Thank you, Bob. I use the ISO and exposition taking in account the histogram graphic. If I use long time of exposure I can get images with high background glow.
If I can use less ISO to reduce the noise... I do it.
If I have some problem or troubles with clouds or guiding ... I use less time of exposure and higher ISO.
I perceive that DSLR is different from others cameras. Some times to use long exposure or high ISO cause saturation of stars and loose of theirs colors.
My best configuration is ISO 400 and 4 minutes. Or ISO 800 and 2 minutes that is very similar. And this, because of skyglow filter. My observatory is under attack from a battery of street lights ! From a radical and idiot group of public workers that install them !
I needed increase the height of the walls of my observatory to protect the OTA.