same11 the flat frames started getting horizonal line across the whole screen.
The panel is probably using pulse width modulation (PWM) to change the brightness. Try changing the brightness of the panel to 100% so that it does not modulate. The other way (not as good; you are simply time-averaging out the problem) is to use a camera exposure time that is many times longer than the PWM rate.
If the modulated scan line problem goes away by doing the above, but the panel is now is too bright to be compensated by reducing the gain of the camera, use some neutral density sheets like these one between the panel and the telescope. Preferably as flat against the panel as possible.
https://www.amazon.com/gp/product/B08GS1CLTJ
Some people use layers of T-shirt to dim the panel, but they may not have the proper spectral response and leads to some color shifts at different parts of the image, and they also may not provide an even illumination to start with.
Or, you can buy flat panels that are actually meant for astrophotography, like the EL (instead of LED) Aurora panel from Gerd Neumann. I have never had mine modulate improperly, even when dimmed sufficiently for f/1.4 lenses. And the spectrum of EL panels are continuous instead of made up of discrete spectrum lines in an LED panel, which may not illuminate a narrow band filter properly. As usual, you get what you pay for.
Chen
