Your Home Observatory Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 19, 2011


Some amateur astronomers recommend that you obtain a pair of binoculars before you spend any money on a telescope. This is an individual choice. Binoculars are good for general star viewing at low magnification. Telescopes are a requirement for resolving detail in the planets, observing lunar terrain up close, or examining Sunspots.

Basic Structure

Figure 20-1 is a simplified functional diagram of a pair of binoculars. You can think of the assembly as two identical telescopes placed beside each other. The eyepieces are spaced to match the distance between the pupils of the observer’s eyes. This spacing is adjustable. In most types of binoculars, when the eyepiece spacing is adjusted, the spacing between the objectives also varies.

Your Home Observatory Location, Location, Location! Basic Structure

Figure 20-1. Functional diagram of a pair of binoculars.

The objectives are farther apart than the eyepieces. This exaggerates perspective for scenes within a few hundred meters but does not affect perspective for celestial objects, which are too far away for parallax to exist relative to any single observation point. The light enters the objectives, passes through a pair of prisms that bring the light beams closer together by means of internal reflection, and finally leaves the eyepieces to enter the observer’s eyes. The principle of operation of each half of a pair of binoculars is identical to that of a Keplerian refracting telescope. The prisms turn the image right-side up and also orient the view properly left to right.

Size Specifications

Binoculars are rated in terms of the magnification (the number of times the apparent diameters of distant objects are increased), as well as in terms of the objective-lens diameter in millimeters (mm). You’ll see a pair of numbers separated by a multiplication symbol, for example 7 × 50, printed somewhere on the assembly. The first number is the magnification, and the second number is the objective-lens diameter.

In general, the light-gathering power of binoculars is proportional to the square of the objective-lens diameter. However, this holds true only when the binoculars are optimized for a particular observer. If you divide the objective-lens diameter by the magnification, you should get a number between approximately 4 and 8. This number is called the exit pupil of the instrument. For best viewing, the exit pupil of a pair of binoculars should be the same as the diameter of the pupils of the observer’s eyes (in millimeters) when adjusted to the darkness. In general, larger exit pupils (6 to 8 mm) are a good match for younger observers, and smaller exit pupils (4 to 6 mm) are better for older observers.

In terms of physical bulk and mass, binoculars range from tiny to huge. At least, this is the impression you’ll get. Some binoculars can fit in your pocket. (But always keep them in a carrying case when you’re not using them). Others are so large that you’ll want a tripod to support them. The most massive binoculars will make your arms tired if you have to hold them up for a long time. High-magnification binoculars, especially those greater than 8×, need the extrasteady support that a tripod can provide.

The biggest binoculars are more appropriately called binocular telescopes or stereoscopic telescopes . These are fabulous for viewing star clusters, galaxies, and nebulae. They also can deplete the average person’s bank account.


The prisms inside binoculars serve as mirrors to reflect the incoming light between the widely spaced objectives and the narrowly spaced eyepieces. Prisms provide better image resolution and contrast than mirrors. The best prisms are called porro prisms (Fig. 20-2 A ). They reflect the light entirely by total internal reflection, which you learned about in Chapter 17. Less effective but still superior to mirrors are roof prisms (see Fig. 20-2 B ), which have aluminized back surfaces that help reflect the light rays. Porro prisms are more expensive than roof prisms because a higher grade of glass must be used to get the highest amount of total internal reflection to occur without aluminized surfaces.

Your Home Observatory Location, Location, Location! Optics

Figure 20-2. The porro prism ( A ) provides better image quality than the roof prism ( B ).

Another factor to consider in binoculars is whether or not the lenses are specially coated to minimize the amount of light they reflect. As you have seen if you’ve looked at the window of a darkened house from the outside during the daytime, all glass reflects light as well as transmitting it. Any light reflected is light that doesn’t pass through the glass. In binoculars or a telescope, you’ll want as much of the light as possible to reach your eyes and not be reflected back into space from the objective(s) or into the internal chamber of the instrument from the eyepiece(s). The best lenses have multiple coatings on both the inside surfaces and the outside surfaces. These binoculars will be specified as having fully multicoated optics .

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