One of the most common aberrations in telescopes that needs to get corrected for is Field Curvature. Field curvature is where objects in a telescopes field of view appear sharp only in a certain part(s) of the frame (usually the center). This happens because of the curved nature of the optical elements in certain telescopes that project the image in a curved manner. In astrophotography film planes and digital sensors are flat and cannot capture the entire image in focus because they don't match the same shape as the curved focus filed. Field Flattners fix this by flattening the curved field to better match the plane of a digital sensor.