Amateur Telescope making

Making a Slitless Foucault Tester


I assume that the reader is familiar with the basic procedure of the Foucault test. If not, I recommend that you read the chapters in Texereaux's, Berry's or any other ATM book dealing with it.
Since this is the first Foucault tester I built, there are of course many things I will do different next time. I also do not have direct comparison to a normal slit-type tester as it is described in most of the ATM books. I only intend to give one example of many possible designs.


Explanation Using a slit instead of a pinhole for the knife edge test has several advantages. One of them is better illumination of the mirror in comparison to the pinhole. But the slit-test requires the knife edge to be exactly parallel with the slit. This can be difficult to achieve.

The slitless tester overcomes this problem. Instead of producing a very fine slit with two razor blades, a round light source (super bright LED for example) is half covered by a razor blade or utility knife. The half-moon shaped light is reversed by the mirror and thus produces in combination with the razor blade a slit of variable width.

Tester Explanation If we arrange in the setup for a sight hole above the light source that is half covered by the same razor blade that covers the light, we eliminate the need for a separate knife edge and by that the problem of getting the knife edge parallel with the slit. Moving the tester sideward until the mirrored half-moon is almost covered by the razor blade, we can produce a very small slit. Moving the tester even further sideward we simulate a knife edge penetrating the light-slit. Since the virtual knife edge is at the same time the object that cuts the light source, it will be automatically parallel to the slit.
Tester LED By changing the distance from the light source to the mirror with the micrometer screw we are able to measure the longitudinal difference between the focal points of 4 or 5 predefined mirror radii. That is basically what the knife edge test is all about.
The data reduction is done in the normal way. We only have to keep in mind that with our slitless tester the knife edge movement will be 1/2 since we move it with the light source.
(Click on the small icon to see larger image)

Description of my slitless Foucault tester

The carriage:
Tester sideview The carriage is made of 10mm plywood and consists of two parts.
The moving part is spring loaded and rides on 4 small wheels on the stationary part of the carriage. To move it forward or backward the micrometer screw is turned. The carriage is made high enough to allow the head to fit behind the sight hole. On the photo you can see a small sideboard which carries the potentiometer and the switch for the light source.
(Click on the small icon to see larger image)
Tester front view The stationary part carries the micrometer and has two wooden guide rails for the moving part.
(Click on the small icon to see larger image)
focuser sideview I would not use the small wheels again to move the carriage. They don't move smoothly. I'd rather use the type of aluminum rails which I used for the focuser (see the side view of the focuser as example).
(Click on the small icon to see larger image)
The micrometer screw:
Tester micrometer Micrometer dial To avoid the investment in an expensive micrometer screw, I made use of the fact that a 6mm metric screw travels exactly 1mm per complete turn. As you can see on the photograph, a wood disk with a circular scale from 1 to 100 is fixed to the end of the screw. Below that is a linear scale in mm.
(Click on the small icon to see larger image)
Micrometer scale One full turning of the screw is equivalent to a forward or backward travel of 1mm on the linear scale. The intermediate values can be read on the circular scale.
The screw pushes the spring loaded carriage and travels in two nuts that are glued in a piece of plywood.
(Click on the small icon to see larger image)

Instead of using two normal nuts, I would use now one 6mm T-nut. When I built the tester, I did not know, where to buy them.

The light source
Tester LED Many people recommend a super bright green LED . as light source for that type of Foucault tester. I used one and was very happy with it. I could imagine that you might need something stronger for a bigger mirror with longer focal length. However, for my 6" F7.16 it was bright enough.
The knife edge is one side of a razor blade. I covered the other side with a small piece of wood to avoid cutting myself in the dark. As you can see on the photo, there is very little distance between the sight hole and the light source.
(Click on the small icon to see larger image)
Tester detail Here is also a close-up of the arrangement of the batteries and the potentiometer. The "Poti" is useful for dimming the LED if necessary.
(Click on the small icon to see larger image)


Testing with a slitless tester works basically the same way as it works with a normal tester. For a general description of the testing procedure, please refer to the common ATM literature.
It is generally difficult if not impossible to get perfect alignment of the mirror and the tester, which means that you will have to shift the tester sidewise often on to maintain the same slit size. One trick to avoid this is to put pressure with your hand on the support where your tester is placed. Unless your support is a heavy table this pressure will make the tester tilt a little bit which provokes the same effect as shifting the carriage sidewise.

Be patient and critical! A self made micrometer will surely introduce some inaccuracy for which you have to account with at least 3-4 reading sequences in each test set.
Another thing you have to avoid is to test in both directions at the same time. You should in each test-sequence either turn the micrometer clockwise or anticlockwise, which means to test either from zone 1 to 4 or from zone 4 to 1. Do not mix both movements.
Try NOT to look at the results of the previous test sequences as you naturally will try to match previous results. You should have independent readings in each set which will be averaged to give a realistic result.

Related pages

A great page about foucault testers!

Making a Ronchi tester

This page has been validated with Valid XHTML 1.0!

Sign Guestbook
View Guestbook

Back to Homepage

© 1996 Contact Berthold Hamburger