This is a quickie, showing some of the methods I use for creating Crowd Images, based on Creative Commons licensed images.

First all images were aligned/registered to a selected "TransformMaster" (a cropped and 200% resampled version of one of Jürgen Kemmerer's images), using distortion correction in PixInsight. Also Generate Masks were enabled, giving a B/W masks of the coverage of each image after alignment.

Version A is all 65 images combined, using equal 1:1 weights. Basically a simple average of all the images. Notice the vignetting (dark towards the edges).

Version B is a stacks of all the masks (coverage of each image), again using a simple 1:1 average.

Version C is A/B using PixelMath in PixInsight. This works like a normal flat calibration. Where you have have white the RGB image is divided by 1 (same-same), and where you have 50% coverage, the image is divided by 0.5 (which is the same as x2). The result is a flat image.

Version D is a full size final.

Version E is a crop, resized to 66%.

I'm currently writing a more thorough explanation which I'll publish at the PixInsight forum. I'll post a link here, once it is ready.

You can also find more information about the concept in the text for the M42 and M31 CI images in my gallery.

Image credits (thanks a lot for sharing your work):

Adam Evans, Alexis Tibaldi, Álvaro Pérez Alonso, Andolfato, Arturo Fiamma, AstroGG, ASTROIDF, Chris Madson, Claustonberry, dave halliday, dyonis, Eric Pheterson, Frank Zoltowski, Fred Locklear, Gaby, Giuliano Pinazzi, Jorge A. Loffler, Juan Lozano, Jürgen Kemmerer, Jussi Kantola, Konstantinos Stavropoulos, Lonnie, Luca Argalia, Luigi Fontana, Michele Palma, Mike Markiw, milosz, Miquel, Morten Balling, NicolasP, Pat Gaines, Paul M. Hutchinson, PaulHutchinson, Peter Williamson, Phil Hosey, Phillip Seeber, Ralph Wagter, Richie Jarvis, RIKY, s58y, Salvatore Iovene, Salvopa, Stephane_B, Steve Yan, stevebryson, Thomas Westerhoff & Werner Mehl