Amateur telescope making is the activity of building telescopes as a hobby, as opposed to being a paid professional. Amateur telescope makers (sometimes called ATMs) build their instruments for personal enjoyment of a technical challenge, as a way to obtain an inexpensive or personally customized telescope, or as a research tool in the field of astronomy. Amateur telescope makers are usually a sub-group in the field of amateur astronomy.
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. Building your own was the only economical method to obtain a suitable telescope for observing. Many published works piqued interest in building telescopes, such as the 1920 book The Amateur's Telescope by Irish telescope maker Rev. W. F. A. Ellison.
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 [1] helped expand interest in the hobby. There was so much public interest, Ingalls began a regular column for Scientific American on the subject (spawning that publications "The Amateur Scientist" column) and later compiled into three books titled Amateur Telescope Making Vol. 1–3. These had a large readership of enthusiast (sometimes called "telescope nuts" [2] ) constructing their own instruments. 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, including Refractors, Schmidt–Cassegrains and Maksutovs, the most popular telescope design is the Newtonian reflector, [3] described by Russell W. Porter as "The Poor Man's Telescope". The Newtonian has the advantage of being a simple design that allows for maximum size for the minimum expense. And since the design employs a single front surface mirror as its objective it only has one surface that has to be ground and polished, as opposed to three for the Maksutov and four for the refractor and the Schmidt–Cassegrain. Typically a Newtonian telescope of 6 or 8 inches (15 or 20 cm) aperture is a standard starter project, constructed as a club project or by individuals working from books or from plans found on the Internet.
Since the Newtonian reflector is the most common telescope built by amateur telescope makers, large sections of the literature on the subject are devoted to fabrication of the primary mirror. The mirrors start as a flat disk of glass, typically plate glass or borosilicate glass (Pyrex). [4] The disk is carefully ground, polished and figured to an extremely accurate shape, usually a paraboloid. Telescopes with high focal ratios may use spherical mirrors since the difference in the two shapes is insignificant at those ratios. The tools used to achieve this shape can be simple, consisting of a similarly sized glass tool, a series of finer abrasives, and a polishing pitch lap made from a type of tree sap. Through a whole series of random strokes the mirror naturally tends to become spherical in shape. At that point, a variation in polishing strokes is typically used to create and perfect the desired paraboloidal shape.
The equipment most amateurs use to test the shape of the mirrors, a Foucault knife-edge test, is, like the tools used to create the surface, simple to fabricate. At its most basic it consists of a light bulb, a piece of tinfoil with a pinhole in it, and a razorblade.
After the mirror is polished out it is placed vertically in a stand. The Foucault tester is set up at a distance close to the mirror's radius of curvature. The tester is adjusted so that the returning beam from the pinhole light source is interrupted by the knife edge. Viewing the mirror from behind the knife edge shows a pattern on the mirror surface. If the mirror surface is part of a perfect sphere, the mirror appears evenly lighted across the entire surface. If the mirror is spherical but with defects such as bumps or depressions, the defects appear greatly magnified in height. If the surface is paraboloidal, the mirror looks like a doughnut or lozenge. It is possible to calculate how closely the mirror surface resembles a perfect paraboloid by placing a special mask over the mirror and taking a series of measurements with the tester. This data is then reduced and graphed against an ideal parabolic curve.
Some amateur telescope makers use a similar test called a Ronchi test that replaces the knife edge with a grating comprising several fine parallel wires or an etching on a glass plate. Other tests used include the Gaviola or Caustic test which can measure mirrors of fast f/ratio more accurately, and home-brew Interferometric testing made possible in recent years by affordable lasers, digital cameras (such as webcams), and computers.
Once the mirror surface has the correct shape a very thin coating of a highly reflective material is added to the front surface.
Historically this coating was silver. Silvering was put on the mirror chemically, typically by the mirror maker or user. Silver coatings have higher reflectivity than aluminum but corrode quickly and need replacing after a few months.
