How do you pull it away? What mechanism is used? How do you test that the sheet doesn't reduce the amount of light received? How do you test that the sheet doesn't increase the rate of buildup of dust? Once you get into the nitty gritty you find out it's a huge engineering problem with lots of costs and time associated with getting it even remotely right, which is why of the three organizations which have recently built Mars landers/rovers they've all decided it's not worth the investment of trying to mitigate dust build up, at least not yet.