Amateur Telescope Making
Beginnings
Ever since Galileo took a Dutch invention and adapted it to 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. There were many published works that also helped spark an interest in building telescopes, including Rev. W. F. A. Ellison’s 1920 book “The Amateur’s Telescope” and several articles in Popular Astronomy by Russell W. Porter, including one appearing in March, 1923 about the “Telescope Makers of Springfield”. In 1925 Albert G. Ingalls featured Porter and the Springfield Telescope Makers. in an article he wrote for Scientific American magazine. There was so much public interest, a whole series of articles were written by Ingalls on the subject. Those articles (and later three books titled Amateur Telescope Making Vol. 1-3) helped people around the world take on the task of constructing their own instruments. Between 1933 and 1990, Sky and Telescope magazine ran a regular column called “Gleanings for ATMs” edited by Earle Brown, Robert E. Cox & Roger Sinnott. In the US, the ready supply of surplus optical components after World War 2 and later Sputnik and the space race greatly expanded the hobby.
“The Poor Man’s Telescope”
A 6 (15cm) Newtonian reflector built by a school student on display at Stellafane
The types of telescopes that amateurs build vary widely. They range from the very simple to complicated designs including Refractors, Schmidt Cassegrains and Maksutovs. The most popular telescope design is the Newtonian reflector, described by Russell W. Porter as he 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 means (for the person creating the optical elements) that there is only 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. A properly designed and built Newtonian is a high performance instrument, fully capable of matching the performance of other designs, a case where inexpensive does not mean inferior. Typically a Newtonian telescope of 6 or 8″ (15 or 20 cm) aperture is considered a good starter project. Some widely available plans found on the Internet include instructions for building 6″, 8″, and 10″ Newtonians.
Mirror making
Grinding a mirror using an abrasive and a smaller tool over 300 mm mirror (“ATM Korenica 2006”, in Korenica, Croatia)
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 mirror has to be carefully ground and polished (figured) to an extremely accurate shape, usually a paraboloid, although 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 are surprisingly 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.
Foucault test
The equipment most amateurs use to test the shape of the mirrors, a Foucault tester, 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.
Foucault test setup to measure a mirror
Shadowing on the surface of the mirror as it appears during Foucault testing, depending on knife-edge position
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 donut 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.
Aluminizing or “silvering” the mirror
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 the coating applied by a thin-film deposition processes (work that has to be done by a firm specializing in the process). Originally the coatings were aluminum. Modern coatings usually contain aluminum and other compounds.
The mirror is aluminized by placing it in a vacuum chamber with electrically heated tungsten or nichrome coils that can evaporate aluminum. 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.
Telescope design
A large fork mounted telescope and several other amateur built telescopes on display at Stellafane
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 obbyist 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 class 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 inish. Since many 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 passable accuracy and stability without requiring strict tolerances in machining.
In recent years there has been an expansion in the commercial production of inexpensive and high quality mass-marketed telescope designs in the smaller 4 to 10 in (10 to 25 cm) apertures, making these instruments far more affordable. This has led to fewer amateurs building instruments in the smaller sizes, as it has become cheaper just to buy such a telescope.[citation needed] Most ATMs focus on building larger instruments, generally from 10 in (25 cm) up to designs as large as 30 in (76 cm) and beyond.[citation needed] Such an instrument can be built for less cost than it would cost to buy, and amateur made telescopes can surpass the quality and craftsmanship of commercially available telescopes.
The difficulty of construction is another factor in an amateur 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 Dobsonian: A case in point
Main article: Dobsonian telescope
An amateur built Dobsonian telescope of moderate size
Since the overall design of a telescope tends to be an exercise in creative problem solving the types of instruments amateurs come up with can be unique to their field. A case in point is the Dobsonian telescope. John Dobson, the originator of the design, wanted an instrument fully optimized for deep sky observing. Since Dobson was a monk living in a monastery at the time, the telescopes he built had to meet the following criteria:
it had to be as large as possible
it had to be easy to build
it had to be constructed with commonly available materials
it had to be inexpensive
it had to be durable
it had to be portable (to get to dark sky locations)
His solution was not only a unique design, it also consequently caused a significant leap in the size of instruments that amateurs build. Based on a simple design the telescope does away with the complex and cumbersome equatorial mount that was typical for earlier telescopes. A simple mount made of plywood, with Teflon bearings allows the telescope to move in azimuth (left and right) and elevation (up and down). The design has proven to be easy to build and highly adaptable. Amateur builders continue to tinker with and improve the design in the true spirit of the amateur telescope maker.
References
^ stellafane.org – A Brief History of Stellafane by Bert Willard
^ Telescope Basics – Mark T. VandeWettering, 2001
^ L. Foucault, “Description des procedees employes pour reconnaitre la configuration des surfaces optiques,” Comptes rendus hebdomadaires des sances de l’Academie des Sciences, Paris, vol. 47, pages 958-959 (1858).
^ L. Foucault, “Mmoire sur la construction des tlescopes en verre argent,” Annales de l’Observatoire impriale de Paris, vol. 5, pages 197-237 (1859).
^ Strong, John (1959). “Aluminizing Mirrors”. in Ingalls, Albert G.. Amateur Telescope Making Advanced. Scientific American. pp. 467482.
Telescope making books and other published information
A 180 mm Texereau standard telescope
Albert G. Ingalls (ed.), Amateur Telescope Making (Vols. 1-3). Orig. edition: Scientific American; new rearranged edition: Willmann-Bell Inc.
Allyn J. Thompson, Making Your Own Telescope, 1947, Sky Publishing, ISBN 0-933346-12-3. (An online version)
Jean Texereau, How to Make a Telescope, Willmann-Bell, ISBN 0-943396-04-2
David Harbour, Understanding Foucault, Netzari Press, ISBN 978-10934916-01-8, (Amazon.com)
David Kriege, Richard Berry, The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes, 1997, Willmann-Bell, ISBN 0-943396-55-7
Richard Berry, Build Your Own Telescope, Willmann-Bell, ISBN 0-943396-69-7
Harrie Rutten, Martin van Venrooij, Telescope Optics, Evaluation and Design, Willmann-Bell, ISBN 0-943396-18-2
Neal Eltinge Howard, Standard Handbook for Telescope Making (Hardcover), Harper & Row, ISBN 978-0061813948
See also
Amateur astronomy
Group 70 Attempt at making the world’s largest amateur-built telescope
List of telescope parts and construction
Mirror support cell
Optical telescope
PLate OPtimizer
Stellafane
Telescope Nut
External links
Wikimedia Commons has media related to: Amateur telescope making
A Little Amateur History (R.F. Royce)
A Manual for Amateur Telescope Makers by Karine and Jean-Marc Lecleire (PDF introduction)
Sacek, Vladimir (July 14, 2006), Notes on Amateur Telescope Optics, http://www.telescope-optics.net/classical_and_aplanatic.htm, retrieved 2009-06-22
The Amateur Telescope Makers Email List also has searchable archives
The ATM Site – A Short History of Amateur Telescope Making
The Telescope Makers of Springfield, Vermont History at stellafane.com.
Categories: Telescopes | Amateur astronomy | HobbiesHidden categories: All articles with unsourced statements | Articles with unsourced statements from February 2007 | Articles with unsourced statements from December 2008
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