Test Jigs

Testing telescope mirrors requires measurement, and we do this with a test jig. The test jig needs to be able to travel and report its position accurately. Moving forward and back along the optical axis, or longitudinal axis, is the z-axis of the system, and we usually try to have at least 0.001” accuracy in positioning. I do this with a micrometer pushing on the jig stage with a spring pulling it back onto the mike securely.

A lateral, or transverse axis, allows the knife-edge and light to move into and out of the optical path and needs no measurement apparatus. However, one of the uses of the test jig is caustic testing, and for this we need 0.0001” measurement accuracy. So my test jig’s transverse, or x-axis, motion is pushed by a micrometer, and resisted by spring force.

The two axes need to be perpendicular for caustic testing, but does not require this perfection for Foucault testing. Since you may improve your skills eventually, you might as well have the jig made with perpendi9cular axes.


For bath interferometry, you will need a vertical axis, or y-axis. When using the bath interferometer, measurement is not performed on the micrometers, but they move the test jig delicately, so micrometers are good to use. You could just as easily use fine threaded bolts opposed by spring force since you are not measuring.

The jig should have a post on it so that you can attached a Ronchi screen, Foucault knife-edge, light source, and other devices with the ability to elevate and lower them. Below is a photograph of my test jig

Toward the mirror is to the left, the post is sticking up and at the top of it is where we attach devices, there are two z-axis micrometers, both are two inch travel, but one is restricted, but the two together offer me over 3” of measured travel. There is no y-axis micrometer on this at the time of the photo, but one is being attached the summer of 2013 for the purpose of interferometry. The original jig was made by Ted Young and Howard Thomas of the Spokane Astronomical Society, then altered by Ted, then finally handed down to me and I’ve altered it from there. Ted used it for Foucault testing, Howard for caustic testing, and me from everything. I have tested over 300 mirrors on this jig in its various forms.

Test jigs do not need to be this complex or finely made. A 1/4x20 bolt pushing against a spring-loaded floating plate of some sort will work. The end of the bolt can have a disk on it with 50 divisions equally spaced on it. 20 times 50 is 1000, so you could measure positions to 0.001”. A simple device atop it could move a razor in and out of the returning light path. Simple to complex, you can make it to your desires.

I have a Radio Shack box with an LED in it. I place a razor over the LED so a half-moon shines outward. It site on the post. As the light from the half moon goes to the mirror and reflects back, the straight part of the half moon returns on the straight part of the razor, and the light passing through the “slit” can be finely miniaturized thinly. When at the center of curvature, the shadow on the mirror will come in from the side of the razor if I’m inside the radius of curvature, and from the opposite side if I’m outside the radius of curvature. The mirror, if it were a sphere, would dim uniformly if I were right at the radius of curvature. There’s more to the story, you can check out the “figuring” page on this set of links.