As we all know, wrinkles on Baader solar film will not affect image quality. However, why is that? Is it because the film is so thin?
If that's the case, I'm wondering if I can use simple red plastic sheet as ERF?
Don't do that with your eyes, however.
Experiment: I shall put a thin piece of transparent plastic (e.g. those food wrap) in front of a refractor and try it out at high power, but this should not be done on the sun in any case.