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Details, Details…

Galileoscope design drawing

Galileoscope design drawing. Courtesy Merit Models.

The Galileoscope is a refracting telescope, or refractor: a long tube with a big lens (the objective) at the front end and a small lens (the eyepiece) at the back end. The great Italian scientist Galileo’s telescopes were refractors, too, but the Galileoscope improves upon his 400-year-old design in several important ways, as described below.

The Galileoscope comes in a cardboard box measuring 19½ by 6¼ by 4¼ inches (49.5 by 15.9 by 11.1 cm) with a shipping weight of 2.6 lb (1.2 kg). A case of Galileoscopes contains 6 kits; each case measures 19¾ by 14 by 13½ inches (50 by 36 by 34 cm) with a shipping weight of 15.6 lb (7.1 kg).

Here are the instrument’s key optical characteristics:

Objective diameter: 50 mm (2 inches)
Objective focal length: 500 mm (f/10)
Eyepiece focal length: 20 mm
Magnification: 25x (50x with Barlow)
Field of view: 1½° (¾° with Barlow)
Eyepiece eye relief: 16 mm (22 mm with Barlow)
Eyepiece barrel diameter: 1¼ inches (31¾ mm)

The field of view is the angular diameter of the circle of sky you see when you look into the telescope’s eyepiece. The full Moon is about ½° wide, so the Galileoscope’s field at 25x is three Moons wide, just big enough to encompass the splendid Pleiades star cluster in Taurus, one of the objects Galileo marveled at 400 years ago. If you double the magnification to 50x using the included Barlow lens, the width of the field of view is correspondingly halved.

Moon, Pleiades, Jupiter

Simulated views of the Moon at 50x, the Pleiades star cluster at 25x, and Jupiter and its moons at 50x in the Galileoscope. Made with Starry Night software and additional images by Rick Fienberg.

Eye relief is the distance behind the eyepiece that you need to put your eye to see the entire circular field of view. The Galileoscope’s ample eye relief of 16 mm at 25x and 22 mm at 50x should enable anyone, including children, to observe comfortably with or without eyeglasses.

The last entry in the table is significant. We’ve designed the Galileoscope to accept accessories with the standard 1¼-inch barrel common to most commercial telescopes. This means that you can substitute your own eyepieces of various focal lengths for the supplied 20-mm eyepiece, giving you additional options for magnification and field of view. (The magnification of a telescope is the focal length of the objective divided by the focal length of the eyepiece in the same units; with a given objective, eyepieces with shorter focal lengths give higher powers, and vice versa.)

Optical Design

Galileo's telescope

One of Galileo’s telescopes on display in Florence, Italy. Photo by Rick Fienberg.

Galileo built each of his telescopes with a convex (magnifying) objective lens and a concave (demagnifying) eyepiece lens. This combination produces an image that is both right-side up and correct left-to-right, but the field of view is uncomfortably narrow — as if you’re looking at the sky through a drinking straw.

Galileo’s contemporary, German astronomer Johannes Kepler, showed that with a convex eyepiece lens a refractor produces a wider, more comfortable field of view — but it also turns the image upside-down. This isn’t a problem for astronomical use, though, because there’s no “up” or “down” in space. (Kepler is best remembered for his three laws of planetary motion, the first two of which were published in Astronomia Nova in 1609, the same year Galileo turned his telescope to the heavens. The 2009 International Year of Astronomy celebrated not only Galileo’s ground-breaking contributions to astronomy 400 years ago, but also Kepler’s.)

Unique among available telescope kits, the Galileoscope can be assembled in either a Galilean configuration or a Keplerian configuration so that you can compare the two designs yourself. The Galileoscope is really two telescopes in one!

Single lenses don’t bring all colors of light to a common focus, so Galileo’s and Kepler’s telescopes showed spurious color fringes around the Moon, planets, and bright stars. This is called chromatic aberration, or false color. In the early 1700s opticians figured out that they could minimize this problem using pairs of lenses of different materials, each of which partially corrects the chromatic aberration of the other. Such lens pairs are called achromats.

Chromatic aberration

Cheap telescopes with single lenses produce blurry images with fringes of false color. The Galileoscope features higher-quality achromatic doublet lenses. Diagrams courtesy Renato Caniatti/Wikipedia; Moon photos by Rick Fienberg.

