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Telescope Information

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Telescope Information

 

 

Types of telescopes

The first decision you'll need to make is what type of telescope you want. All have their strengths and weaknesses. If you want to look at both birds and the Moon, for example, you'll want to stay away from a Newtonian reflector. If you're more interested in viewing deep-space objects on a tight budget, the reflector might be the perfect solution. It all depends on what you plan to do with your new telescope.

 

 

Refractors

Refractors are standard tapered telescopes with a large objective lens on one end and a small eyepiece on the other. Light passes through the large objective lens, which focuses it into a narrow beam that strikes the eyepiece, where final focusing can be adjusted to produce clear images.

 

Any time a lens is used to collect and focus light, imperfections can cause visual aberrations. Since light must pass through the lens, some light is dispersed to other parts of the lens in inferior glass designs, creating a colored ring around the object being viewed and other chromatic problems. Most inexpensive refractors now use an achromatic design, where the objective lens has two elements that cancel out much of this problem. The best refractors use an apochromatic design that uses even more lens elements, new kinds of lens materials like extra-low dispersion (ED) glass, or a combination of the two to entirely eliminate chromatic aberrations. Don't ever confuse an achromatic design with an apochromatic one. The latter is much more accurate, and therefore much more expensive, than achromatic refractors with similar aperture sizes.

 

Advantages

 

Refractors are popular because their sealed design rarely requires adjusting, and they are very easy to use. With the right optics (especially where apochromatic designs are concerned), refractors offer better overall image quality than competing designs. They are capable of displaying sharp, highly detailed images with incredible contrast since light passes straight through them without encountering any solid obstructions.

 

Disadvantages

 

As we've already discussed, apochromatic (and to a lesser extent, achromatic) designs are much more expensive than refractors with standard lenses, meaning these telescopes cost more than reflector designs with similar specifications. The design also makes refractors more heavy than reflectors, especially when the aperture size gets into the 6-inch or higher range.

 

A minor disadvantage these telescopes have is that the eyepiece is located at the very end of the tube. This can make them less comfortable to use than competing designs, but a special eyepiece adapter that reflects the light at a 90-degree angle (included with most refractors these days) greatly alleviates this problem.

 

 

 

Reflectors

Most reflector-type telescopes sold to consumers use a Newtonian design, where light enters one end of a large, straight tube, bounces off a concave focusing mirror at the other end of the tube, then travels back up the tube, where the concentrated beam is reflected off a secondary mirror and through the telescope's eyepiece for final viewing. Unfortunately, there is no way to just suspend the secondary mirror in the middle of the tube, so it is held in place by one or more spokes that block some of the light coming into the tube. It appears that any image you view with the telescope will be intersected by the spokes (and blocked by the secondary mirror), but once focused a reflector can produce crisp, clear images. The obstructions do mean, however, that contrast theoretically will never be quite as good as that of a refractor with similar specifications. In practice, few people will notice the difference if the mirrors are properly aligned.

 

Advantages

 

The main advantage of a reflector is its relatively low cost, but that's not all this design has going for it. The focuser is mounted near the front of the telescope, making Newtonian reflectors generally more comfortable to use than refractors (there's less stooping and bending over involved). Since reflectors use mirrors instead of lenses, they have none of the color reproduction problems associated with refractors and, therefore, don't require the expensive coatings and exotic materials used in refractors to overcome the problem. The other advantage to the mirror design is that it saves weight. Reflectors are always easier to lug around than refractors of the same aperture.

 

Disadvantages

 

Newtonian reflectors project upside-down images. This doesn't matter at all when looking at objects in the sky (well, it can make charts of the Moon confusing to read), but makes the telescope pretty much useless for looking at objects on the ground. The fact that light has to pass by the secondary mirror before hitting the primary mirror and bouncing back also causes some minor light loss that can impact the contrast of an object you are viewing. This usually has minimal impact, but can cause some problems in inferior telescopes.

 

Another drawback to Newtonian reflector designs is that the secondary mirror has to be perfectly centered for them to work as advertised, and occasionally users have to make manual adjustments (called collimating). With practice, these adjustments are simple, but it is an extra concern that refractor owners don't have to worry about.

 

 

 

Schmidt-Cassegrain

A big problem with reflector and refractor designs is that as the aperture increases, the length of the tube must also increase for the light to be properly focused. Once you get up to bigger aperture sizes of 8 inches or larger with these designs, they really become unwieldy.

 

A terrific solution if you want a large-aperture telescope is a catadioptric design that combines lenses and mirrors to simulate a larger tube size than actually exists. The most popular catadioptric design by far is Schmidt-Cassegrain, which reflects incoming light several times inside the telescope before it finally is focused in the eyepiece. Light enters the far end of the tube via a Schmidt lens, is reflected off a focuser mirror to a secondary mirror, then is reflected in a more tightly focused beam to the eyepiece. This combination of refracting and reflecting technology yields terrific image quality, yet lets a telescope with an extremely compact tube have a very high focal length. A Schmidt-Cassegrain design with an 8-inch aperture size can have a focal length of over 2,000mm compressed into a tube only 16 inches long. Compare this to a much smaller 4.5-inch-aperture Newtonian reflector, which needs an 34-inch tube for its 910mm focal length.

