Piston (optics)
In
mean value of a wavefront or phase profile across the pupil of an optical system. The piston coefficient is typically expressed in wavelengths of light at a particular wavelength. Its main use is in curve-fitting wavefronts with Cartesian polynomials or Zernike polynomials
.
However, similar to a real engine piston moving up and down in its cylinder, optical piston values can be changed to
actuators that translate the Fizeau interferometer
reference surface along the optical axis by precise fractions of the test wavelength, usually by one quarter of a wavelength. This changes the interferometric fringe patterns and allows direct calculation of the exact wavefront error.
Piston and
Defocus
is the lowest order true optical aberration. If piston and tilt are subtracted from an otherwise perfect wavefront, a perfect, aberration-free image is formed.
References
- Malacara, D., Optical Shop Testing - Second Edition, John Wiley and Sons, 1992, ISBN 978-0-471-52232-4.