Astronomy

How to Choose a Telescope



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A telescope is a powerful tool used in exploring objects throughout the universe. They literally enable us to see the invisible! Telescopes are also a time machine, allowing us to peer into the past. The word telescope was derived from the roots tele, which means "distant," and skopos, which means "to see." So a telescope is an instrument that allows us to see distant objects, such as the Moon, planets, stars and star clusters, nebulae and remote galaxies. 2009 marked the 400th anniversary of the first telescope pointed to the night sky by the famous mathematician, scientist and astronomer Galileo Galilei. Modern telescopes are far superior in optical quality than these earlier instruments. Just like the pupil of our eye gets larger in the dark to let in more light, the larger the telescope's optics the more faint light from distant stars and galaxies appear brighter, allowing us to see further and deeper into space. A second benefit with a larger telescope is its ability to resolve smaller and finer details on extended objects like the Moon and planets, and permit the clear separation of close double stars.

Perhaps you are interested in purchasing a new telescope. Basically, there are two types of telescopes to choose from. A refractor uses lenses to collect and bend light as a cone to a focus. Binoculars are merely two refractor telescopes mounted side by side. Reflectors use a set of mirrors to gather light, which is brought to a focus by virtue of a concave curve (inward like the scoop of a spoon) on the front surface of the primary (largest) mirror. Light enters a mostly hollow tube and reaches the primary mirror at the bottom. As the reflected cone of light (due to the curve) travels up the tube, it is intercepted by a smaller flat (plane) diagonal mirror set at a 45 degree angle with respect to the light path. 45 + 45 = 90 degrees, so the light is sent outside the tube at a right angle for the observer to inspect a focused image through an eyepiece (ocular). This is a classic Newtonian reflector, named after another famous scientist, Isaac Newton who created its design. The distance between the primary objective (lens or mirror) and the eyepiece where the focal point is reached is called focal length. This is determined by how steep or shallow the curve in the glass is. A greater curve will focus light in a short distance, so the telescope tube will be correspondingly shorter as well. A shallow curve will extend this distance, calling for a longer tube assembly. Many reflectors are referred to as compound telescopes because of their short, stubby tubes. This cassegrain design uses a steeply curved primary mirror and a convex (curved outward like a ball) secondary mirror mounted near the top center of the tube. When light reflects off of this convex curved mirror, the steeply converging (come together) rays of light are made to diverge (spread apart), thereby effectively extending the focus further so the light path will continue through a central hole in the primary mirror (like a donut) and focus outside the rear of the tube assembly. Many cassegrains use a special glass plate at the front of the tube to "correct" the light path from different problems inherent in this design. They may be called a Schmidt Cassegrain or a Maksutov.

In order to point the telescope's optical tube assembly at a particular location in the night sky, it will need a mounting. There are basically two types of telescope mountings. An altazimuth mounting has two axes at right angles to each other where one axis allows the telescope to pivot up and down (altitude) and the other axis left and right (azimuth). It's the simpler of the two. The other type is called an equatorial mounting. It also uses two axes at right angles to each other, but one of them, called the polar axis, is set in line with the earth’s axis of rotation. Once accomplished, you simply set the declination (north-south) axis and right ascension (east-west) on the polar axis to point at a particular object, then just rotate westward on the polar axis to track an object in the sky as it appears to move due to the earth’s rotation. Setting circles may be attached to both axes for locating objects using their celestial coordinates (right ascension and declination). If the polar axis has a clock drive motor, it will automatically guide this tracking at the same rate the earth is turning. Many commercially-made telescopes now come with computer controlled guiding systems and a push-button hand paddle known as "Go-to"capability. This is great for taking pictures through the telescope, known as astrophotography. If not, hand knobs with worm and gears are typically used to manually guide the instrument. Either way, the mounting is supported typically on a pedestal or tripod. Some common types of equatorial mountings include the German, fork, English yoke, and others.

Many accessories are available or required to properly operate a telescope. A finder is either a small refractor telescope with a wide field of view and crosshairs or a laser device used to point the main telescope accurately and "find" the object sought for viewing. Eyepieces come in different types and sizes. Magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. For example, a telescope with focal length 900mm using an eyepiece with a focal length of 20mm will yield a magnification of 45X (900/20=45). Another way to say this is 45 power, which means that objects will appear 45 times larger in diameter than with the unaided eye. A common misconception is that magnification determines how powerful a telescope is. Since magnification can be adjusted using different focal length eyepieces for any telescope, the telescope's true "power" is determined by its size (aperture or diameter). A Barlow lens can amplify the magnification of any given eyepiece by effectively extending the focal length of the telescope. Commonly, they double or triple the magnification of the eyepiece used, so in our previous example we now have 90X or 135X with the same eyepiece. Filters are typically threaded to screw into the barrels of eyepieces and come in different colors to enhance specific features of planetary detail. Lunar filters work well to reduce glare and increase contrast on the Moon. Solar filters block all the harmful rays allowing safe viewing of sunspots. Other filters are used for reducing scattered light as in city areas permitting views of faint extended objects such as nebulae. A star diagonal prism or mirror changes the position of the eyepiece by creating a right angle to the normal light path. This works well with refractors or cassegrain reflectors, especially when viewing objects high overhead. Dew caps extend the tubes length preventing dew from forming on the surface of lenses or corrector plates.

What advantages/disadvantages exist for either type telescope design? Well, if you want as large and powerful a telescope as feasible within your budget, you must consider whether you intend to place it in the car to transport it to a dark sky site or you intend to permanently mount it in your back yard or observatory. Again, a long-focus telescope will have a longer tube, possibly prohibiting it from fitting in the car easily. A short-focus telescope, sometimes called richest field telescopes or RFT's, are great for looking at wider fields of view in the sky, capturing beautiful views of rich star clusters and several objects in relation to each other simultaneously. Long-focus telescopes are usually superior for examining fine details on planets and splitting close double stars. The trade-off is loss of portability. Refractors have an optical advantage over reflectors in that they don't suffer diffraction (light scattered or bent around the edge of a barrier) caused by the central obstruction to the incoming light by the diagonal or secondary mirror. If the lens is of high optical quality, the refractor will typically outperform an equal sized reflector in producing sharply-defined images. Refractors are commonly made with a long focus, making the tube length a concern. Also, refractors are far more expensive than reflectors of equal size. Since the size of the telescope's lens or mirror is a function of its light-gathering power, the reflector is preferred overall when larger sizes are desired.

How about some tips for enhancing your observational experience through a telescope? When beginning any observing session, always start with the longest focal length eyepiece and the widest field of view to help spot objects more easily in the telescope. Simply align the telescope by sighting along the top of the tube and pointing it in the general direction of the object in the sky. If you have an equatorial mounting with setting circles, you can look up the celestial coordinates of the object and adjust accordingly. Of course, if you have Go to capability and have done a 2 or 3 star alignment, you may simply push a button on the hand paddle. Either way, your next step is to spot the object in the finder and align so it appears in the center of the crosshairs (like a gun sight). If your finder is optically aligned with the main telescope, the object should appear in the field of view of the eyepiece. When observing faint objects, try using a technique known as averted vision. Instead of looking directly at an object, look off to the side a bit and see if you notice that the object appears brighter. That's because that part of your eye's retina has more cones, which are sensitive to light and dark. Become familiar with the night sky by using a planisphere, commonly referred to as a star finder. Get to know the brightest stars and seasonal constellations by name. Attend a planetarium show to learn their relative positions in the sky. It takes time, but the universe is a very patient place, one that doesn't mind waiting while we take the first steps towards understanding.

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