Before
1920
GOTO RONCHI INDEX
Copyright – P. J.
Smith
But permission is given
to distribute this material in unaltered form as long as it is not sold for
profit.
Pre Ronchi Techniques
Strictly speaking, the first
Ronchi test was performed in the early 1920’s, but the state
of the art is an important
backdrop to its development. The
following attempts
to give a summary of major
tests developed beforehand.
1738. Compleat
System of Opticks, published at Cambridge, England, in 1738. Robert Smith
gave instructions for speculum making that were destined to be the beginner's
guide for Herschel the amateur 35 years later. To find the center of curvature
of a speculum Smith's method was to set it on edge opposite a candle. Selecting
a tiny pinhole near the edge of the tin, he shifted candle and tin until he
could simultaneously focus in the eyepiece the edge of the tin and the image of
the pinhole reflected from the speculum. How he then tested the speculum is
described in his book. . Reported by Ingalls,
Scientific American, Oct 1949.
1773. Constance A.
Lubbock, in The Herschel Chronicle tells how Herschel uses Zonal testing
with masks. Reported by Ingalls,
Scientific American, Oct 1949.
1777. How Mudge used
masks was described in the same periodical, Volume 67, page 335, in a paper
that Mudge delivered in March, 1777. He placed a separating mask having one
eighth the diameter of the mirror opposite a zone midway between the center and
the edge, and tested first the inner zone and then the outer one both for
definition and for coincidence of focus. If the two images were equally sharp
and of equal focus "the speculum," he said, "is perfect and of
true parabolic curve." . Reported
by Ingalls, Scientific American, Oct 1949.
1840. Lord Oxmanton,
the third Earl of Rosse, born William Parsons. Philosophical Transactions of the Royal Society of London,
Volume 130, Part 2 (1840). Methods
of testing telescope mirrors. He describes his method of testing as used on a
36-inch speculum. Describes how the dial plate of a watch is suspended from a
high tower, face downward. At the bottom of the same tower is the speculum,
face up on its machine. Lord Rosse used masks "as Mudge did." Reported by Ingalls,
Scientific American, Oct 1949.
1859, Reported in Sky and Telescope, No 50. of a reprint of an atrticle by R. S. Luce published in the Royal Astronomical Society publication 1958 – 1959. It purports to be a translated reprint of Foucaults original article. Even if the translation is not quite accurate, this is the publication through which most English speakers would have been introduced to the Foucault test so is quite historic. It says that Leon Foucault made use of three methods.
The first consisted of imaging a pinhead. Problems were best diagnosed by examining the inside and outside of focus rays with an eyepiece. This parallels the star test we all know and which was used by others even before Foucault.
The second is rather vague but it consisted of imaging a small object having parallel sides illuminated from behind. If the curvature of the mirror is not uniform, the object is not imaged in the same plane and the parallel sides will be deformed and the images of the edges are deformed in the shape of a hyperbola. This has a lot of the elements of a Ronchi test.
The third method is the familiar Foucault test we all know. Unfortunately, old descriptions usually lack diagrams so we will never really know the exact set-up of method 2.
1859. Reported Sky & Telescope, May, 1982. and July 1975. Leon Foucault never used zonal mask testing. He preferred null tests.
1859. REPORTED by Ingalls, Scientific American, Oct 1949. Leon Foucault’s described the
"Foucault test".
1861. REPORTED by Ingalls, Scientific American, Oct 1949. William Herschel's son Sir John, in his Encyclopaedia Britannica article on the telescope, reprinted as a book, ‘The Telescope’, in 1861, briefly outlines Foucault's "peculiar method." But he prefers the diffraction-ring test, supplemented by his father's test of matching-three zones for focus on the stars, and the watch-dial test of Lord Rosse.
1862. Reported by Twyman. “Prism and Lens Testing”. Fizeau described the Fizeau Interferometer – usually used to test flat plates. It can, however, be used on curved surfaces. It is really a modification of a system using Newton’s fringes across a larger air gap.
1883. Reported by Twyman. “Prism and Lens Testing”. Laurent modifies the Fizeau Interferometer to make it more suited to workshop use. Production models were described by Schulz (1912 and 1914). The 1914 model was made by Goertz. Other Optics firms made instruments like these in house (eg., Hilger).
1880’s or earlier. Reported by Twyman, “Prism and Lens Testing” Fraunhofer introduced the practice of using test plates which make use of Newton’s rings. He kept the process secret.
1887. REPORTED by Ingalls, Scientific American, Oct 1949. With, (‘With’ is the name of a man) the English professional, wrote the following about a mirror: "Among my choicest of the choicest, I find one recorded thus: '8lh focus 5 feet 3 inches. Absolute Perfection; Not for Sale.'" F. J. Hargreaves, Britain's foremost optician, who was once an amateur, states in the Journal of the British Astronomical Association that With had no knowledge of Foucault's knife-edge test. Hargreaves found in 1941 that this mirror "gave images as nearly perfect as any I have ever seen, even with a magnification of 500 diameters." He tested it by the Foucault method and found that it "showed no imperfection, apart from a narrow turned-down edge."
1887. Reported by Twyman. Reported by Twyman, “Prism and Lens Testing”. Voit gave an account of Fraunhofer’s various methods including the use of test plates.
1920. R. V. Occhialini. Riv. Ottica. meccan. precis. 1, 99 (1920)
1891. Dennis Taylor.
“The adjustment and testing of Telescope Objectives.” Describes in great detail how to use the
Star Test to diagnose problems. This is
an excellent account of what had been used by experts well before the publication
date.
1921. Dennis Taylor (for T Cooke and Sons). A reprint of the book “The adjustment and testing of Telescope
Objectives.” The
copyright was owned by Grubb Parsons.
It is significant that this has recently been reprinted yet again. Dick Suiter’s book is an up to date coverage
of the same topic.
1866, 1867.
Reported by Twyman, “Prism and Lens Making”. Toepler modified the
Foucault test. It was often then called
the Schlieren test.
1900, 1904.
Reported by Twyman, “Prism and Lens Making.” Hartmann developed his test. This was a very powerful but painstaking
test involving taking a series of photographs at different positions with on
objective mask in place.
1916, 1918, 1823.
Reported by Twyman.
Twyman and Green developed testing methods based on the Michelson
Interferometer.
It is obvious that
there were various precision ways to evaluate optics before Ronchi.
What the Ronchi
test held out was the promise of a very simple and cheap test which
would make special
and often patented gear unnecessary.
GOTO RONCHI INDEX