Amateur Telescope Making

Lore & Background

The origins of the ATM books trace to 1923, when Ingalls saw an article by Russell Porter in Popular Astronomy about the Telescope Makers of Springfield, Vermont. Porter convinced Ingalls to use Scientific American to spread the telescope making hobby. The first article appeared in November 1925; after nearly a thousand reader letters, Ingalls and Porter collaborated on two more articles in February and March 1926. The first edition of ATM appeared in 1926, containing 102 pages and consisting of the two Porter articles plus extracts from a 1920 book by Rev. W. F. A. Ellison. The 820 copies of the first printing sold out by mid-1928. The books promoted a philosophy that eschewed standardization and rote rule-following, emphasizing intellectual involvement, creativity, and high optical and mechanical standards. In the preface to the 3rd edition (1932) of Volume 1, Ingalls wrote that telescope making 'exacts intelligence; requires patience and sometimes dogged persistence' and that 'a hobby should be a way to waste time, not to count it.' The later volumes contained chapters by James Gilbert Baker, George Ellery Hale, George Willis Ritchey, and others on topics from lens grinding to photoelectric photometry. Much information, including Porter's articles on the Springfield mount and articles on Schmidt cameras, appeared for the first and only time in these books.

Reader's Guide

The ATM books are widely credited with having initiated the amateur telescope making movement in the United States. They have remained in constant use by both amateurs and professionals due to the uniformly high caliber of contributions. The first volume was essentially a reprinting of articles from Ingalls's column 'The Backyard Astronomer' (later 'The Amateur Scientist'), with numerous drawings by Porter. The two later volumes were based largely on articles that first appeared in that column, plus some commissioned chapters. The books were republished in a reorganized form by Willmann-Bell in 1996, with chapters rearranged into more logical groupings, expanded tables of contents and indices, and the deletion of articles on pendulum clocks and photoelectric photometry. The final edition of Volume 1 in 1980 had 510 pages; Volume 2 reached its 17th printing in 1978, and Volume 3 its 10th printing in 1979.

Did You Know?

• The 820 copies of the first printing of Volume 1 sold out by mid-1928.
• The ATM books are often referred to collectively as the 'bible' of amateur telescope making.

From Necessity to Passion: The Roots of ATM Culture

Telescope building as a personal pursuit stretches back to Galileo Galilei, who adapted a Dutch optical device for astronomical observation. For generations, astronomers constructed their own instruments out of practical need, yet the transformation of telescope making into a recreational and educational pastime became prominent only in the twentieth century. Before modern mass production, the price of even a modest telescope was often beyond the reach of an aspiring stargazer, so building one's own was the sole economical route to a functional instrument. A cluster of publications catalyzed widespread interest. In 1920, Irish maker Rev. W. F. A. Ellison published The Amateur's Telescope, and in the early 1920s, Russell W. Porter and the Springfield Telescope Makers captured readers' imaginations through articles in Popular Astronomy and Scientific American. Albert G. Ingalls' recurring column in Scientific American evolved into the enduring "The Amateur Scientist" feature and was later gathered into three volumes called Amateur Telescope Making. From 1933 to 1990, Sky & Telescope magazine kept the community engaged with its "Gleanings for ATMs" column, edited by Earle Brown, Robert E. Cox, and Roger Sinnott. Surplus optical components from World War II and the Space Race era further accelerated the hobby's growth.

The Newtonian as the ATM's Workhorse Design

While amateur builders experiment with refractors, Schmidt-Cassegrains, and Maksutov designs, the Newtonian reflector remains the most widely constructed telescope in the hobby. Russell W. Porter aptly dubbed it "The Poor Man's Telescope," a label that captures its core appeal: a straightforward optical layout that delivers the largest possible aperture for the smallest possible cost. The design's practical advantage is structural simplicity. Because the Newtonian relies on a single front-surface mirror as its objective, the builder must grind and polish only one optical surface. By contrast, a Maksutov demands three surfaces and a refractor or Schmidt-Cassegrain requires four. This reduction in labor makes the Newtonian an ideal first project. A six- or eight-inch (15 or 20 cm) aperture Newtonian is the standard starter build, whether undertaken as a group effort at a telescope-making club or by an individual working through books or plans found online. The combination of low material cost, manageable optical work, and generous light-gathering power has kept the Newtonian at the heart of amateur telescope making for well over a century.

Grinding, Figuring, and Testing the Primary Mirror

Because the Newtonian dominates the hobby, much of the ATM literature is devoted to fabricating the primary mirror. The process starts with a flat disk of plate glass or borosilicate glass like Pyrex, which is ground, polished, and figured to an extremely accurate shape, typically a paraboloid. The tooling is remarkably humble: a similarly sized glass tool, a series of finer abrasives, and a polishing lap made from a type of tree sap. Through a whole series of random strokes the mirror naturally trends toward a sphere; the builder then varies the stroke pattern to sculpt and perfect the paraboloidal figure. To verify the result, most amateurs use a Foucault knife-edge test, which at its simplest requires a light bulb, a pinhole in tinfoil, and a razorblade. A perfectly spherical mirror appears evenly lit; a paraboloid shows a doughnut or lozenge pattern; and surface defects are dramatically magnified. Alternative methods include the Ronchi test, which swaps the knife edge for a fine wire grating, the Gaviola or Caustic test for fast mirrors, and modern interferometric approaches enabled by affordable lasers, webcams, and computers.

Coating the Mirror: From Fragile Silver to Durable Aluminum

Once the mirror surface achieves its correct figure, a thin layer of highly reflective material must be deposited on the front face. Historically, that material was silver, applied chemically by the mirror maker or the end user. Silver coatings offered higher reflectivity than aluminum but carried a critical drawback: they corroded quickly and required replacement after only a few months of use. Since the 1950s, the standard practice shifted to aluminizing, a thin-film deposition process typically performed by a specialized firm. In the aluminizing process, the mirror is placed inside a vacuum chamber alongside electrically heated tungsten or nichrome coils that evaporate aluminum. In the vacuum, the hot aluminum atoms travel in straight lines; when they strike the mirror surface, they cool and adhere. Modern coatings usually consist of an aluminum base layer overcoated with protective transparent compounds to guard against oxidation. This shift from a fragile, short-lived silver finish to a durable, professionally applied aluminum layer transformed mirror maintenance from a recurring chore into a one-time step, removing a significant barrier for amateur builders and extending the functional life of their instruments.