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Less than ONE e- per ADU in a CMOS/CCD specs? does it make sense?

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gblasco's picture
Less than ONE e- per ADU in a CMOS/CCD specs? does it make sense?


I am thinking in buying a CMOS 14bit camera based in IMX178 MONO sensor.(ZWO ASI178M or QHY178M)

The specs for the QHYCCD Cold peltier model are here:

I will follow an example of a GAIN of 10, to the purpose of my question....

It is a 14bit camera, and you can read the SYSTEM GAIN at a graph in this image

It says that sensitivity is at GAIN 10: 0.28e- per ADU

Also you can find the Readuot noise about 1.2per ADU at GAIN=10

I have checked also some specs for 16bits CCD cameras. there are many models with specs of 0.2-0.3 e- per ADU at a nominal GAIN


Does it make sense have such much precision in the A/D converter at this gain? At first glampse the well at this GAIN is only about 3k e- , but ... Can we measure with a CMOS device less than 1e- ? Can we measure with a CCD device less than 1e-? Does it makes sense following the quantum mechanic principles?

It is a little mindblowing for me measure less than a electron or a photon. Does it make sense?

I have read about how to compute RMS noise, and we are near 1e- noise after summing with the Square root formula of several noises in this case. and I think is not very unlikely have about 1 to 1.5e- of noise.

With QE in IMX178 of about 80%. We can say that 10 photons are 8e-.... but if we have only 4 photons in the well? or does it make sense a difference between 1000 photons and 1004 photons?


If we have a pixel with a value in the 14bit converter of 20e- and another value in other pixel of 24e- with a RMS of 1.5e- , Are really a REAL difference between the two pixel measurements following the quantum mechanics rules, with a significant SIMGA value of confidence?

If not, it will be equivalent have a 12bits AD converter?

Why Am i doing this questions?

Does it really makes the difference a 14bit AD converter in this case or will we have the same confidence in the results with a 12bit AD converter?

This appy the same to the CCD 16bits sensors with a 0.2e- per ADU , does it make sense have a 16bit converter here, if we arent able to measure less than one photon/electron?

Thank you very much,





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