Part 4: Light pollution correction and background calibration of the Mosaic Tutorial – Milky Way to Rho Ophiuchi by Mabula

Light Pollution correction & Background Calibration are both performed
with the Remove Light Pollution tool.

You need to place area select boxes on parts of the image where
there is no nebulosity or galaxy.

The area select boxes may contain a lot of stars, APP will disregard the
stars in the calculations, so the stars will not affect the outcome
of the correction model calculations.

Probably, the main key in getting a good correction, is to create the correction model, step by step, focusing on just part of the image.

Placing the area select boxes at the right place, takes some practice and experience.

The model flexibility can be adjusted, the model that looks correct with the lowest flexibility tends to be the best model, since it will be less vulnerable to
having placed some area select boxes at the wrong place.

The stiffness of the least flexible model allows for not placing the area
select boxes at exactly the right place.