-- welcome to
The Spot Test
By John Sherman
The Spot Test is an improved way to test telescope mirrors, complete telescopes, and other optical equipment. A century and a half ago Jean Bernard Léon Foucault showed the world a way to see errors measuring into the millionths of an inch. In his time other people had already been examining telescope mirrors from their radius of curvature (roc) with a pinhole and an eyepiece. (A telescope mirror's reflecting surface is curved, right? So if you think of that curve as part of a sphere, the radius of curvature is the center point of the sphere.)
In the photograph on the left, the shadow of the spot is not exactly in the center, and there is a disturbance in the diffraction pattern on its right. If I adjust the setup in order to center the spot, then the dark ring shifts over and is not centered. This persistence is a demonstration of the astigmatism caused by the small lateral separation between the source and the spot of an inch or so. A straight-edge device is not likely to show you that.
Most of the many versions of the Foucault test can be done in two-dimensions. This mini website will briefly describe some of them. Presented are my findings, some or all of which may be in error. After all, I am just a beginner at all of this. Please do not think that I am an expert. There are no formulas, math or rigorous analysis here. Most of that stuff has already been figured out in relation to the 1D tests. Those familiar with mirror testing should not have a problem understanding how to do it in 2D.
The essence of the Spot test is to illuminate the mirror with a round pinhole, and examine its image with a round spot on a piece of glass. The setup is the same as in the diagram above. A spot has an edge pointing in every direction, and functions as a 360° "knife" edge. The Spot test is not actually a new test, it is just a way to convert the existing tests into two dimensions. You still do the same test you've been doing, except now you have your eye wide open.
Happy telescope making!!
Page 2 shows testing methods at the radius of curvature
Page 3 are methods inside or outside of the radius of curvature
Page 4 deals with astigmatism in a spheroid mirror
Page 5 deals with astigmatism in a paraboloid
Page 6 gives my ideas for a robo-tester
Page 7 is about measuring zones
copyright ©2002 - 2004 by John Sherman