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Optical Design of Spotting Scopes

There are basically two types of optical design in spotting scopes: the refractor and the catadioptric.The difference between the two is the material used to focus the image.The refractor uses optical glass lenses to bend light where the catadioptric primarily uses mirrors to reflect the light. This basic difference in materials, however, translates into several distinctive optical characteristics.

Refractors

The refractor can be as simple as a long tube with an objective lens at one end and an eyepiece at the other. Those designed for terrestrial use will incorporate some type of image-erecting prism system (either Porro or roof prism) in the optical tube between these two lenses. This prism system provides an image orientation that is not only erect but correct right to left, also. For terrestrial use, the focal length is typically short to provide a compact, lightweight scope with a wide field of view. Eyepieces available for this type of refractor are usually of the interchangeable, fixed focal length and are attached by either the screw or bayonet type mount. Zoom eyepieces are usually available which provide flexibility and convenience in the field at the expense of some loss in brightness, resolution and field of view. These eyepieces normally deliver a magnified image in the range of 15 to 60X, but give the best performance in the 20 to 40X range.Usually the focusing mechanism is of the helical or knob type which is internal and protected from dust and moisture.

Refractors which have astronomical capability are similar to the terrestrial type, with the main differences being in focal length, eyepiece placement and focusing mechanism. To be capable of higher magnifications for celestial viewing (up to 200X), a longer focal length is usually employed. Eyepiece placement is usually at 90 degrees for comfortable viewing above the horizon and the eyepiece is inserted into an image erecting prism which is attached at the rear of the optical tube. The observed image with this arrangement is erect, but reverted (right to left reversed). Eyepieces are of the fixed focal length type and are easily interchangeable by means of a set screw.The focusing mechanism that operates best with this eyepiece configuration is the rack and pinion type which offers smooth, accurate focusing on high quality instruments. Because these type of refractors are often used at higher magnifications, a small, low power finder scope fitted with crosshairs may be attached for assistance in locating the desired object.

In general, refractors are of simple design and are therefore easy to use, durable and reliable, and require little or no maintenance. Their objective lens is permanently mounted and aligned. The high cost and bulk of optical glass limit their practical useful aperture size to about 100mm. Some color aberration may be noticeable with refractors but models using apochromatic lens designs or those with fluorite or extra-low dispersion glass elements, virtually eliminate this problem. Refractors deliver sharp, high contrast images and overall, are the most popular for birding, hunting and general outdoor activities.

Catadioptrics

The catadioptric design uses a combination of a corrector lens and a pair of mirrors to bring an image into focus. This optical system typically has a long focal length which is folded by means of these mirrors into a compact optical tube. Catadioptics are capable of high magnification and exceptional telephoto lens performance without excessive bulk (lightweight and portable). The long focal length, coupled with a fairly large aperture size (90 to 100 mm is typical) allows for useful magnifications of up to200X or more on some models. On the other hand, this long focal length also tends to restrict the field of view.

Catadioptrics are usually supplied with a 45-degree or 90-degree erecting prism which is fitted into the rear of the optical tube. The 45-degree prism will deliver a correctly oriented image in all dimensions while the image from a 90-degree prism will be erect, but reverted. Both work well for viewing objects above the horizon, with the 90-degree actually working better for astronomical observation. Interchangeable fixed focal length eyepieces are inserted into the prism to change the magnification (zooms are not readily available). Since catadioptrics operate at higher magnifications with the resulting narrow fields of view, a finder scope is usually supplied for centering the main scope on a distant object. Catadioptric systems will generally suffer less from chromatic aberrations since the light is reflected and not bent (as in a refractor) through the optical system. Some models are offered with rubber armor coating for durability and protection from the elements.

The secondary mirror in the optical system is located in the center of the correcting lens at the front of the optical tube. Although this is actually an obstruction in the center of the light path, normally the observed image is bright and clear. Diffraction, however, may cause some loss in image contrast and sharpness. Also, at lower magnifications (20 to 30X), especially on a bright day when the eye pupil is constricted, a faint shadow from this mirror may be observed at the center of the image. The location of this secondary mirror also makes catadioptrics with an aperture of less the 80mm not very practical.

The result of all these characteristics is that catadioptrics are best suited for long distance terrestrial applications, telephotography and astronomical observing. Because of the materials used in the optical design, they are generally not as rugged as a refractor. They have more limited use for scanning purposes such as fast-moving sporting events and close-in nature observation.One exception is that the optical design allows for focusing down to as close as 5 to 15 feet, so it an excellent instrument for long distance microscopy.

These various characteristics, features and specifications of telescopes can help you determine which one will best meet your needs and requirements. A fine optical instrument will bring the fascinating world of nature and the heavens closer to you and can provide a long term investment in satisfaction and enjoyment.

Thanks to our friends at Eagle Optics for providing this information!

See also:
Bird Watching Binoculars—Tips for choosing the right birding binocular
Birding Tripods—Tips for choosing the right tripod for you binoculars and scopes

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