The Floppy Foucault tester

Analysis of the surface profile of a mirror is done by means of a testing device, which can be made with fairly basic tools. The easiest is the knife edge test, invented by Leon Foucault (aka Foucault test). A point or slit shaped light source located at the Radius of Curvature (twice the focal length) is reflected by the mirror under test. The image of the lightsource is then analyzed by means of a knife edge.
What you need for this test is a way to move the source and/or knife edge and measure its relative position in axial and preferably also in sideways direction. Then you need a proper source and a straight edge for analysis and a mirror stand which allows for tilt adjustment. There are many sources on the web describing such devices. I made an X-Z stage from two old floppy drives.
Alternatives that are similar to the KE test are for example a slit test, wire test or a Ronchi test.

This tester platform is based on two magnetic head slides from old 5.25" floppy drives. These slides consist of a plastic carriage with bronze sleeves that moves over two precision machined stainless steel bars. There is no noticeable slop and only very little friction.
These parts can easily be taken out of the floppy drive (together with the stepper!), and put in an aluminium framework to serve as a micrometer stage for example for a Foucault tester. Adding some more aluminium and a second slide will even give you a X-Y stage.

The way I have constructed the X-Y stage, has resulted in a very low profile tester, with 30mm travel in both directions, and a base for attachment of several test methods. The lightsource is also removable, so the tester can be used for any kind of test such as wire, knife-edge and grating test methods.
Tester Front view
What you need to construct this tester:
  • Two magnetic head slides from 5.25" floppy drives.
  • Some 40x40x3 L profile aluminium.
  • Some 30x30x3 L profile aluminium.
  • Some 30x3 bar aluminium.
  • An M6 and an M3 bolt, 45mm long.
  • An M6 T-nut.
  • Some 32mm pvc pipe.
  • Three rubber feet.
  • A bunch of little screws and epoxy glue.
Tester Parts

You only need simple hand tools for metal work, such as a saw, some files, taps, screwdrivers etc. Holes may be drilled with a hand drill, but to achieve sufficient accuracy you really need a drill press.
For better definition, the CAD files are available for download from the resources page.

The Base

Tester Base

The base is constructed from pieces of stock aluminium, as can be seen in the drawing. The construction is placed on three rubber feet, at the three extremities. This provides a sufficiently stable and well defined support for the tester base.

The following aluminium parts are needed:

  • 166mm, 30x10x3mm L-profile
  • 164mm, 30x3mm bar
  • 90mm, 30x30x3mm L-profile (twice)
  • 90mm, 30x3mm bar
  • 74mm, 30x30x3 L-profile
The first L profile can be generated from the standard 30x30, by cutting off 20mm on one side. The holes for the slider bars need to be 4mm across, they should not be completely through the aluminium but only 2mm deep. The bars should then fit exactly, but to eliminate residual play they will be clamped in, or if neccessary epoxied. The other aluminium parts need no special handling.

Further bits and pieces:

  • a 45mm M6 bolt
  • an M6 T-nut
  • moderate tension spring, pulling the carriage against the screw
  • a slice of 32mm PVC tube
  • a 3mm aluminium disk snugly filling up the slice
  • a pair of slider bars
  • a handful of short M3 screws
  • three rubber feet plus mounting screws
  • epoxy glue

Construction is pretty straightforward, take care of squaring all angles, epoxy all aluminium together, except from one of the short L profiles (otherwise you wont get the slider bars in).
The M6 bolt has a speed of 1mm per revolution. Since it loosely fits in the T-nut, it is advisable to make the hole in the L-profile a bit too tight (like 5 or 5.5 mm), so that the bolt can tap its way through. Together with spring-loading this will decrease the amount of slop.
The knob itself is made from the 32mm slice, filled with an aluminium disk. This assembly is mounted onto the bolt, and filled with epoxy when square. The nice thing of 32mm diameter is that the circumference is almost 100mm. This means that you can make a scale with a CAD program, and tune the printer scaling to get it fitting. The true length should be about 101mm, you need to experiment a little to get it right.
The bolt has to be ground of into a 45deg centered tip. This tip will be catched in a dent in the longitudinal stage, and pulled tight with a spring.
If the tester is too light to your taste, you can attach the base to a heavier substrate using the same three mounting points.

The Carriage assembly

Tester Carriage

The carriage is a bit harder to make, you need to work pretty accurate to keep the cumulative error down. First the two sliders need to be freed from all loose bits, so that only the plastic and the brass sleeves remain. Then all plastic parts that stick out of the box shape need to be cut off, and finally the surfaces must be sanded flat. The result is a component as shown in the drawings, a 7x30x52mm flat box with a curved elevation sticking out 1.5mm on one of the short ends. One of the sliders must be cut in length, exactly through the middle of the elevation.
The two halves are used for the longitudinal stage, and the other slider is used for the transversal stage.

The following aluminium parts are needed:

  • 90mm, 40x10x3mm worked L-profile
  • 40mm, 30x15x3mm L profile (twice)
  • 74mm, 30x30x3mm L-profile
  • 52mm, 30x30x3mm L-profile
The first L-profile is the base for the transversal stage, and this needs to accommodate the two slide halves. The positioning of these is critical, especially the half with the two bronze sleeve bearings should be pretty well squared to the profile. A good piece of the profile needs to be cut/filed away to give room for the slides. Keep the surface smooth to prevent unneccesary tilting of the slide halves.
The two short L profiles need holes for the slider bars, and these should be made in the same way as described for the tester base.

Further bits and pieces:

  • a 45mm M3 bolt
  • moderate tension spring
  • a slice of 32mm PVC tube
  • a 3mm aluminium disk snugly filling up the slice
  • a handful of various M3 screws
  • one complete slider and one cut in two
  • a pair of slider bars
  • epoxy glue

The assembly of everything follows from the drawings. Take care of the order in which to do things. Note that the spring-loading of both stages is not in the drawings!

Tester Left view
This shows the tester as seen from the left side.

The Accessories

It is possible to equip the top stage with any measuring device that you need. Examples are a knife edge for Foucault test, a wire holder with or without a loupe for wire or caustic test, or a grating for Ronchi testing. As a light source a piece of square aluminium tube is used, that houses a 10W halogen light bulb, a matte piece of glass and a slit. Other light sources, like a LED or a solid state laser, are also possible.

Below, the XY stage with a ToUcam 840 mounted behind the KE is shown, as it is used for PC assisted testing.

Tester with CCD, front view
With camera mounted, LED on.
Tester with CCD, rear view
Rear view, camera taken off.