Shutters for astronomical instruments are specifically designed to open and close evenly, so that each pixel of the detector receives the same amount of exposur

Shutters for astronomical instruments are specifically designed to open and close evenly, so that each pixel of the detector receives the same amount of exposure time (as good as is possible). Otherwise it becomes very difficult to calibrate the image, since some parts of the image received more light than others. Even opening and closing the shutter multiple times during one exposure will cause issues - but probably still possible to get under control. But covering parts of the detector will create significant variants that will be extremely hard to control for. Keep in mind we don't really want pretty pictures, but we use these images to measure the brightness of objects in those images (photometry).