU per second versus total exposure time, corrected for
shutter delay. By using a stable light source, the scatter becomes small
enough to reveal a linearity deviation of 0.7% over the dynamic range
for amplifier A in high gain mode.
Full well:
Saturation occurs at approx. 105.000 e
for both channels.
At 90.000 e
, corresponding to 30.000 ADU in low gain mode and
above the dynamic range of high gain mode, no saturation effects
are seen.
Surprisingly, the full well is independent of the mpp mode.