 

Advantages

 

Schmidt-Cassegrain telescopes are extremely portable, and their closed designs make them rugged and reliable. With wide apertures compared to their minimal overall lengths, Schmidt-Cassegrain telescopes produce some of the clearest, most colorful images available and have great contrast if the internal optics are of high quality. Unlike Newtonian reflectors, Schmidt-Cassegrain designs can be used for terrestrial viewing.

 

Disadvantages

 

High construction costs make these expensive compared to Newtonian reflectors with similar specifications. Other than that, Schmidt-Cassegrains offer a superb blend of performance and portability that generally justify their high cost. Another similar design, the Maksutov-Cassegrain, uses slightly less accurate optics to provide similar performance to Schmidt-Cassegrain designs at a much lower cost.

 

 

 

Types of mounts

A good telescope can never be used to its full potential when placed on an unstable mount. Most beginner telescopes come with fairly stable tripod mounts, while entry-level Schmidt-Cassegrains usually require users to buy a separate mount, and, if you are looking for one, you'll need to be familiar with the several different types.

 

 

 

Altazimuth

The most common type of mounts is of an altazimuth design, meaning it moves freely both horizontally and vertically. Tracking objects requires moving the scope on both axes, making things a little tougher than they could be. On the other hand, the freedom of an altazimuth mount makes the design ideal for viewing ground objects, and a solid altazimuth mount with slow-motion controls (or electronic controls) works well for beginners with only a little practice.

 

 

 

Equatorial

Once a celestial object is centered in a telescope using an equatorial mount, it can be tracked by merely moving the telescope right or left. Equatorial mounts are calibrated to follow the polar axis, obviating the need for vertical adjustments when tracking objects. They do require an extra setup consideration, as they must be pointed at Polaris (the North Star) before they work, but even beginners should have no trouble becoming familiar with the design within a few hours of use.

 

 

 

Dobsonian

An increasing number of reflector telescopes come with Dobsonian mounts, which use a simple design to save a lot of money. Dobsonian mounts sit flat on the ground on a swiveling base, with two "arms" extending upward. Pads on the side of the telescope fit into half-circle cutouts at the top of the arms, and the telescope is then free to rotate up, down, left, and right. Dobsonian mounts are incredibly stable, have few moving parts, and can really cut down on the overall cost of a new telescope.

 

 

 

Specifications

There are many specifications to consider when purchasing a telescope, and not all are of equal importance. Here's a breakdown of the types of numbers and features you'll see:

 

 

 

Magnification

Far too many first-time telescope buyers fall into the trap of comparing telescopes based on their advertised magnification levels. In reality, this is the least important specification to consider. A telescope's power is determined by the quality of its optics, its aperture size, its focal length, and the eyepiece you use. As a rule of thumb, never expect clear images from your telescope at more than 50x the size of its aperture in inches (a maximum of 150x for a 3-inch telescope, for example).

 

 

 

Aperture

All things being equal (optics quality, mounting system, etc.), a telescope's aperture is of primary importance. Aperture, usually measured in millimeters (but sometimes in inches), tells you the diameter of the light-gathering end of the telescope. The more light a telescope can gather, the better its performance will be, and, since telescopes are circular, small increases in aperture lead to enormous gains in light-gathering ability. For example, a 70mm telescope can take in 36 percent more light than a 60mm model.

 

 

 

Focal length

A telescope's focal length is the length of the path light takes before a focused image appears in the eyepiece. Don't mistake the length of the tube for the focal length, as Schmidt-Cassegrain designs reflect light inside the tube to artificially lengthen the true focal length of the telescope. Focal length is an important figure to know when determining the magnification power of your telescope.

 

 

 

Resolving power

Measured in arcseconds, this number tells you how well (in theory) the telescope can separate a closely spaced pair of binary stars. Smaller numbers are better.

 

 

 

Limiting visual magnitude

Objects in the sky are classified by brightness, expressed in levels of magnitude with successive levels being about 2.5 times brighter or fainter than the next level down or up. Smaller numbers represent brighter objects, and the brightest stars in the sky even have negative values. The average person can see stars to about the sixth magnitude, so you can use that for comparison purposes when looking at telescope specifications.

 

 

 

Focuser size

Eyepieces, the interchangeable lenses you look through, come in a variety of diameters. The focuser size tells you what diameters with which the telescope is compatible. Generally, telescopes that accept only 0.965-inch eyepieces are inferior to those that accept the more common, higher-quality 1.25-inch and 2-inch sizes.

 

 

 

Eyepiece types

Eyepieces have many names and specifications that you should be familiar with before making a purchase. First, there's the diameter of the lens, measured in millimeters. This figure, divided into the focal length of the telescope, determines the magnification power you'll achieve. A 9mm eyepiece used with a 910mm focal length telescope, for example, yields a magnification of just over 100x. A 25mm eyepiece used with the same telescope magnifies objects by about 36x.

