The Testing of Telescope Optics in Historic Times By Peter Abrahams Presented to the 1994 Convention of the Antique Telescope Society. Telescope making techniques have evolved since Galileo but the problems that he faced remain to challenge telescope makers of today. The literature on historical telescopes includes a meager supply of information on how they were made. This article will briefly discuss both the techniques used by early makers and the testing of antique instruments using modern optical techniques. There were many techniques used to test early telescope optics. In The Historv of the Telescope, Henry King notes that the telescope makers of the seventeenth century observed a page of printed type set at a distance. This technique is still useful for a quick indoor test in a close-focusing instrument. In the 1600s, the periodical Philosophical Transactions were favored for this test because many other books were so poorly printed that the aberrations seen might be from the print and not from the lens. Star tests were also used, but their full meaning was not clear. Even the eminent Johannes Hevelius thought that the spurious discs produced by stars in his 150 foot telescope were measurable stellar diameters, and he used a micrometer to quantify his instrumental error. There are in fact many varieties of false test results, including simple mismeasurement, erroneous interpretation, misapplied test procedures, inadequate test equipment, and the famous 1990 Space Telescope ‘failure to test as an assembled instrument’. John Mudge was a mid eighteenth century English amateur telescope maker who might have been the first to use zone tests on mirrors. He observed a distant object, using high power, masking the edge and then the center of his speculum. If the object stayed in focus at all positions of the mask, the mirror was judged properly corrected. Also in England during the 1700s, the Reverend John Edwards tested his Gregorians by observing a half inch black ring drawn on a card, from a distance of around 200 feet. By racking in and out of focus and comparing the defocused images on either side, the primary could be judged over-corrected or under-corrected. If the blurring was equal on both sides, the mirror was considered parabolic. Most early reflectors were f10-f12 or more and did not require parabolizing, the exceptions being mainly tabletop telescopes, the beautiful (and scarce) handheld Gregorians, and enormous instruments such as those by Herschel or Rosse. John Hadley was a more professional English telescope maker of the 1700s who tested mirrors with a lighted pin hole at the focus and an adjacent eyepiece to inspect the image. If the out of focus image was any shape other than a circle, then the various diameters of the speculum (at one o'clock, five o'clock, etc.,) were judged to have different curvatures. Comparing the two images on either side of focus shows the degree of parabolic correction. Henry King quotes Hadley at length, from Robert Smith's A Compleat System of Opticks. This comprehensive 500 page compilation from 1738 includes much material on telescope making. The Earl of Rosse was a notorious tinkerer and gadgeteer who used Mudge’s zone test on his enormous mirrors. Even before the overall figure is true, determining the focus of areas of the mirror will show where further work is needed. One of his final tests was to view a watch dial from 50 feet. Rosse also made speculum flats, and tested them by observing with a telescope the reflection of a distant object in the flat. Considering how difficult it is to make an accurate flat, the inaccuracy of this test was probably appropriate. Another chapter in the false test saga could be, inadequate tests applied to inaccurate optics for results that go beyond false. In contrast to Rosse’s achievements is the inspired career or Leon Foucault, his French contemporary. One of Foucault's many accomplishments was the publication, in 1859, of his development of the older pinhole technique into the Foucault test. Viewing the reflected pinhole across a straight knife edge causes a pattern of shadows that clearly shows the figure of the mirror. This simple improvement granted an enormous increase in the perfection of telescope optics. Foucault went on to analyze the patterns made by mirrors as they are polished from spherical to parabolic form. He also experimented with moving the light towards the mirror and the straight edge away from it, and with the various ellipsoidal mirror forms that can be made in this way. In his fifty year lifetime, Foucault also devised the famous Foucault pendulum that was the first experimental demonstration of the rotation of the earth. He was also responsible for an accurate determination of the velocity of light, measured the distance to the sun with improved precision, and pioneered the silver on glass telescope mirror. In 1864, Henry Draper published an authoritative guide to the Foucault test, in his book, On the Construction of a Silvered Glass Telescope. This was very popular throughout the remainder of the nineteenth century, and in 1904 was reprinted with additional material by George Ritchey, a true master of the art of optical testing. Ritchey also relied on the Foucault test, and developed an elaborate mask with small arcs cut out of it at different radii. He made many hyperboloidal secondaries for Cassegrain configurations and tested these against the intended primary mirror with a full size flat in an autocollimation test. One of Ritchey’s colleagues at Yerkes was Frank Wadsworth, who in 1902 published a pioneering article on the caustic test. The shorter focal ratios of modern telescopes use deeper parabolas that are very difficult to test with a Foucault tester. The caustic test uses a mask with holes cut in it to reduce the area under test and is capable of great accuracy. The testing of refractor objectives is not as well documented in the historical literature. Many makers had a pragmatic, hands-on approach that was not easy to describe, and many were very reticent concerning their work. The highly secretive telescope makers of yore, laboring in their closed-door shops, allowing employees to work only in selected stages of the process, and keeping no written