Hello,

If I setup filter bandwidth=8 nm, the exposure time is 20 minutes ! And more, if I increase the SQM, the exposure increase also ! Curious no ?

F/D=6.5, ASI183MM, gain=120

Yes, the exposure calculator considers the effects of light-pollution.

The 8nm filter is limiting the light reaching the sensor, so the image sensor is "seeing" less pollution, and this allows for a longer exposure. By increasing the value of the SQM input you are reducing the amount of light pollution, and this also allows for longer exposures. (And raising the focal ratio will also cause the exposure time to increase.

The calculator can produce some extreme exposure times.
Factors that raise the exposure time are: dark skies (a high SQM value), long focal ratio, narrow-band filtering, and camera gains that produce more read-noise.
So in very dark skies, with a long focal length, and narrow-band filtering a exposure calculation may be many hours.

Conversely, in heavy light pollution, with a short focal length, broad band filter, and camera gains that produce less read-noise; the exposure times can be extremely low, even sub-second.

Hi,

Thank you for your answer. I understand your reasonning.
When the conditions are better, I don't change the exposure time, but the total exposure, meaning more captures. And primaly, I check the 3 sigma rules in order to determine the minimum time exposure.