The Galileoscope’s 50-mm f/10 objective lens is an achromat made from two types of glass. The 20-mm (25x) eyepiece employs two achromats — a total of four lenses — made from two types of plastic. This four-element configuration is similar to that of the popular Plössl eyepiece, a high-quality design rarely seen on any telescope costing less than $100.

In addition to the main eyepiece, the Galileoscope comes with another plastic achromat that can be used in either of two ways: as a 17x eyepiece in the Galilean configuration, or as a 2x Barlow lens to double the magnification of the Galileoscope to 50x in its normal Keplerian configuration, as described below under Accessories.

Mechanical Design

We’ve designed the Galileoscope so that it can be assembled in 5 minutes or less by young children with some adult supervision. Assembly requires no tools, no glue, and no adhesive tape — everything snaps into place easily.

As already noted, the Galileoscope’s objective is a two-lens glass achromat. It comes as a single unit with the two elements glued together. The twin two-element plastic achromats for the main eyepiece, and the two-element plastic achromat for the Galilean eyepiece/Barlow lens, come as individual lenses, so that’s six more pieces. Except where noted below, all other parts in the Galileoscope kit are made of ABS plastic, a durable material commonly used in injection molding.

Here’s the complete 31-piece parts list, working approximately from the front (sky) end to the back (eye) end:

Lens shade/dew cap Eyepiece-barrel halves (2)
Objective lens Eyepiece field-stop ring
Main tube halves (2) Eyepiece clamp ring
¼-20 mounting nut Eyepiece eye-end ring
V bases/stands (2) Barlow tube
Main-tube clamp ring Galilean eyepiece/Barlow lenses (2)
Main-tube O-rings (2) Galilean eyepiece tube halves (2)
Focuser tube halves (2) Galilean eyepiece clamp ring
Focuser tube O-rings (2) Galilean eyepiece/Barlow eye-end ring
Eyepiece lenses (4) Sun-warning sticker
Galileoscope Kit

Galileoscope telescope kit before assembly. Photo by Joson Images.

All parts except the lenses (of course) and mounting nut are black. The tube halves fit together with tongues and grooves to prevent light from seeping in through the sides, and the main tube includes multiple internal baffles. In addition, the insides of the telescope and focuser tubes are roughened to minimize internal reflections. These features, designed to reduce glare and the amount of stray light reaching the eyepiece, make the Galileoscope an ideal urban telescope for use in even the most light-polluted cities and are usually found only on telescopes costing much more.

The Galileoscope can be put together in 5 minutes or less. A 10-year-old child can assemble the telescope with minimal adult supervision, and younger children with a bit more supervision. Moreover, the kit can be assembled and disassembled multiple times — making it well suited for classroom use.

To assist with aiming the telescope at celestial targets, the main tube has a traditional notch-style sight along the top edge.

Eyepieces and other accessories are held securely in the focuser by self-adjusting spring-loaded tabs molded into the tube. The focuser assembly slides in and out with a comfortable amount of friction and can be moved in very tiny steps to achieve precise focus. If you’re nearsighted or farsighted and prefer to look through a telescope without wearing eyeglasses, you’ll appreciate the Galileoscope’s extra-long focuser tube, which accommodates a wide range of focus positions.

The ¼-20 mounting nut, made of zinc-coated low-carbon steel, enables the Galileoscope to be affixed to a standard photo tripod (see below) or any other mount with a ¼-20 threaded post.

The bases/stands serve several purposes. They can simplify assembly by cradling the main tube on a table top or display the telescope by supporting it on a shelf or mantel. They also can enable students to partially assemble the telescope on a table and see what’s happening inside the instrument as it forms an image of a distant object. For more about using the Galileoscope as an optics-education aid, click on Support for Educators in the menu at upper right.

No tools are needed to assemble the Galileoscope — all the parts fit together by hand in just a few minutes, with the tube halves held together by snap-fit plastic rings and/or rubber O-rings. A sheet of English-language instructions is included, but a set of graphical instructions is available in our Assembly Instructions section, so you don’t need to read English to assemble the scope. There is also a vinyl sticker for the back end of the main tube with a graphical warning not to point the telescope at the Sun.

