Shack-Hartmann vs. Hartmann test

Setup for Hartmann screen vs Shack-Hartmann test
Hartmann screen test vs. Shack-Hartmann test setup

In the classical Hartmann test, a screen with holes is placed before the element being tested. The spots corresponding to these holes are recorded on a CCD, before or after (as in the figure at right)the focus. The positions of these spots are mapped back onto the optical element, and the difference between the positions of these spots and the holes of the screen are used to compute the aberrations. This method is mostly obsolete now, as the Shack-Hartmann test has replaced it.

In the Shack-Hartmann test, a lenslet array is placed in the transferred pupil: light from the optical system being tested comes to a focus and is made parallel by the collimator, which also images the pupil on the array. The displacements (shown by red arrows) due to the aberrations from the optical system, with respect to the ideal wavefront (shown by black arrow) are used to compute the aberrations (in terms of Zernike polynomials) and the wavefront. The Shack-Hartmann technology was first applied to the  ESO 3.5mt NTT Astronomical telescope . Since ESO is a Scientific European Consortium the technology cannot be patented. We acknowledge ESO scientists R. N. Wilson and L. Noethe for being the first who applied this technology to Astronomy.

The Shack-Hartmann system is easier to calibrate and is of much higher precision compared to the Hartmann method. The use of a lenslet array leads to higher sensitivity.

SpotOptics  Shack-Hartmann wavefront sensors that can test optical systems on-axis as well as off-axis. Some models are also suitable for laser metrology. Our instruments can test at different wavelengths in the range 193nm to 10.6 microns. We also offer a complete set of accessories to make easier your optical setup.