|Some facts about precision in well-built Shack-Hartmann wavefront sensors
We would like to take this opportunity to dispel two widespread myths about Shack-Hartmann technology. The first being “crossing spots,” or the claim that these wavefront sensors can be misled when measuring highly aberrated wavefronts.
This first erroneous claim purports that large aberrations in the source would be misinterpreted by the device as light from a different microlens and therefore cause the sensor to provide a false measurement. Not only is this myth unfounded but the simple truth is that it’s impossible in wavefronts with a continuous derivate.
In the case where a wavefront were to be so aberrated that it could cause the light from one microlens to stray into an adjacent measurement zone on the CCD camera’s detection grid, the wavefront’s local radius of curvature would render it undetectable due to a simple lack of contrast (see illustration above). Theoretically, for two spots to cross or to superimpose on the detection grid, the wavefront’s local radius of curvature would have to be such that it would be exactly equal to the microlens’ focal distance. In this case, the microlens’ focal point would not be on the detection grid but far in advance, creating an image on the detection grid equal to or superior to the size of the microlens. This complete defocalization of the light from that microlens would be considered by the HASO™ wavefront sensor as non-measurable and no data would be provided for that point.
Although HASO wavefront sensors have an exceptional dynamic range, no sensor can claim to measure any and all aberrations regardless of their magnitude. When a HASO device encounters this type of situation, the data is discarded and no measurement is provided for that measurement point. In contrast, inferior devices may provide erroneous data based on false “assumed” data.
The second erroneous claim is based on the supposition that devices on this type are subject to provide erroneous data due the chromatic effects introduced by the microlens array. Yes, it is true that any optical medium induces chromatic effects. What really matters is understanding the effects on measurement and working around them. Imagine Optic’s HASO wavefront sensors are built using patented fabrication techniques to ensure precision measurement, no matter what wavelength you work with.
First, HASO wavefront sensors are precision built devices with a fixed and known distance between the principal image plane (microlens array) and the CCD detection plate (“d” in the first illustration). Using the distance between those two elements is the only sure way to correctly calculate the wavefront’s local slope. Second, the optical geometry chosen by Imagine Optic employs a microlens array that places the microlens’ convex face towards the CCD detection plate. This enables HASO wavefront sensors to calculate by means of a fixed parameter where the distance between the principal image plane and the detection grid never changes, making them achromatic.
By means of these two constants, HASO wavefront sensors measure correctly no matter the chromatic properties of your source.
In short, HASO Shack-Hartmann wavefront sensors are the most reliable means of using wavefront sensing and analysis to achieve precision results every time you use them – even on the most highly aberrated, convergent or divergent beams.
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