Telescope Accessories Help (page 2)

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

Star Diagonal

With refractors and SCTs, the eyepieces are normally in line with the telescope tube. Viewing can be uncomfortable when such a telescope is aimed at objects high in the sky; you have to crouch down and crane your neck. However, there’s a simple and common solution to this problem: the star diagonal . This device bends the light path without introducing distortion, although it flips the image laterally, as a mirror flips your reflection.

A simple star diagonal employs a prism that causes the light to turn a 90-degree corner because of total internal reflection. The principle is the same as in binoculars. A cutaway view of a basic 90-degree star diagonal is shown in Fig. 20-7. More sophisticated star diagonals provide smaller angles, such as 45 degrees. Some star diagonals use two prisms rather than one, so the image is not laterally reversed.

Your Home Observatory Telescope
Accessories Star Diagonal

Figure 20-7. A star diagonal uses a prism to bend the light, making it easier to view some celestial objects.

The lateral-reversal feature of basic star diagonals makes it rather inconvenient when you try to find objects in the sky using a star map. You have to imagine everything on the map backwards. With a little practice, however, most people can overcome this mental obstacle.

Sighting Device

If you’ve ever used a telescope in an attempt to locate a planet or star and you didn’t have some sort of aiming or sighting device, you know how frustrating such an exercise can be. Except at the very lowest magnifications, you can end up searching for a long time. The simplest sighting devices are similar to gunsights. You aim the telescope as if it were a high-powered rifle. The more advanced type of sighting device has a small laser diode inside; it shines on a slanted glass to produce a variable-brightness red dot in the center of the view field. This dot is used to align the telescope with the object you want to observe.

Before you use it for celestial observations, the sighting device first must be aligned on a terrestrial target that is at least a couple of kilometers away. Find some object on the horizon that is large enough to see through the sighting device (that is, at 1×) yet small enough to fit into the view field of the telescope. Get the object centered in the view field of the telescope, fix the telescope in position, and then adjust the sighting device until the object lines up in it. Then check the view through the telescope again to be sure the object is still centered there. For good measure, go back and check the sighting device again too.

Finder Scope

A more precise device for telescope aiming is a finder scope , often called simply a finder . This is a small Keplerian refractor. Most finders have objective diameters of 40 to 60 mm and magnify several times. The eyepiece has a pair of fine threads or wires, called cross hairs , placed at its focus. These produce a + or × pattern in the view field. The intersection point of the cross hairs is at the center of the view field. The finder position is adjusted until a star that falls at the cross-hair intersection point also shows up in the center of the view field of the main telescope at high magnification.

A finder can be aligned using the same technique as is used for a simple sighting device. The best finders are mounted in a pair of rings, both of which are attached to the main tube of the telescope near the eyepiece. Each ring has three or four adjustment screws. These should be fairly tight (but not so tight that the finder is damaged or the screw threads are stripped). A few finders have single-ring mountings. These are unstable. It is best to stay away from them.

Barlow Lens

A concave or planoconcave lens can be inserted in any telescope between the eyepiece and the objective, and the effect is to increase the apparent focal length of the objective. This type of lens is called a Barlow lens . It is placed close to the eyepiece. The lens is mounted inside a cylinder designed to fit into the eyepiece barrel of the telescope at one end and around the barrel of the eyepiece at the other end (Fig. 20-8).

Your Home Observatory Telescope
Accessories Barlow Lens

Figure 20-8. A Barlow lens increases the magnification obtainable with a given eyepiece.

Because the Barlow lens increases the effective focal length of the objective, it provides increased magnification when a given eyepiece is used. Most Barlow lenses are rated at 2×. This means that they double the magnification for each eyepiece used. Some Barlow lenses are rated at 3×; these triple the magnification.

A Barlow lens can be useful in two situations. First, it eliminates the need for using eyepieces with extremely short focal lengths when high magnification is desired. An 8-mm eyepiece can be used in place of a 4-mm eyepiece, for example, when a 2× Barlow is inserted in the light path. Most people find an 8-mm eyepiece more viewer-friendly than a 4-mm eyepiece. Another asset of the Barlow lens is that it can double the number of obtainable levels of magnification, provided that you have chosen your eyepieces wisely. Suppose, for example, that you have a telescope whose objective has a focal length 1,000 mm and you have eyepieces whose focal lengths are 20 and 28 mm. This provides magnifications of 50× and 36×, respectively. If you obtain a 2× Barlow lens, you can obtain magnifications of 100× and 72× with the same two eyepieces. This gives you four well-spaced degrees of magnification.

A Barlow lens should not be used in an attempt to get extreme magnification. For example, if you have a telescope whose objective has a focal length of 2,000 mm and you use a 4-mm eyepiece with a 3× Barlow, you can theoretically obtain 1,500×. However, Earth’s atmosphere generally makes it futile to try for anything more than 500×, even with the largest telescopes. The slightest vibration will cause terrible wobbling of the image. In addition, the brightness of an observed image in any particular telescope decreases as the magnification increases. Remember the formula for the highest useful power you can get out of a telescope: approximately 20× per centimeter of objective diameter, or 50× per inch, with a maximum of 500× at sea level and most land-based locations.

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