A

David

Darling

focal plane

The focal plane is the flat plane at right angles to the optical axis onto which a lens will focus an image. Curvature of field results in an extended image forming on a curved surface known as the focal surface. A focal plane scale is the relationship between angles on the sky, in arcseconds, and millimeters of size at the focus of the telescope, i.e., the number of arcseconds per millimeter.