Maximum operating temperature versus exposure time versus read noise 

The maximum practical exposure time is that amount of time that it takes for the accumulated dark shot noise to match the read noise.

If you want to expose longer, then you should cool more to avoid adding noise to the image.

These images were exposed for 30 minutes through an [SII] and an Halpha filter in my suburban backyard.

Note that the read_noise^2 component is significant compared to the shot noise of the signal.

This is true for the brightest parts of the nebula, the faintest parts and the background. 

Also note how little difference in DN value there is between the faint parts of the nebula and the background

In these simple backyard examples after a half hour exposure the read noise still is a significant term in the noise equation.

In this case the camera was an FLI CM10 with a read noise of 5.55 e-

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