Making and using Testplates

I have mentioned these often before, but without an explanation as to what they are and how you use them. It lies beyond the scope of this webpage to explore their many uses in detail. Interested readers can find extensive discussions of testplating in H. Karrow, "Fabrication Methods for Precision Optics," (Wiley and Son, ), pp., a goldmine of information on making fine optics; and Malacara, "Optical Shop Testing,".. Additional information of a more basic nature can be found Texerau, chapters 3-3 ("Interference Test for Flat Mirrors"), 3-4 ("Making the Interference Test"), and 8-4 ("Testing the Secondary Against a Concave Reference"); and in T. Waineo, "Making and Testing Optical Flats," in "Amateur Telescope Making Journal," vol. 8, pp. 4-8.

A testplate is usually an optic with a concave surface that is used to test another optic with a convex surface. The major exception to this rule is that flat mirrors under fabrication are often tested by means of other "master" flats whose surface quality it without question. Convex surfaces cannot be tested directly by means of a knife-edge, Ronchi, or some more advanced test, because they do not converge light. Instead, one fabricates a concave surface equal in radius of curvature to the convex surface one wishes to test, and then after testing this concave carefully by one of the above methods to guarantee its radius and smooth sphericity, one places it in contact with the convex surface and examines it in monochromatic light, for example, that produced by a low pressure sodium (LPS) vapor lamp. Dark and light bands will appear caused by the interference of light reflected from both contacted surfaces. By analyzing the shape of the bands, called "fringes," one can deduce the nature of the curve on the convex piece, whether for example it is spherical or conic (elliptical, parabolic, or hyperbolic), whether its radius of curvature is shorter or longer than the concave piece and by how much, etc.

Testplates can be extremely useful in optical fabrication and are extensively employed in industry. But for the patient amateur they can be useful too. And if you lack an autcollimation flat and the ability or equipment to make one, making and using testplates can be a way to test the convex surfaces of refractor lenses and to assure that they are in fact spherical and of a particular radius of curvature. Making the testplate is just like making a small telescope mirror, but not as hard. You don't have to polish it out completely and you don't have to parabolize it. In fact it doesn't even need to be rigorously spherical, just close to spherical in order to be useful.

Alas, there is no good single, convenient source of information for ATMs concerning testplating. The most extensive discussions that I know of are in the Karrow and Malacara books mentioned above. But both Texerau and the Waineo article can give you the basics.

For an amateur wanting to make a refractor but lacking a suitable flat, probably the SF15 lens mentioned above in Appendix I is the most practical proposition. Two testplates are needed, one for r1/r2 and another for r4. R3 can be tested directly by knife-edge. The testplates can either consist of 6" disks of Pyrex or two 4.25" disks. It will be challenging but doable to fabricate them to within +/-1mm of the nominal radius for r1/r2, and about +/-5mm for r4. To do this right, you would first want to somehow get the radii on the testplate close to the lens radii and then repeatedly tweak the testplate radii by grinding with 9micron and then putting a slight ("flash") polish on the surface, enough to make it visible by knife-edge or Ronchi, and then measuring the radius with a good tape measure. Once you have the correct radii on your test plates, you would need to polish out their front and back sides enough so that you can see clearly through them. The figure on the back doesn't matter at all. You just need to see through the glass. The front, concave radii need to be good spheres. Turned edges and narrow zones don't matter, but having a conic surface can't be allowed. All the errors in your testplates will be superimposed on your lens when you test it, and it will be left to you to figure out which error is where!

I would urge you to read several of the above sources and do additional research on testplating, before you undertake this project. In particular you need to be aware of the general shapes that the fringes are likely to take, and what those shapes tell you about your lens. It is beyond the scope of this page to discuss that. But I wanted to alert interested, energetic ATMs to the possibility of testing concave surfaces by means of testplates.