Since the 1950s most mirror makers have an aluminum coating applied by a thin-film deposition process (work is done by a firm specializing in the process). Modern coatings usually consist of an aluminum layer overcoated with protective transparent compounds.
The mirror is aluminized by placing it in a vacuum chamber with electrically heated tungsten or nichrome coils that can evaporate aluminum. [5] In a vacuum, the hot aluminum atoms travel in straight lines. When they hit the surface of the mirror, they cool and stick. Some mirror coating shops then evaporate a layer of quartz onto the mirror, whereas others expose it to pure oxygen or air in an oven so that the mirror will form a tough, clear layer of aluminum oxide.
The telescopes amateur telescope makers build range from backyard variety to sophisticated instruments that make meaningful contributions to the field of astronomy. Instruments built by amateurs have been employed in planetary study, astrometry, photometry, comet and asteroid discovery to name just a few. Even the “hobbyist” end of the field can break down into several distinct categories such as: observing deep sky objects, observing the planets, solar observing, lunar observation, and astrophotography of all those classes of objects. Therefore, the design, size, and construction of the telescopes vary as well. Some amateur telescope makers build instruments that, while looking crude, are wholly suited to the purpose they are designed for. Others may strive for a more aesthetic look with high levels of mechanical “finish”. Since some amateur telescope makers do not have access to high-precision machining equipment, many elegant designs such as the Poncet Platform, Crayford focuser, and the Dobsonian telescope have evolved, which achieve functionality and stability without requiring precision machining.
The difficulty of construction is another factor in an amateur's choice of project. For a given design the difficulty of construction grows roughly as the square [ citation needed ] of the diameter of the objective. For example, a Newtonian telescope of 4 inches (100 mm) aperture is a moderately easy science fair project. A 6-to-8-inch (150 to 200 mm) Newtonian is considered a good compromise size since construction is not difficult and results in an instrument that would be expensive to purchase commercially. A 12-to-16-inch (300 to 410 mm) reflecting telescope is difficult, but still within the ability of the average amateur who has had experience building smaller instruments. Amateurs have constructed telescopes as large as 1 metre (39 in) across, but usually small groups or astronomy clubs take on such projects.
The history of the telescope can be traced to before the invention of the earliest known telescope, which appeared in 1608 in the Netherlands, when a patent was submitted by Hans Lippershey, an eyeglass maker. Although Lippershey did not receive his patent, news of the invention soon spread across Europe. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo improved on this design the following year and applied it to astronomy. In 1611, Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens. By 1655, astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.
A reflecting telescope is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Many variant forms are in use and some employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a catoptric telescope.
John Lowry Dobson was an American amateur astronomer and is best known for the Dobsonian telescope, a portable, low-cost Newtonian reflector telescope. He was also known for his efforts to promote awareness of astronomy through public lectures including his performances of "sidewalk astronomy". Dobson was also the co-founder of the amateur astronomical group, the San Francisco Sidewalk Astronomers.
A Dobsonian telescope is an altazimuth-mounted Newtonian telescope design popularized by John Dobson in 1965 and credited with vastly increasing the size of telescopes available to amateur astronomers. Dobson's telescopes featured a simplified mechanical design that was easy to manufacture from readily available components to create a large, portable, low-cost telescope. The design is optimized for observing faint, deep-sky objects such as nebulae and galaxies. This type of observation requires a large objective diameter of relatively short focal length and portability for travel to less light-polluted locations.
The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newton's first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope. The Newtonian telescope's simple design has made it very popular with amateur telescope makers.
The Stellafane Observatory is an astronomical observatory in Springfield, Vermont, founded in 1920 by Russell W. Porter. The Pink Clubhouse was built in 1923 at the site by the Springfield Telescope Makers Club. The name Stellafane, suggested by Porter at the club's December 1923 meeting, is derived from the Latin words stella and fanum meaning "Shrine to the Stars", and originally referred specifically to the clubhouse, but has since come to refer to all of the club's land and buildings on the summit of Breezy Hill, west of downtown Springfield.
