DirkT I used a LED lighting module pointing to a white ceiling.
Hi Dirk,
That's probably the problem. The PWM that feeds the LED lamp is beating with the camera sensor.
There are special LED panels that are meant for astrophotography applications that do not use PWM to adjust brightness.
To avoid the beat effect, you can use a much slower exposure time, say 10 seconds, while using something to reduce the amount of light that enters the objective side of the OTA. A couple of layers of T-shirt cloth is often sufficient.
However, if you are going to use the ceiling as your flat source, make sure the objective lens is very close to the ceiling, and the ceiling is perfectly neutral with no texture, and there is no chromatic gradient (for example, two light sources one at 2700ºK and one at 5000ºK at different parts of the room, or even a large color object on one side of the room that can change the hue of the ceiling or wall). If there is either a luminance gradient, or chromatic gradient, your processed image will reflect the reference that is given to it. I.e., it is a very bad idea to use an arbitrary ceiling or wall as your flat illumination source when taking a flat image.
Chen
