The next frame shows the result of dark frame subtraction. The raw image with the dark frame subtracted. Notice that the background is now more even and smooth. This is the flat frame that will be used to correct for various optical defects, vignetting and dust on the optical surfaces. A new flat must be taken each time the optical configuration is changed.
Dark frames are more an art than science, with many techniques used to obtain darks.
I also use a white board placed in front of the telescope an illuminated by six Christmas lightbulbs placed around the front of the telescope. These are caused by dust particles or other imperfections. They have this shape because the telescope is a Schmidt-Cassegrain with a central obstruction. Different telescopes will cause these artifacts to look different. The short answer is that this is impossible.