Light pollution removal - completely confused

Posted by: @brendan

Hi,

I've just spent an hour trying to figure out the light pollution tool and I'm getting nowhere.

I have to draw some boxes. Why?

Because APP doesn't know where to place them. It can't really be sure about what the true background is and needs user input to sample the background in places where there is no nebulosity for instance.

Some boxes are 'right', in green. Some are 'maybe right' in yellow. Some are 'wrong' in red. What are these probabilities based on? How do I choose which boxes are right? All I can glean is that I should choose some sky without nebula or galaxy. 

The colors can be a bit confusing as they don't reflect them being really wrong or not. APP uses the boxes to calculate a background model. This model is rigid as true gradients are usually gradual and thus having a rigid model will make life a bit easier for the user. You also then don't need a lot of boxes usually. This can be done, but usually is a sign that there is an issue with the data. When a box is yellow or red, it only means that the sample in that box doesn't reflect the model very well. However, if it looks good to you, you can leave them.

Even the tutorial says 'Probably' the best step is to focus on one part. Well, is that probably or definitely or what?

That depends on the data and since every data set is different, the advice can change a bit. The best approach is to place about 5 boxes all around the image, when possible. Press calculate and look at the result. Also select a big stretch (the 30% preset at the right of the APP window) to really bring out the gradients. Then, you "sneak up" on areas that are problematic, not placing boxes directly on top of a bright spot, but on the edges of it, press calculate again and work your way up like that. Normally you wouldn't need more then 10 boxes on average. But that depends on the data.

Then there are the other settings on the right with the cryptic labels ST: and BA: and so on. What do these do? What on earth does '15% BG, 3 sigma, 2.5% base' even mean?

When you click on the red question mark next to that setting, you get an overview of that side of APP. Maybe that makes it clearer? If not, happy to help.

Thanks for this.

So I draw boxes where there is no nebulosity, or bright spots, but ideally on the edges between brighter and darker areas?

And yes, the colours are confusing! As is the entire interface on the right. I didn't even know the question mark in red (never put user prompts in red, it's a warning colour) would show me extra information.

And we still really need a user guide, right?

You draw boxes on parts that you think are background, stars in them is fine, but nebulosity should be avoided (if not possible, I only place one of the most faint part I can see).

Yes, a manual is worked on as well and will arrive with the next stable release (in between we will have a beta version).