3) Good questions
You need to select areas where there is only sky background with stars. So no nebulosity should be there. Dark nebula shouldn’t be used as well. APP uses very robust statistics for the tools where you need to create area select boxes. So it’s no problem if you select area’s with a lot of stars, as long as there is no nebulosity. I’ll make a short movie of this, correcting one of your frames. I will give you a signal when it’s uploaded.
The flexibility slider plays a very important role here. It damps the correction and this will help you if you place your area select boxes on wrong spots. Due to the model damping, a misplaced select box will have less influence on the model and it could also turn red, indicating it’s a possible misplaced select box.
1 is least flexibility, largest damping
16 is no damping at all
Indeed, calculate will create a model and show you the image corrected for the model. Note that APP immediately shows your image background calibrated using the supplied are select boxes. The tool starts off with showing your data background neutralised. The background calibration tool should always be used after the remove LP tool.
And finally, clicking OK, will apply the current model to your lights and save the result with a -lpc postfix.
About the colors of the select boxes.
Green are all select boxes with a deviation to the model within 1 standard deviation.
Yellow is everything between 1 and 2 standard deviations
Red is larger than 2 standard deviations.
They way to use these colors is the same as the 123 sigma rule or
https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule
The select boxes are samples of a distribution.
68% should be within 1 standard deviations so roughly 2/3s of the select boxes should be green at least.
95% shoudl lie within 2 standard deviations, so roughly 19/20 of the select boxes should be green or yellow.
The remaining 5% can be red. So 1 in 20 select boxes can be Red and not pose a problem.
So yellow select boxes are in most case fine select boxes, and don’t need to be removed.
If a red select box pops up, double check it, if you think you really sampled onlu background and stars without nebulosity, keep it. If in doubt, remove it, and place a new select box somewhere else.
In really difficult cases, you can use this tool iteratively. But in most cases this is not necessary.
I will explain this in the manual as good as I can ?