He is known to be the developer of 'Foucault currents', 'Foucault polarizer', 'Foucault knife-edge test
' which is still used in amateur telescopes.
Various tests are used for this, from the very simple Foucault knife-edge test
to very elaborate tests based on laser interferometers.
The venerable method of checking the figure of a homemade mirror is the Foucault knife-edge test
. To better quantify the readings from such a test, amateur telescope maker Larry Gilstrap has created the freeware ATM Foucault Test--Mirror Surface Profiler for Windows 95 and higher.
As described in my September 2000 article ("Understanding Thermal Behavior in Newtonian Reflectors," page 125), the schlieren test (a close cousin to the more familiar Foucault knife-edge test
used by mirror makers) permits us to directly observe what is going on, and it reveals a persistent thermal chaos around the primary mirror with effects that are more damaging and long-lasting than most telescope users realize.