On the Construction of Telescopes with Silvered Glass

by Leon Foucault
Translated by Guy Brandenburg
Copyright 2000
      This is the frontpage for an article that Leon Foucault did on the subject of making telescopes. I've divided the article into sections so that it will be a bit more readable on the web as a single 118K+ page is rather large. I have tried to divide the article by the various sections so that each may be viewed easily. If you plan on printing this article, I'd suggest pasting it all back together if desired (in any case, cut the text out of the web page and put into your wordprocessor so that the page can be nicely formatted).
       Guy Brandenburg translated the article so if you know where the original is and have read it, any transcription errors are not my responsibility, only the conversion from a single text file to these HTML pages.
      I might note that at some point in fhe future, the various drawings may be forthcoming from Guy.

Part 1Introduction
Part 2Optical Examination of Concave Surfaces
Three Different Procedures
Positive and Negative Aberration
Part 3Practical Details on the Shaping of Glass
Mirrors and Performing Local Re figuring
Part 4Definition of optical power
Determining its numerical value
Part 5Silvering Glass
Application to Telescope Mirrors
Part 6Construction details for large telescopes
Setup of eyepieces
Changing power
Mounting the mirror
New wooden equatorial mounting

      The interesting thing about this article is that Leon Foucault goes through three different tests for the quality of a mirror but never actually does what we now call the Foucault test! The other thing is that he does a more complex version of the Ronchi Test where he uses a square array to look at the surface rather than the lines we now use for that test. The version he does is the point source of light with the grating between the viewer and the mirror and this is a valid test as it is doing both directions at the same time as the source is a point rather than a line as is often done these days.
      The closest he comes to what we use as the Foucault Test is to do a null test of the mirror at varying amounts of ellipse (light source at one focus and the KE at the other focus point) rather than sitting down and figuring the amount of change of ROC for each of the zones. When you have access to a long tunnel, this test can show the errors of the figure when you're at the right point of the ellipse with the parabola always showing a bit of overcorrection. It might be noted here that the extreme elongation of the ellipse is often indistinguishable from that of a parabola at some point.
      It's also interesting that he's talking about 8" (20cm) F6 mirrors as a starting point for his work.

      Part 5, which shows how to do silvering on the glass, is replete with the old "artistic" names for some of the chemicals used to do the process along with some old names for equipment used for the process. It is an interesting reading for the history of mundane things in some regards.

      I'd like to thank Guy F. Brandenburg for his efforts in getting this article on the web. He did the translation of the work and got the images into computer form (when he gets them to me!) where they could be cleaned up and presented to us all.