Flats ADU value decreasing

[david_taurus83]

As titled, when I take flats, the ADU value reported in the FITS viewer decreases with every frame. I've been using EKOS but it's not a software acquisition problem as I noticed the same thing happen with Sharpcap. I've attached a video. Watch the median value. I started off with a value of around 22000 ADU. I just left the camera looping 5s exposures. With each exposure the value decreases. I use an Artesky flats panel with an adjustable voltage regulator. Not a cheap piece of kit so I'd like to think it's not reducing brightness.

 

 

[alan potts]

I use a light box from my photography days of film and neg viewing, don't see how they could with one of those.

Alan