With the growing number of complex-shaped lenses, aspheric and freeform surfaces, the demand for an appropriate and
cost effective measurement technique to test these high quality components is still very high. Ferrofluid deformable
mirrors (FDMs) offer a promising alternative. However, high accuracy profiles produced by FDMs have only been
demonstrated in a closed-loop system which is inappropriate for metrology applications as it requires an additional
measurement instrument and complicates the setup. Consequently, a FDM open-loop driving technique which maintains
good precision while being simple, robust and stable, is required. In the following paper, we present a new active null
test system based on a FDM for the testing of deep aspheric surfaces. We show a new driving method which provides an
accurate open-loop operation mode of a FDM. We demonstrate that the method gives a significant improvement in
comparison with the normalized influence function method. The results are promising enough to consider an active null
test configuration for measuring optical components having high sag departures or complicated continuous profiles.
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