Special Features

Focuser: As noted above, the Galileoscope’s 1¼-inch (31¾-mm) barrel enables the use of standard commercial accessories such as additional eyepieces, Barlow lenses, and camera adapters. A 1¼-inch focuser is one of the key factors that distinguishes a serious telescope from a toy. (Another is achromatic optics, and the Galileoscope has those, too!) There are thousands of 1¼-inch accessories on the market today, giving the Galileoscope virtually limitless versatility.

One accessory that does not work with the Galileoscope, at least not in most cases, is a star diagonal. That’s a right-angle mirror or prism that goes between the telescope and eyepiece and enables you to avoid having to crane your neck when observing celestial objects high overhead. The Galileoscope is designed for straight-through viewing. There’s not enough “in focus” to permit the use of most star diagonals. We recommend sitting in a chair with the Galileoscope on a tripod (see below) that can be extended to a height of at least 150 cm (60 inches). That way, observing objects high in the sky will be comfortable even without a star diagonal.

Galilean Eyepiece: Included in the Galileoscope kit is a custom accessory consisting of a plastic two-element achromat with a two-piece plastic holder, a plastic ring, and a plastic tube. The lens has a negative focal length of 30 mm. This accessory can be used in either of two ways. If you assemble it without the tube, you get a Galilean eyepiece that gives a magnification of 17x and a right-side-up field of view less than ½° wide. With this eyepiece, you can have the rather frustrating “Galileo experience” of looking at the heavens one very tiny piece at a time. This will make you really appreciate the advance represented by today’s wider-field designs!

2x Barlow Lens: If instead you assemble the parts using the tube, you get a 2x Barlow lens which, when used with the regular supplied eyepiece, increases the Galileoscope’s magnification to 50x. In addition to providing more detailed views of the Moon, Jupiter, and other targets, the Barlow lens will make plain the rings of Saturn. When Galileo looked at 20x to 30x, he couldn’t quite figure out that he was seeing a ringed planet — he described Saturn as “tri-form.”

Optional Accessories (Not Included)

Tripod: As experienced stargazers know, a telescope is only as good as its mounting. Because a telescope gives a highly magnified view, the tiniest vibration looks like a major earthquake in the eyepiece. Even at a relatively modest 25x, the Galileoscope needs to be firmly attached to something stable. Because the instrument is so lightweight, an inexpensive photo tripod — the type you’ll find at most discount stores — should be sufficient. The tripod should have a pan head that moves smoothly in altitude (up-down) and azimuth (left-right), so that you can aim the telescope anywhere in the sky and make small adjustments without jerking things around.

Photo tripodThe included ¼-20 mounting nut on the bottom of the Galileoscope will fit any standard photo tripod, and if you already own such a mount, you’re all set. We are not packaging a tripod with the Galileoscope, but you can find suitable models by manufacturers such as Sunpak, Vanguard, Sakar, and Tiffen (Davis & Sanford) on the Internet. We particularly recommend the medium-duty tripod sold by Edmund Scientific and the Celestron photo-video tripod sold by Oceanside Photo & Telescope. Especially if you plan to use the Galileoscope while standing up — sitting in a chair is way more comfortable! — we recommend attaching it to a tripod that extends to a height of at least 60 inches (150 cm). Otherwise, you’ll find it difficult to get your head under the eyepiece when the telescope is pointed high in the sky.

Looking for a cheap alternative to a tripod? Astronomy blogger Shannon Murphy (University of Michigan) has posted detailed instructions for building a cardboard box mount.

Solar Filter: There are several ways to observe the Sun safely with a telescope. One is to use a special-purpose filter that fits over the front opening of the Galileoscope to block all but a minuscule fraction of the Sun’s light (including solar ultraviolet and infrared radiation) and create a safe, comfortably bright image in the eyepiece. If you already own a Galileoscope, you can buy a single, inexpensive 70-mm-diameter solar filter for it from Rainbow Symphony, one of the best-known manufacturers of safe solar filters. If you don’t already own a Galileoscope and want to buy a single kit, Rainbow Symphony sells the kit bundled with a solar filter at an attractive price.