On the Construction of Telescopes with Silvered Glass
by Leon Foucault
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 word processor so that the page
can be nicely formatted).
Translated by Guy Brandenburg
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
I might note that at some point in fhe
future, the various drawings may be forthcoming from Guy.
||Optical Examination of Concave Surfaces
Three Different Procedures
Positive and Negative Aberration
||Practical Details on the Shaping of Glass
Mirrors and Performing Local Re figuring
||Definition of optical power
Determining its numerical value
Application to Telescope Mirrors
||Construction details for large telescopes
Setup of eyepieces
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
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.