The piston is the 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.
IGE (IGES, filesize: 6.9 MB), SLDPRT (SolidWorks, filesize: 3.03 MB)