What's wrong with my dark frames?

Here is my screen capture (I’m sorry if Chinese is unreadable for you). The number of cold pixels always stays at 3113042, no matter what I set the sigma to. This doesn’t seem to be a problem, as long as I uncheck it. But the number of hot pixels seems to be too large, and I’d like to know what should I set the sigma to and what on earth goes wrong.

When sigma is 3, the number of hot pixels is 64321.
When sigma is 6, the number of hot pixels is 27,288.
When sigma is 10, the number of hot pixels is 12499.
when sigma is 20, the number of hot pixels is 2727.
When sigma is 30, the number of hot pixels is 292.

My camera is Nikon Z6. The resolution of the picture is 3952*2640 (using APS-C).
Light and dark frames are all at ISO 800, 3 seconds exposure.

Did you subtract the bias to the dark?

Yes, I did.

So this is the issue.

Why do you do that? Because you want dark optimization?

:sweat: It was some Chinese tutorial that taught me to do this. And yes, I did check the dark optimization box as the tutorial said.

OK. The problem here it that because of the noise in the bias frame, your dark frames have a lot of negative pixel values.
You should follow this tutorial: Siril - Manual pre-processing

:ok_hand:t3: Thank you a lot. Now I know I should calibrate the light frames with darks and flats. Does it means that the biases should only be used to calibrate the flats?