The rule to fix the MINIMAL time exposure is:
Std_dev of a light background must be at least 3.5 time the std_dev of a biais.
So no problem for the biais, we have to measure the std_dev of an area without star on a light.
This could be done on the view image.
The exposure time is increased until to reach the 3.5 value.
Today, we have to get a light capture and do this measure into a software like Siril or Pixinsight.
#78643 said "without star on a light".
Should this (#78646) read "the area must be without any stars" ?
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Yes the area must be without start.
We have just be able to zoom into the image and draw a right box.
I think you are going to automatize this function. It is not mandatory on my thinking.
Juste a possibility to draw a box into the image and obtain its statistic would be great.