David, I forgot about the graduated filters! Yes indeed, I went through The Big Box and found two graduated color filters - one yellow (true grad) and one red (semi-transparent silvered 'star' in the middle). Found two heated windows, too.

My worries are over. I can test for focus shift. I was concerned about the rest of the things I am ignorant of.

Back to the work bench - with more joy. I appreciate the help!