focal plane tilt

Focal plane tilt is a type of problem that optical instruments (such as telescopes) can have: a focal plane that is tilted in relation to its intended or ideal attitude. Typically the focal plane is intended to be perpendicular to the optical axis, and for a telescope symmetric around its optical axis, such a tilt represents a misadjustment or a flaw in construction. If the sensor is aligned with the tilted focal plane, some distortion results, but if not aligned, then not much of the image will be focused: the focus will vary over the extent of the image. The term focal plane tilt may also be used for a quantification of the difference between the focal plane and some reference plane, often meaning from the focal plane's ideal attitude. Some off-axis telescope types have an inherent focal plane tilt: the telescope might be designed to reduce it, e.g., through a long focal length.

The term sensor tilt refers to a tilting of the sensor (e.g., CCD or film), again, often meaning its tilt away from its ideal position; or alternately, the term is used to describe some intended tilt of the senor that isn't perpendicular to the optical axis. A sensor tilt may be a flaw or a misadjustment of the equipment. Or a tilt may be deliberately used to partially compensate for a problematic focal plane tilt. Atmospheric seeing can result in an approximation of focal plane tilt, and methods used to overcome this include adjusting the sensor tilt to best match the resulting tilted focal plane or compensating through optics.

Focal plane tilt and/or sensor tilt is sometimes deliberately used in (non-astronomical) photography, e.g., to bring your attention to one portion of the picture by degrading the focus of the rest of the picture.