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Read noise v sensor temperature


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My camera (ASI 2600MC Pro) has a specified read noise of 1 - 3.3 e

Am I right in thinking that the sensor temperature determines which end of that range I will actually get? Generally I cool the sensor to -15°C but I'd be interested to know if it would be worth going lower, so does anyone know where I can find a graph of read noise v temperature?

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8 minutes ago, vlaiv said:

No - read noise has absolutely nothing to do with temperature. It is the same for cooled and non cooled version of the camera.

Read noise depends on gain used.

Ok thanks @vlaiv

So what is the reason for cooling the sensor ? I thought this was to reduce noise?

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It is thermal shot noise (not read noise) that is reduced by cooling the sensor - I'm pretty sure I've seen a graph from ZWO which shows how thermal noise is reduced by cooling the sensor.

Also be aware in the adverts that the lowest read noise is at the highest gain, whereas the highest dynamic range is at the lowest gain, so you can't get both simultaneously on your camera. 

I think you get diminishing returns past -5C, so I tend to use that value (you will still get lower thermal noise by going colder, but this is at the expense of more cooling power to the sensor and more chance of ice issues on the sensor). 

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18 minutes ago, StuartT said:

Ok thanks @vlaiv

So what is the reason for cooling the sensor ? I thought this was to reduce noise?

It does reduce noise - just not read noise.

There are several noise sources and one of them is thermal or dark current noise.

Cooling helps bring down that noise, but more importantly - keeps sensor on set temperature so that dark current can be properly measured (dark calibration). Dark current varies with temperature in non linear way (doubles about every 6°C) and it is very hard to remove that dark current signal when you have varying sensor temperature.

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4 minutes ago, iantaylor2uk said:

It is thermal shot noise (not read noise) that is reduced by cooling the sensor - I'm pretty sure I've seen a graph from ZWO which shows how thermal noise is reduced by cooling the sensor.

Also be aware in the adverts that the lowest read noise is at the highest gain, whereas the highest dynamic range is at the lowest gain, so you can't get both simultaneously on your camera. 

I think you get diminishing returns past -5C, so I tend to use that value (you will still get lower thermal noise by going colder, but this is at the expense of more cooling power to the sensor and more chance of ice issues on the sensor). 

Thanks Ian.

Fortunately my camera has a very convenient second peak at the same gain as the read noise drops off. So there is clearly a 'sweet spot' at about gain=110-120 where you get the best of both worlds

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