tiptilt

Tiptilt refers to the rapid angular correction of an optical element, typically a flat mirror, around two orthogonal axes to compensate image motion caused by atmospheric turbulence or tracking errors. The corrections are described as tip and tilt because they rotate the surface rather than translating it.

In practice, tiptilt systems use a fast steering mirror mounted on two actuators, often piezoelectric or voice-coil devices, to pivot the mirror and produce small angular deflections. The mirror is light-weight to minimize inertia, and the control loop operates at tens to hundreds of hertz, with some systems reaching kilohertz bandwidth. The resulting angular deflections translate into image motion on the detector and can be used to stabilize the image or provide pointing information.

In astronomy, tiptilt is commonly the first stage of adaptive optics, correcting the largest, lowest-order component of atmospheric distortion—overall tilt of the wavefront. Residual higher-order aberrations are addressed by a deformable mirror in a subsequent corrective path. Beyond astronomy, tiptilt mirrors are employed in laser communications, free-space optical links, and precision imaging where fast pointing stability is required.

Sensing is typically achieved by measuring the centroid of a reference star or beacon within the field of view, or by a dedicated guide sensor. The error signal is fed back to drive the actuators to suppress image motion. Limitations include dependence on guide star brightness and distribution, mechanical bandwidth, tilt range, and non-common-path errors; tiptilt cannot correct higher-order wavefront errors.

See also: adaptive optics, deformable mirror, fast steering mirror, wavefront sensing.