Flats stray light and all that... ...seeing what's going on.

[andrew s]

There have been a number of posts on flats not working, stay light and the like.

I came across the attached some years ago but was prompted to look it out as it describes an easy way to see what's going on by using your rig and a pinhole to image its internal and see where the issue are. 

Here is the paper Detection of scattered light in telescopes.

Regards Andrew