Telephoto sensor arrays and whisker drones are variations on passive EM sensors. Unlike the standard PESA, Telephoto systems detect a narrower EM band, and use lenses and filters to manipulate the light before it strikes the receptor. These systems are similar to historical cameras or telescopes, and are typically used to assess the dimensions and reflective properties of objects. A whisker drone is a tiny self-propelled camera that moves along with the ship and transmits back information back to the ship. Capital ships whisker drones have a wide variety of sensors, but smaller ships generally only employ a telephoto system.