records of their techniques, could make a fascinating chapter in this history; with an appendix on the cloak and dagger techniques of their spying competitors. Lens testing was very complicated because the low quality of glass available to craftsmen meant that the final lens curvature was very different than the mathematically derived figure. Testing was followed by local correction of the lens and more testing. Alvan Clark and Sons used a pinhole test and many hours of labor to test and correct their objectives. With the advent of astronomical photography, lenses had to be tested at the blue wavelengths that film was most sensitive to, and the human eye is least sensitive to this color range. The Clarks used a spectroscope for testing photographic lenses, and a corrected lens showed that blue light was brought to the same focus as the other colors. John Byrne, an apprentice of Henry Fitz (a self-taught apprentice due to Fitz's secret ways,) suggested in a catalog, circa 1880, that astronomers themselves test their refractors using a star test. He continues, if a star under test is red on one side and green on the other, the lenses are not on center with each other. An objective with low chromatic aberration will be fringed with reddish blue inside focus and yellow green outside focus. Spherical aberration can be star tested by examining the out of focus discs for even roundness inside and outside of focus. Collimation of lens elements can be checked by removing the eyepiece and viewing through the objective at a candle held in front of the tailpiece. The four small reflected images of the flame should be centered in the actual flame. In a final note from the 19th century, John Brashear was unique among telescope makers in publishing details of his testing procedures, which necessitates a separate article on his techniques. There are many interesting topics in modern telescope testing. The Ronchi test was published in 1922 and uses a grating to form an image of the mirror's surface as a series of lines. The Hartmann test was the standard for large observatory instruments earlier in this century. A mask with holes cut in it is placed over the mirror, and photos are taken from prime focus. By shifting the mask and taking pictures in front of focus and behind focus, a very accurate picture of the mirror's surface can be deduced. The two hundred inch at Mount Palomar went through years of Hartmann testing as engineers first hoped it would sag under its own weight into a proper figure, then refigured it, and finally made a corrector lens for it. The Space Telescope is a paradigm of how to test and how not to test. The most accurate objective does not a telescope make. Since historical aspects of telescope making are the theme of this article, it should be noted that one thing HST did right was in realizing the historical importance of the project and including, from the beginning, an organization devoted to recording and documenting the program. In 1982, the Space Telescope History Project was founded, and in 1989, Robert Smith's book, The Space Telescope, was its first product. The testing of antique optics using modem techniques is a subject that could fill a book. Polarized light was used by nineteenth century makers to check their optical glass and the finished lens. Inhomogenous glass would usually reveal streaks or splotches where the molten glass was not thoroughly mixed. An antique lens can be placed between crossed polarizers and will frequently reveal imperfections. Autocollimation tests are performed on assembled telescopes and also reveal inhomogenous glass; by placing a flat in front of the objective and a Foucault knife edge at the focus, inhomogenous glass will show areas of light and dark corresponding to high and low density of the glass in the lens. This test is recommended by several optical engineers as the most sensitive test of glass quality in antique retractors. The Foucault test can be used with filtered, monochromatic light to show aberrations at each wavelength. Spherical aberration in particular varies with wavelength, a phenomenon known as spherochromatism. Finally, interferometers are of great use in analyzing the surface quality of old lenses and mirrors. Interferometric testing of reflectors can reveal all types of aberrations. Since most interferometers use monochromatic laser light, chromatic aberration in a lens will not be shown unless several different colored lasers are used. Coma, spherical aberration, and astigmatism can all be seen as variations in the parallel lines seen on an interferogram. Many antique lenses have been tested in this way, including several of Galileo's lenses, as reported in the July 9, 1992 issue of Nature. Two eyepieces and two objectives were examined. One eyepiece had a flat side polished to a fraction of a wavelength of light, a virtuoso feat that was not required to maintain image quality. One objective was considered nearly diffraction-limited. One eyepiece showed a ring shaped pattern that might indicate that it was made on a lathe, and it should be noted that here we have optical testing used as a tool for historical investigation of early technology. The chromatic aberration that was not revealed by this test is the imperfection that the lenses suffer from, and that could not be reduced until the doublet lens was developed. At single wavelengths, Galileo's telescopes are of very high quality. There are many other fascinating aspects of the history of optical testing. The Reverend W.R. Dawes derived his Dawes limit for the theoretical resolution of a point source, and gave Alvan Clark an important career boost by publicizing the quality of his telescopes. George Airy realized the importance of the rings of light that surround a star when viewed through a telescope, and the Airy discs are an important aspect of evaluating an instrument. Fraunhofer used monochromatic sodium light in optical testing, a major advance in the field. However, if the literature on historical telescope making is sparse, the available references on the testing procedures of early makers is extremely so. Henry King, a notable exception, included a number of details on the subject of optical testing in his history of the telescope.