Eagle Nebula without stars
I love the backgrounds that narrowband filters give me. The swirly, mysterious clouds that you find in nebulas, combined with false colour that highlights the different elements can produce the most spectacular and aesthetic images.
The problem is that the stars you find in these images are awful.
It's well known that narrowband stars have a purple tinge that makes them look wrong. If you look closely at the first of these images you'll probably see what I'm talking about. Yuck.
If only I could get the beautiful false colour backgrounds along with visible colour stars. Well, recently I've found a way of getting just this.
Out in the field, I have to collect data for two images - a false colour image using narrowband filters, and a true colour image using red, green and blue filters.
Back at home, I process the narrowband filters to get a nice background, and then give them to a program called StarNet++, which - get this - removes the stars, replacing them with its best guess for what the background for those pixels would have been.
Incidentally, what do you think of the starless image? Lots of people love them. I don't. I think leaving the stars out crosses the line between objective astrophotography and drawing. This has been controversial and divisive in the field.
I know I'm making myself vulnerable to accusations of hypocrisy here, as I'm mixing true and false colour images anyway.
Once I get over myself, I use the difference between these two images (starry and starless) to identify where the stars were, and overlay this "mask" onto the true colour image to erase the background.
So now I've got a nice narrowband background, and a transparent image with nicely coloured stars. Combine the two (making sure I've got the stars in the right place) and voila! The final image.
Now I just need a lot of practice to get the hundreds of processing options right...