Amateur telescope making

Lore & Background

Ever since Galileo Galilei adapted a Dutch invention for astronomical use, astronomical telescope making has been an evolving discipline. Many astronomers after the time of Galileo built their own telescopes out of necessity, but the advent of amateurs in the field building telescopes for their own enjoyment and education seems to have come into prominence in the 20th century. Before the advent of modern mass-produced telescopes, the price of even a modest instrument was often beyond the means of an aspiring amateur astronomer, making building your own the only economical method to obtain a suitable telescope for observing. Published works such as the 1920 book The Amateur's Telescope by Irish telescope maker Rev. W. F. A. Ellison piqued interest in the hobby.

In the United States in the early 1920s, articles in Popular Astronomy by Russell W. Porter and in Scientific American by Albert G. Ingalls featuring Porter and the Springfield Telescope Makers helped expand interest. Ingalls began a regular column for Scientific American on the subject, later compiled into three books titled Amateur Telescope Making Vol. 1–3. Between 1933 and 1990, Sky & Telescope magazine ran a regular column called 'Gleanings for ATMs' edited by Earle Brown, Robert E. Cox, and Roger Sinnott. The ready supply of surplus optical components after World War II and later Sputnik and the Space Race also greatly expanded the hobby.

Although the types of telescopes that amateurs build vary widely, the most popular design is the Newtonian reflector, described by Russell W. Porter as 'The Poor Man's Telescope'. It has the advantage of a simple design that allows for maximum size for minimum expense, employing a single front surface mirror as its objective. Typically a Newtonian telescope of 6 or 8 inches aperture is a standard starter project. Amateurs have constructed telescopes as large as 1 metre across, but usually small groups or astronomy clubs take on such projects.

Reader's Guide

Amateur telescope making is significant as a means for individuals to obtain an inexpensive or personally customized telescope, and as a research tool in astronomy. Instruments built by amateurs have been employed in planetary study, astrometry, photometry, comet and asteroid discovery. The hobby's legacy includes the development of elegant designs such as the Poncet Platform, Crayford focuser, and the Dobsonian telescope, which achieve functionality and stability without requiring precision machining. The difficulty of construction grows roughly as the square of the diameter of the objective, making a 6-to-8-inch Newtonian a good compromise size. The literature on the subject is extensive, with large sections devoted to fabrication of the primary mirror, including grinding, polishing, and figuring to a paraboloidal shape. Testing methods such as the Foucault knife-edge test and Ronchi test are simple to fabricate and widely used. The tradition of building telescopes has been sustained by published works and magazine columns, and the ready supply of surplus optical components after World War II and the Space Race greatly expanded the hobby.

Did You Know?

• The most popular telescope design among amateurs is the Newtonian reflector, called 'The Poor Man's Telescope' by Russell W. Porter.
• The Foucault knife-edge test, used to test mirror shape, can be made from a light bulb, tinfoil with a pinhole, and a razorblade.
• Historically, mirror coatings were silver, which corrode quickly and need replacing after a few months; since the 1950s, aluminum coatings applied by thin-film deposition have become common.
• Amateurs have constructed telescopes as large as 1 metre (39 in) across, often as club projects.

From Necessity to Passion: The Roots of the Hobby

Telescope construction has deep roots stretching back to Galileo Galilei, who repurposed a Dutch optical device for observing the heavens. In the centuries that followed, astronomers frequently crafted their own instruments out of sheer need. Yet the transformation of telescope building into a genuine leisure pursuit—one driven by curiosity, technical satisfaction, and the desire for a personalized instrument—really took hold in the twentieth century. Before affordable mass-produced optics became available, even a modest telescope was often financially out of reach for an aspiring stargazer, making self-assembly the only realistic path to a usable scope. A 1920 publication by Irish maker Rev. W. F. A. Ellison helped spark wider interest, and in the early 1920s, American writers Russell W. Porter and Albert G. Ingalls brought the craft to a broad readership through articles in Popular Astronomy and Scientific American. Ingalls went on to maintain a regular column that evolved into the well-known 'The Amateur Scientist' series and eventually compiled three volumes titled Amateur Telescope Making. The post–World War II flood of surplus optical parts, followed by the excitement of Sputnik and the Space Race, further fueled a community of enthusiasts who proudly called themselves 'telescope nuts.'

The Newtonian: A Design Built for the Many

Among the many optical configurations an amateur might attempt—refractors, Schmidt–Cassegrains, Maksutovs—the Newtonian reflector consistently stands as the most popular choice. Russell W. Porter aptly nicknamed it 'The Poor Man's Telescope,' a label that captures its core appeal: a straightforward architecture that delivers the largest possible aperture for the smallest possible budget. The practical advantage is significant. Because the Newtonian relies on a single front-surface mirror as its primary light-gathering element, the builder must grind and polish only one optical surface. By contrast, a Maksutov demands three such surfaces and a refractor or Schmidt–Cassegrain requires four. This simplicity makes the Newtonian the natural first project for newcomers. A six- or eight-inch (roughly 15 to 20 cm) aperture scope is the classic starting point, whether tackled as a group effort at a local astronomy club or pursued solo by someone following instructions from a book or plans shared online. The design's accessibility has kept it at the heart of the hobby for generations.

Grinding, Polishing, and Proving the Mirror

The heart of any Newtonian build is its primary mirror, and the literature devoted to this single component is vast. The process begins with a flat disk of plate glass or borosilicate glass such as Pyrex. Using a similarly sized glass tool, a progression of increasingly fine abrasives, and a polishing lap made from a natural tree-sap pitch, the builder works through countless random strokes. These strokes naturally drive the surface toward a sphere. Once sphericity is achieved, a deliberate change in stroke pattern sculpts the subtle paraboloidal figure that most telescopes require, though high-focal-ratio instruments can get away with a purely spherical mirror. Verifying the result is where the Foucault knife-edge test shines. At its simplest, the apparatus is a light bulb, a pinhole punched in tinfoil, and a razor blade. Positioned near the mirror's radius of curvature, the tester reveals the surface shape: a perfect sphere glows uniformly, a paraboloid shows a doughnut or lozenge pattern, and any local defects are dramatically magnified. More advanced builders might turn to a Ronchi grating test, a Gaviola caustic test for fast mirrors, or modern interferometric methods enabled by inexpensive lasers and webcams.

Coating the Mirror and Building a Community

Once the mirror's figure is verified, a reflective coating must be applied. In the early days, builders chemically deposited a thin layer of silver directly onto the glass. Silver offered superior reflectivity, but it corroded rapidly and had to be reapplied every few months. Beginning in the 1950s, the standard shifted to aluminum, applied through a vacuum thin-film deposition process typically performed by a specialist firm. In the vacuum chamber, electrically heated tungsten or nichrome coils vaporize aluminum; the atoms travel in straight lines, strike the mirror surface, cool, and adhere. Modern coatings add a protective transparent overlayer on top of the aluminum for durability. Beyond the bench, the hobby thrives on community. Groups like the Springfield Telescope Makers in the 1920s gave early builders a shared identity, and the nickname 'telescope nuts' stuck. For decades, Sky & Telescope ran a dedicated column called 'Gleanings for ATMs,' edited successively by Earle Brown, Robert E. Cox, and Roger Sinnott from 1933 to 1990, ensuring that practical knowledge kept flowing to a worldwide audience of self-taught opticians.