Hi Dan!
The artificial flat was made with the software
PixInsight
PixInsight has a process called DBE - DynamicBackgroundExtraction.
Thats a very simple to use but mighty process which creates an
artificial flat and divides it with your image. All
image formats are supported!
It was a 30 second job to make the image "flat".
If you are interested, PI can be downloaded and used 1 month
for free with ALL functions supported.
Good luck for finding the obstructions in the image train.
Gerald (PI-Fan)