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About Spotting Scopes

Spotting Scopes

A spotting scope is a compact telescope designed primarily for terrestrial observing and is used in applications which require magnifications beyond the range of a typical binocular usually with a magnification power between 15x and 250x. Ideally, the objective lens should be at least 60 mm in diameter to provide a bright image.

Spottingscopes are part of a bigger group of optical instruments called telescopes. In order to get a better perspective on spottingscopes, we look deeper into telescopes.

Difference between Spotting Scopes and Telescopes

From a design point of view, a spotting scope and a telescope are very similar. Both are used for magnification and are constructed with an eyepiece lens and an objective lens. Both are used for clear long distance viewing.

The basic purpose of a spotting scope is to observe terrestrial targets, whereas that of a telescope is to study celestial objects. Spotting scopes are lighter to make them portable and are armed with rubber reinforcements. They also sport a more ergonomic design, as compared to telescopes, and are more rugged and durable. As compared to an astronomical telescope, a spotting scope has lower magnification capabilities and a smaller aperture. However this enables the spotting scope to have a wider field of view.

Telescopes are optimized for being placed at one place and hence tend to be heavier than a spotting scope. A major difference between the two is that spotting scopes are designed to be mounted on generic camera tripods while telescopes require special mounts, which are quite expensive, to support them. The image produced by a telescope is always inverted and a spotting scope always produces a right side up image.

How to decide whether to use a telescope or a spotting scope? As mentioned earlier, a spotting scope is meant for terrestrial observations. Typically, when there is a need for high magnification, a spotting scope is chosen over a pair of binoculars. Due to the wider field of view that spotting scopes offer, as compared to telescopes, they are better suited for observing moving targets. This makes them ideal for birding, hunting, surveillance etc. Spotting scopes are used by marksmen and can be paired with a camera to shoot pictures of distant objects. In fact a spottings scope can also be used for star-gazing but will not provide as much clarity as a telescope would.

Astronomy telescopes are far better suited at observing stationary objects due to their narrower field of view. Also, due to their greater aperture and magnification than a spotting scope, they are suited for viewing objects at great distances such as stars, planets and the moon. Although a telescope can be used to view terrestrial objects, it is not a recommended practice as not only is a telescope bulky but also produces an inverted image.

In order to get a better perspective on spottingscopes, we look deeper into telescopes.

Background on Telescopes

A telescope is a device with primary function of capturing as much radiation as possible from the observed area and concentrate it into a focused beam for analysis.

Different types of telescopes capture and analyze radiation in different regions (classified by wavelengths) of the electromagnetic spectrum. According to the electromagnetic spectrum, telescopes can be classified among others into the following categories:

a)   Gamma Ray Telescopes

b)   X-Ray Telescopes

c)   Ultraviolet Telescopes

d)   Optical Telescopes

e)   Infra Red Telescopes

f)    Microwave Telescopes, and

g)   Radio (FM, TV and Shortwave) Telescopes.

The visible spectrum ranges from less than 400 to more than 700 nm, with Violet the shortest, and Red the longest wave length. We shall look deeper into optical telescopes in the next section.

1. Optical Telescopes

Optical telescopes are designed to collect wavelengths visible to the human eye.

There are three basic types of optical telescopes:

a) Refractor Telescopes

b) Reflector Telescopes, and

c) Catadioptric Telescopes.

All of these telescopes are designed to collect light and bring it to a focus point so that it can be magnified by an eyepiece, however each design does it in a different manner. Each of the designs have the potential to perform very well, and all have their own virtues, as well as faults.

1.1 Refractor Telescopes

The Refractor, also known as the dioptrics, is a telescope that uses lenses to refract, (bend), the light that it collects. This refraction causes parallel light rays that converge at a focal point at the opposite end, where they can be magnified by an eyepiece. The large lens at the front is called the objective lens. The objective lens usually comprises of two or more individual lenses that are bonded and or arranged together to make up what is called the objective lens cell. The glass material used can also vary which will help in the overall performance of the objective lens. Refractor telescopes are very well suited for terrestrial viewing due to their weight, size and optical qualities.

1.2 Reflector Telescopes

The Newtonian Reflector, also known as catoptrics, is a telescope which uses a spherical or concave parabolic primary mirror to collect, reflect and focus the light onto a flat secondary mirror (diagonal). This secondary mirror in turn reflects the light out of an opening in the side of the tube and into an eyepiece for focus and magnification.

1.3 Catadioptric Telescopes

Catadioptrics are telescopes that use a combination of mirrors and lenses to fold the light path and direct it for focus and magnification through a hole in the primary mirror. There are two popular designs, the Maksutov-Cassegrain and Schmidt-Cassegrain. Both designs have similar advantages and disadvantages.

In Maksutov designs the light enters a thick meniscus correcting lens with a strong curvature. The light then strikes the primary mirror and is reflected back up to the secondary mirror that reflects the light out an opening in the rear of the instrument. The secondary mirror is usually an aluminised spot on the back of the meniscus corrector. The Maksutov secondary mirror is usually smaller than the Schmidt's thus giving the Maksutov better resolution for planetary observing. The Maksutov is usually heavier than the Schmidt and the thicker correcting lens takes longer to reach thermal stability.

In Schmidt designs the light enters a thin aspheric Schmidt correcting lens. The light then strikes the primary mirror and is reflected back up to the secondary mirror that reflects the light out an opening in the rear of the instrument. Schmidt's usually have shorter focal lengths thus making them more suitable for fainter deep sky objects. The thinner corrector plate means the Schmidt is faster to reach thermal stability.

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