Conventional wisdom told us that the wrinkles on the filter does not affect the image quality much, however, I found that it's not the case for a Maksutov, it is because the internal reflections between the secondary and the filter could become very complicated due to those wrinkles!
We have two testing sample behind which the aperture was over 50% blocked by my window frame, but still it's enough to show the improvement on contrast. The first shot is taken at prime focus of my 127mm Maksutov:
Next is taken with a 2x barlows, i.e. 3000mm focal length:-
I'm longing for a chance which I could image with full aperture.