Foucault & Ronchi tester

A Foucault meter is a forgiving and inexpensive way to test optical surfaces. It is also very accurate. A resolution of 1/8 of a wavelength, or the size of 300 atoms of aluminum is easily achieved.

The instrument described here is 3 in one: Foucault meter, Ronchi tester and, as a bonus, Schlieren photography set up. I’ll talk about the first 2 only here, Schlieren photo should be an entire separate article on its own.

The Foucault meter was first described by Leon Foucault in 1858. A century later, when Texereau wrote his reference book “How to make a Telescope”, it seemed like the definitive textbook discussion on a well understood topic, with noting to add. And yet, many innovations have improved the Foucault meter since. It is worth discussing them, as many tutorials on the net or examples in the litterature are still based on old school designs from the last century:

  • The slit-less concept could have been invented by Foucault or Texereau a long time ago. However it is a relatively recent discovery. It removes the necessity to build a slit, which is tricky to do well. The same knife edge masks the source and also acts as the Foucault knife. This simplifies the system, making the apparatus even more easy to build, without loss of accuracy.
  • Today, we further benefit from cheap and bright LEDs,
  • x-y-z linear stages, mass produced in Asia, with an accuracy unbelievable for the price.
  • Macro rails” with a stepper motor controlled by a computer are now accessible to amateurs offering the ability to take pictures at regular interval of a few microns along a linear axis.
  • For mirror support, aluminum extrusions (like 80/20) allow to easily build very sturdy and versatile structures.
  • For interpretation, the Couder masks (physical cardboard masks put in front of the mirror to obscure some zones) can be replaced by modern computer analysis of the image, cutting though the time necessary to perform a test, build the masks and mess with them during the measurements.
  • The Ronchi grating, or for this matter any slit or pinhole can be a computer generated image printed on a slide, which is very accurate and inexpensive.
  • Last, anybody with a laptop and a cad software (I use OpenSCAD) can design very complex parts to either 3D print for a few dollars in plastic, or have CNC machined to very high precision.


  • x-y-z-r linear stage to hold the knife edge and light source
  • camera, with a medium telephoto lens, in manual focus mode. I use the same Samyang 135 so popular among astro-photographers
  • a wee photo macro rail or equivalent, connected to the camera, and to a computer for control.
  • a mirror support made of 80/20 extrusion or plywood.
  • A power supply, to vary the source brightness.
  • The free software Foucault Unmask for image processing.

The knife edge is made from an aluminum L extrusion big enough to support a razor blade or a slide (for the Ronchi and color Schlieren). The source light is an high brightness green LED (plus its resistor), housed in some 1/4 nuts (led fit exactly and lock perfectly in 1/4 nylon insert nuts).

Front of the Foucault/Ronchi tester, here in Ronchi mode.

Side view: the Macro rail provides micrometric movement along X, controlled by computer, while the x-y-z block allows precise positioning of the source and kife edge.

The image collection is then preprocessed with a Python script (rotating, cropping, renaming, size adjust), and fed as input to Foucault Unmask.

Collection of images taken by the computer controlled macro rail, all automatically.

The same can be use for Ronchi testing, by replacing the razor blade with a grating slide.

Ronchi 200 lpi slide moved along the x axis.
This mirror (a 10 inches f/4.5 parabolic mirror rescued from a Meade Starfinder) shows a Strehl ratio of .86 and a big wash out in the center, also very visible on the Ronchi test.