Knowledge Corner

SpotOptics is pleased to present information on the principles of their wavefront sensors and wavefront sensor based systems. You can learn more on Zernike polynomials, Shack Hartmann wavefront sensor comparison, aberrations and photo gallery on aberrations. FAQ on SH test is also available

Importance of a long focal length for the lenslet array

 

Shack-Hartmann image of aspherical lens fl=50mm taken with OMI lenslet array fl=22mm and fl=11mm

 

Shack-Hartmann image taken with OMI-22 (lenslet array with fl=22mm). The diamond turning marks are clearly visible

Shack-Hartmann image taken with OMI-11 (lenslet array with fl=11mm). The diamond turning marks are hardly visible

We explain the importance of using a long focal length for the Shack-Hartmann lenslet array using a simulation computed with our Sensoft simulation package.

10 nm of Coma are added to a perfect wave (Standard Zernike coefficients). The spot displacements on the normalized pupil are computed for two different cases:

  • assuming our standard lenslet array with focal length = 22 mm
  • assuming a generic array with focal length = 5 mm

Questions on Shack-Hartmann test

What is the Shack-Hartmann test and how is it different from the Hartmann test?

It is an extension of the Hartmann test. In the Hartmann test, a screen with holes is put in front of the primary mirror, and a photographic plate is placed near the telescope focus to record a pattern of spots. An analysis of the position of these spots gave the aberrations of the telescope.

Since it is inconvenient to put a screen in front the of mirror every time the test has to be made, Shack in 1980 suggested that the test could be done on the transferred pupil, by putting a lenslet array instead of a screen, which could be of a much smaller size. This also allowed the use of CCD detectors, making the process of recording the SH pattern and its subsequent analysis much easier.

The SH method is now standard on most telescopes, and can be used both in the off-line and on-line mode (active optics).

Who needs a wavefront sensor?

%d bloggers like this: