ABOUT SPOTTINGSCOPES Background on Telescopes

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1 22 November 2010 ABOUT SPOTTINGSCOPES 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. 1. 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 as shown below, 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.

2 As seen above, 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. In the image below at can be seen that different colours of light are refracted through different angles. This is due to their different wave lengths. This explains why in refractor optical instruments, phase correction is part of the equation. In the next section we shall focus on optical telescopes. 1.1 Optical Telescopes Optical telescopes are designed to collect wavelengths visible to the human eye. There are three basic types of optical telescopes: 1.1) Refractor Telescopes 1.2) Reflector Telescopes, and 1.3) 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.

3 Figure 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. Figure 2 (a) shows that refraction by a prism changes the direction of a light ray by an amount that depends on the angle between the prism s faces. Figure 1 (b) shows that a lens can be thought of as a series of prisms. A light ray traveling along the axis of a lens is unrefracted as it passes through the lens. Parallel rays arriving at progressively greater distances from the axis are refracted by increasing amounts, in such a way that all are focused to a single point. Figure 3 shows the path of light through a refractor telescope. One advantage of refractor telescopes is that very sharp images are possible. One disadvantage is the high cost per unit area of aperture.

4 Figure 2. Figure 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. Figure 4 shows how light is focused by a reflector telescope. Figure 5 shows the path of light through a typical reflector telescope. Figure 4. Figure 5.

5 Figure 6. Figure 6 shows Four reflecting telescope designs: (a) Prime focus, (b) Newtonian focus, (c) Cassegrain focus, and (d) Coudé focus. Each design uses a primary mirror at the bottom of the telescope to capture radiation, which is then directed along different paths for analysis. One advantage of reflector telescopes is that it has the lowest cost per unit area of aperture. Disadvantages are the requirement for frequent collimation and long cooldown time. 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. Figure 7 and 8 show the path of light through catadioptric telescopes. One advantage is the long focal length in a short tube. Disadvantages are the narrow field of view and long cooldown time.

6 Figure 7. Figure Difference between Spotting Scopes and Telescopes From the point of view of the design, a spotting scope and a telescope are not very different altogether.both are used for magnification and are constructed with an eyepiece lens and an objective lens. Both are used to be able to see long distance images clearly. 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 lesser magnification 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 telescope, they are better suited for observing moving targets. This makes them ideal for birding, hunting, surveillance etc. Spotting scopes are also used with guns for targeting. They can also be paired with a camera to shoot pictures of distant objects. In fact a spotting 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. We trust that the above gave you some background on spotting scopes. With compliments Optics International. (