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.
Related category OPTICS AND OPTICAL PHENOMENA
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