[PKay (Peter)]

After looking at ChrisV's image of M16 (deep space).

Colin did his version using the same data, and kindly explained all of the steps.

Then, seeing M20 made image of the day at Astrobin I had a good reference.

So extracting Luminance, stretching it to suit and then using as masks is a very useful tool.

You have better control on portions of the image.

One never stops learning in this game...