Build Mirror Test Equipment

Building a mirror tester is not hard, and these pages will show you how to construct a mirror tester and test stand with simple hand tools and common hardware store items. Simple, basic "glue & screw" wood fabrication techniques are used.

With this tester, you will be able to figure your mirror to high accuracy. Below we will describe the requirements for each component of the test setup; other pages will provide construction details. Using and interpreting the test results is covered in another topic.

The Tester

There are several variations of testers. A basic tester needs to be able to move a knife edge or Ronchi screen smoothly and slowly along the optical axis of the mirror, and be able to measure the movement to about 0.001 inch (not as hard as it may seem!). Additionally, the knife edge must be able to move across the optical axis, but this motion does not have to be measured. A trick that keeps tester complexity down is to use a slight rotational motion to do this crossing motion which is much simpler to build than a second linear motion.

Originally, testers used a fixed light source and a moving knife edge, mainly because light sources were not compact nor cool (kerosene lamps in metal shields were initially used). Today, miniature electric lamps or Light Emitting Diodes (LED's) make it easy to mount the light source on the same stage that moves the knife edge, creating the moving source tester. It is helpful to keep separation of the light source and knife edge small, and moving source testers make this easy.

Early Foucault testers used a pin hole or slit in front of the light source, and a separate knife edge. It is difficult to make an accurate, small pin hole. It also can be difficult to make a small, parallel slit, and to make sure the slit is parallel to the knife edge. An excellent innovation has been the slit-less tester, where a single knife edge is used in front of the light source and is also used to cut the returned rays from the mirror - parallelism is automatic and there are no pin holes or slits to make.

To make a Ronchi tester, you remove the knife edge and cover both the light source and viewing area with a Ronchi grating - a fine grid of parallel lines, between 50 and 150 lines per inch. These gratings can be purchased on plastic film inexpensively, or on a precision glass substrate for a little more money. You can also use a computer drawing program and printer to print your own Ronchi grating onto overhead transparency film.

Many testers are not very beautiful, but they work very well Concentrate on smooth motion, rigidity and measurement accuracy - beauty should be your last consideration.

The Mirror Test Stand

The mirror test stand must be able to support the mirror vertically. The mirror will need to be aligned with the tester, which may require adjusting the overall height of the mirror with respect to the tester, adjusting the vertical tip of the mirror and adjusting the yaw, or side-to-side angle with respect to the tester.

For a basic test setup, the height adjustment is handled by building the test stand and tester to compatible heights, with final height adjustments done by placing stacks of paper or thin wood blocks under the lower object. The yaw adjustment is made by physically turning the whole test stand with respect to the tester. The tip adjustment is made by using an adjustable leg, that screws up and down to change the tilt of the entire test stand.

Mirror Test Stand Construction Plans

The Couder Mask or Pin Stick

In order to do final figuring of your mirror with the Foucault test, you need to be able to identify selected zones of your mirror. Zones are nested doughnut shaped rings with certain radii; the central zone is a circle (the "hole" of the innermost doughnut). A Couder mask is a paper or cardboard mask that is placed on the mirror, with holes cut out to expose selected zones. A pin stick is just what it sounds like: a small piece of softwood that is hung by a wire across the diameter of the mirror; straight pins are pushed into the wood to delineate the zone edges and centers. Which one you use is your personal preference.

Make a Couder Mask or Pin Stick
and
Calculate Mirror Zone Radii

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