Research with a Shack Hartmann Sensor
A Shack Hartmann sensor uses a two dimensional array of small lenses of the same focal length, called lenslets, to characterize an incoming optical wavefront. It does this by using the telescope's aperture objective optics along with a additional lens to relay the aperture wavefront to the lenslet array. In this way the aperture is optically divided into sub apertures with each lenslet focusing its particular sub aperture's optical wavefront onto its respective quad-cell, position sensing device, or in this case, camera's focal plane array location. Each lenslet's focused blur spot experiences x-y positional displacement that is proportional to each sub aperture's local wavefront tilt. The x and y positions of all blur spots are collected into a column vector and multiplied by a reconstruction matrix to reconstruct the two dimensional aperture wavefront Since there is considerable computation in the reconstruction process the entire wavefront reconstruction occurs off-line, frame by frame, from a recorded movie of the array of Hartmann spots.
The paper describes the design of the Shack Hartmann sensor, a very effective optical tool for characterizing and subsequent compensation of atmospheric turbulence. Attaching this tool to the telescope exit pupil allows one to 1) visualize the shape of the optical wavefront seen at the telescope aperture as well as find: 1) the strength of turbulence (Fried parameter ), 2) the time rate of change (Greenwood frequency) which is useful for computing turbulence mitigation bandwidth, 3) turbulence layer wind velocity, and 4) measuring scintillation, the spatially small deep shadows that fly across a telescope aperture produced in the upper atmosphere and carried by its wind velocity.
Links to a recorded blur spot movie and its reconstructed wavefront are also shown below.