Field curvature is an optical aberration caused by a difference of curvature in the focal planes. Because of this, the lens is unable to focus the edges and center of the field at the same time, resulting in a curved distortion.
What Causes Field Curvature?
Field curvature is caused by a difference in the lens curvature. Because of this, rays of light will be reflected in a bowl shape. This causes the focal point to change as you near the edges of the field of view.
How Does Field Curvature Affect Astronomy?
Field curvature can be present in nearly any telescope design. Fortunately, the spherical imaging surface of the eye compensates for this aberration, so it isn’t detrimental to astronomical viewing. However, it can be problematic when taking photos of space. This is because the flat surfaces of most camera lenses cause this aberration to become more noticeable, showing off-axis stars as being out of focus.
What are the Types of Field Curvature?
Most telescope objectives experience a positive curvature, where edges of the field are focused closer to the objective than the center. To counter this, the eyepiece typically has a negative curvature. If the curvature of the eyepiece is stronger than the objective, the curvature will be negative. In negative curvature, the edges of the field are focused farther from the objective than the center.
How to Fix Field Curvature
Some telescope designs, like the Petzval telescope, are designed to produce flat fields. This is done by using a doublet and second pair of lenses. In designs with expected field curvature, you can purchase a field flattener to minimize this aberration.