This is part 3 in my first PixInsight Tutorial series of videos. In this third video I will show you how to process the light frames for each filter in a linear stage. This will include how to crop all light frames, how to extract the background, how to do a linear fit for each frame and finally how to do some initial sharpening and noise reduction. As a last step in this video, I will show you how to permanently stretch the light frames to prepare them for the final stage. In the tutorial I will use a ASI294MM Pro camera and Ha, O-III and S-II filters, but the principles will apply to other scenarios. I will show you the complete steps for the entire processing in PixInsight.