That was the only time I got to talk to her, so not sure how she ended up solving it. She was searching for transiting exoplanets, so looking for a star's dip in brightness of less than 1%. Even a bug temporarily flying in front of the star could throw it off.
Some people do use tissue paper, or a t-shirt over the lens, and that works for making pretty pictures. But she said none of them come close to 1% accuracy.
But now you have me wondering how the big telescopes do it. Things like the JWST or HST, or even large ground based scopes don't really have any of those options.
I guess you'll need something like an integrating sphere? https://en.wikipedia.org/wiki/Integrating_sphere
Photography reviews often measure vignetting by putting a piece of tissue paper on the lens lol.