I am testing a ASI 183 MM Pro in the lab. I am taking exposures of a stable light source and vary the exposure time. I expect to see the signal on the sensor increasing linearly. This works for exposure times shorter than 1s. If the exposure time is longer than 1s it looks like the exposure time is increasing in steps of 0.2s.

The upper panel shows the signal increasing with exposure time (blue) as measured of the images.
I am increasing the exposure time in steps of 0.01s and the light source is a LED (it is on for more than 1h so that the output is stable) feeding into an integrating sphere.
The orange dots are my first attempt to correct for the steps in exposure time.
The green line is a linear fit to the corrected orange dots.

The lower panel shows the deviation of the measured points (blue) from the linear fit.
The orange points show the deviation from the linear fit using a step size of 0.2s for the exposure time and a time offset of 0.095s.
You see that for some points the correction does not work.

Also for short exposure times I find a deviation from linearity.
The camera is operated at -10°C.

I am using a raspberry pi 3b+ with Ubuntu Mate and pyindi to control the camera.
Pyindi_client 0.2.3
Indi Library 1.8.0

Is this a feature or a bug?
In case it is a feature, where do I find the precise size of the steps and an possible offset?
Why do I not find this modified exposure time in the header of the fits images?

Thanks a lot in advance.

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