Russell Williams Porter was an American artist, engineer, architect, amateur astronomer, and Arctic explorer. He was a pioneer in the field of “cutaway illustration" and is sometimes referred to as the "founder of amateur telescope making."
A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope designed to provide wide fields of view with limited aberrations. The design was invented by Bernhard Schmidt in 1930.
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as searchlights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as "catadioptric", such as surveillance catadioptric sensors.
The Maksutov is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly "spherically symmetrical". The negative lens is usually full diameter and placed at the entrance pupil of the telescope. The design corrects the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting chromatic aberration. It was patented in 1941 by Soviet optician Dmitri Dmitrievich Maksutov. Maksutov based his design on the idea behind the Schmidt camera of using the spherical errors of a negative lens to correct the opposite errors in a spherical primary mirror. The design is most commonly seen in a Cassegrain variation, with an integrated secondary, that can use all-spherical elements, thereby simplifying fabrication. Maksutov telescopes have been sold on the amateur market since the 1950s.
Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often silver, the term is used for the application of any reflective metal.
The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer James Gregory in the 17th century, and first built in 1673 by Robert Hooke. James Gregory was a contemporary of Isaac Newton, and both often worked simultaneously on similar projects. Gregory's design was published in 1663 and pre-dates the first practical reflecting telescope, the Newtonian telescope, built by Sir Isaac Newton in 1668. However, Gregory's design was only a theoretical description, and he never actually constructed the telescope. It was not successfully built until five years after Newton's first reflecting telescope.
The Houghton telescope or Lurie–Houghton telescope is a catadioptric telescope. Houghton's original design uses a two-lens corrector at the front of the telescope and a spherical mirror at the back; it was patented in 1944. Instead of the hard to make intricately shaped compound curve Schmidt corrector plate, or the heavy Maksutov-type meniscus corrector lens, the Houghton double-lens corrector is relatively easy to make.
Figuring is the process of final polishing of an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.
Optical Mechanics, Inc. or OMI is a high-end American telescope and optics instrument manufacturer. OMI was founded in 2002 and produces observatory telescopes, Lidar telescopes, optical tube assemblies, telescope mirrors and reflective coatings for mirrors. OMI mirrors are used by other telescope makers such as Obsession Telescopes. Also taking on custom projects, they produced the 48-inch Dob, a 48.875-inch-diameter (1,241.4 mm) aperture, f/4, Dobsonian telescope called "Barbarella" and featured in Astronomy Technology Today magazine. OMI is located in the US state of Iowa. OMI procured the assets of the former optics company Torus Technologies. OMI has an optics shop where it does work on telescopes.
Amateur Telescope Making (ATM) is a series of three books edited by Albert G. Ingalls between 1926 and 1953 while he was an associate editor at Scientific American. The books cover various aspects of telescope construction and observational technique, sometimes at quite an advanced level, but always in a way that is accessible to the intelligent amateur. The caliber of the contributions is uniformly high and the books have remained in constant use by both amateurs and professionals.
Robert Edward Cox was an American optical engineer and a popularizer of amateur telescope making. He conducted the popular "Gleanings for ATMs" column in Sky and Telescope magazine for 21 years.
In astronomy, a mirror support cell - more commonly mirror cell - is a component of a reflecting telescope that supports the mirror in place to hold optical alignment, allow collimation adjustment, and protect it from falling out. The common usage of the word denotes the cell that holds the primary mirror (M1), however technically it could also be used to denote the support assembly for the secondary mirror (M2) or other mirrors.
The Foucault knife-edge test is an optical test to accurately measure the shape of concave curved mirrors. It is commonly used by amateur telescope makers for figuring primary mirrors in reflecting telescopes. It uses a relatively simple, inexpensive apparatus compared to other testing techniques.
The Porter Garden Telescope was an innovative ornamental telescope for the garden designed by Russell W. Porter and commercialized by Jones & Lamson Machine Company at the beginning of the 1920s in the United States.
