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Galileo’s Legacy

Telescope Making > ATM Diaries 1

Galileo’s Legacy

By Chris Stewart

You can travel in time and space safely, cheaply and enjoyably with an astronomical telescope, which really is a time machine. Since light travels at a finite speed, the farther away you look the further back in time you are seeing. The bigger the telescope, the farther you can see. With a decent instrument, you can travel the universe without leaving your backyard.

Professional instrumentation requires teams of specialists and huge sums of money but a telescope suitable for an individual can be built by an enthusiastic amateur. Some of the most prized commercial instruments are finished by hand. Crafting surfaces to tolerances of a fraction of a wavelength of light is done routinely, using just bare hands.

Throughout the telescope’s 400-year history, amateurs have been near the forefront of construction and development. Amateurs’ time is “free”, so they can work to arbitrary levels of precision or explore arcane concepts. Sometimes, their efforts attract commercial attention. The current off-the-shelf availability of computer-controlled telescopes is an example of this.

One should start with a basic telescope, but even that will give access to more celestial objects than can be comprehensively observed in a lifetime. The cost of imported items presents a barrier, so building a simple telescope is an economical way of gaining access to the sky. Parts can even be junk, with obvious environmental benefits. The satisfaction of accomplishment is enormous.

Astronomical telescopes are both simpler and more complex than terrestrial ones. Simpler, in that the number of optical surfaces and components is minimised to preserve every photon. More complex, because the instrument must be stable, and able to track and point with precision.

Optically, the simplest design commensurate with good quality has only two components. The Newtonian (named for its inventor, Sir Isaac Newton) comprises a concave paraboloidal primary mirror and a flat secondary mirror. The primary collects incoming light and focuses it to an image; the secondary merely brings the image to an accessible point. Reflective (mirror-based) optics have certain advantages over refractive (lens-based) designs, in terms of simplicity and materials. A good reflector costs much less than an equivalent refractor, the ratio increasing rapidly with size. Large astronomical telescopes, both amateur and professional, are almost invariably reflectors.

Most amateurs would start with a Newtonian. Making a mirror involves several phases: rough grinding, fine grinding, polishing, figuring and aluminising. In grinding, successively finer grades of abrasives are introduced between two disks of glass, which are moved so as to cause one (the mirror) to become concave and the other (a tool) convex. Polishing is much like grinding, except that a slightly yielding “lap” replaces the tool and a polishing agent is used instead of abrasives. Figuring, the process of coercing the polished surface into the optically correct shape, proceeds as for polishing, the movements being altered in response to actual surface defects as revealed by ingenious test equipment. This is the most interesting but often the most frustrating part. Aluminising is accomplished by evaporating pure aluminium in a vacuum chamber, allowing a film just a few molecules thick to condense on the face of the mirror. This provides the reflective surface.

Mechanically, the telescope comprises an “optical tube assembly” and a mounting. The tube (or equivalent structure) holds the optical components in the correct relative positions, shields extraneous light and avoids air currents in the optical path. A cell to support the primary mirror and allow it to be collimated (adjusted for optical alignment) lies at the bottom end. At the top, a “spider” assembly hangs in the middle of the tube, providing for secondary mirror support and collimation. On the side is the focuser, which positions the eyepiece. A “finder” - a small refractor used like gun sights to locate objects - is typically attached to the tube since the main instrument has a comparatively narrow field of view.

Mountings fall into two main categories: alt-azimuth (up/down and left/right) and equatorial. The latter has two axes, one being placed parallel to that of the Earth. By turning it in the opposite direction to the Earth’s rotation, at one revolution per day, the apparent movement of the sky is cancelled and the instrument is said to be “tracking”. Stability is always a primary consideration, as the telescope magnifies every wobble or vibration. “Dobsonian” alt-az mounts  - an intrinsically stable form devised by John Dobson - can be made from readily available materials. To track, or to slew from one object to another, merely involves pushing the scope gently. The simplicity, effectiveness and low cost of this design makes it extremely popular. Aalt-az mounts can be made to track automatically, but this is complicated and generally requires computer control.

Eyepieces are essential. They match the converging cone of light at the focus of the instrument to the needs of the eye. By changing eyepieces, magnification and field of view can be changed to suit the subject. While some make their own eyepieces from scratch, and quite reasonable eyepieces can be scrounged or assembled from available lenses, a really good eyepiece is typically a complex and expensive.

Amateur Telescope Making (ATM) boasts a large Internet community. A few small groups are active in South Africa, including the Astronomical Society of Southern Africa (ASSA), Johannesburg Centre. For the past decade, it has run an ATM class in which people proceed at their own pace, with much sharing of expertise and resources. People of all ages and walks of life have successfully completed a fair number of telescopes. Key to this are perseverance and a willingness to follow instructions.

The ASSA has had several exhibitions including ScopeX, an annual event celebrating astronomy and telescopes. This is held at the Military History Museum adjacent to the Zoo in Johannesburg. Please pay it a visit.

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Original article © 2004 EE Publishers (Pty) Ltd.  
http://www.eepublishers.co.za/
Reproduced with permission of the publishers. First published in the March 2004 engineers@leisure column of the Energise and Vector journals of the South African Institute of Electrical Engineers.


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