Keep in mind that the 2 megasecond exposure was not done all at once. It consists of many shorter exposures (probably about 10-20 minutes apiece) added togethe

Keep in mind that the 2 megasecond exposure was not done all at once. It consists of many shorter exposures (probably about 10-20 minutes apiece) added together. If the exposure had been done all at once, it would have saturated. Moreover, cosmic rays would ruin large regions of a single long-exposure image.