 

Bear in mind that an eyepiece with a larger diameter provides a wider field of view, so it isn't always better to scan the skies at high magnification levels. Eyepieces also come in several types, which helps determine the maximum field of view. The most common types of lenses found with beginner telescopes are Kellner lenses and Plössl lenses. Kellners use a three-element design and top out at a 40- to 50-degree field of view. Plössl lenses use a more advanced (and expensive) four- or five-element design and provide clearer optics with a 50- to 52-degree field of view. There are many more types of lenses, using more or less elements and better or worse optics, but just remember that more elements and a wider field of view generally correspond to a higher-quality eyepiece. Also know that you don't have to use eyepieces made by your telescope's manufacturer.

 

Many telescopes come with a Barlow lens, which is used in conjunction with one of your other eyepieces to double, triple, or even quadruple the standard magnification. A 2x Barlow lens is a great upgrade for beginning telescope users, as it basically doubles the number of eyepieces you already have without costing a lot of money. There also are several filters you can use with your existing eyepieces, such as solar filters for observing the Sun and nebula filters that improve deep-sky contrast.

 

 

 

Finderscopes (viewfinders)

Those small telescopes you see piggybacking on large telescopes are called finderscopes or viewfinders. They are designed to provide a little magnification and a wide field of view for easily finding objects in the sky. Once calibrated, whatever is centered in the finderscope will be centered in the main telescope, making them invaluable tools for all types of viewing, but especially for looking at objects using high magnification levels.

 

 

 

Motorized controls and auto-finders

Many amateur telescopes come with motorized controls, which let users move the telescope up, down, left, and right by pushing buttons on a handheld remote control. If you opt for controls like this instead of a standard manual slow-motion control, be sure to get a unit that can move the telescope at a variety of speeds. The best units let users lock on to a target and then automatically follow it to compensate for Earth's rotation.

 

Auto-finders (also referred to as go-to devices) couple a computerized database with the motorized controls, giving the telescope the ability to automatically point to objects stored in the database. A good example of this is Meade's Autostar series, which "knows" the location of over 1,400 objects in its base version (the best version has more than 14,000 objects stored in memory). A guided-tour feature points out related objects and lists information about each object on the Autostar's small display. Autostar is a great way for beginners to see the most objects in the shortest amount of time, but be aware that it isn't perfect. Sometimes it points to the general part of the sky where an object is supposed to be, but it's up to you to actually get it centered. That's where star charts and planispheres, discussed in the next section, come in.

 

 

 

Other things you'll need

Beginners are sometimes frustrated when they take their new telescope out and discover that all they can find is the Moon. If you want to find double stars, galaxies, nebulae, and sometimes even planets, you'll have to invest in a good star chart or planisphere (star wheel). Even if you have something like the Autostar system that points out objects automatically, you'll have to know where a few stars are to align the instrument each time you use it.

 

Most beginners purchase a planisphere, which uses a rotating wheel to show what the sky should look like at the time you are viewing it. Planispheres are latitude specific, so make sure the one you buy will work where you live (there generally are six versions of the same planisphere available for users living in the Northern Hemisphere). A few of the books for beginning astronomers available here on the site come with planispheres that can get you started.

 

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Terms

 

Reflectors

Most consumer-grade reflector telescopes use a Newtonian design, where light enters one end of a large, straight tube, bounces off a concave focusing mirror at the other end of the tube, then travels back up the tube, where the concentrated beam is reflected off a secondary mirror and down through the telescope's eyepiece for final viewing. Since the secondary mirror cannot be suspended in the middle of the tube, it's held in place by spokes, which block some of the light coming into the tube. Despite the spokes, a reflector can produce crisp, clear images, and if the mirrors are properly aligned, few people will notice the difference between them and theoretically superior refractor designs.

 

 

Refractors

Refractors are standard, tapered telescopes with a large objective lens on one end and a small eyepiece on the other. Light passes through the lens, which focuses it into a narrow beam that strikes the eyepiece, where final focusing can be adjusted to produce clear images.

 

 

Primary aperture

Determines the telescope's ability to gather light. Usually measured in millimeters (but sometimes in inches), aperture is the diameter of the telescope's main optical element, be it a lens or a mirror. The more light a telescope can gather, the better its performance will be, and, since telescopes are circular, small increases in aperture lead to enormous gains in light-gathering ability. For example, a 70mm telescope can take in 36 percent more light than a 60mm model.

 

 

Focal length

Focal length is the distance the path of light takes from the front of the telescope until a focused image arrives in the eyepiece. Don't mistake the length of the tube for the focal length, as certain designs reflect light inside the tube to artificially increase the true focal length of the telescope.

 

 

Advertised maximum power or magnification

The magnification a telescope is theoretically capable of providing. This is mostly an irrelevant number; if you switch eyepieces, a telescope can be made to magnify at almost any power, but there is an absolute limit to the resolution possible for each aperture. As the magnification is pushed beyond this limit, the image will stop revealing any additional detail.

 

 

Resolving power

A number amount indicating how well (in theory) a telescope can separate a closely spaced pair of binary stars. Measured in arcseconds; smaller numbers are better.

 

 

Limiting visual magnitude

A way to specify the light-gathering power of a telescope by indicating the faintest star the telescope is capable of showing, though this is really determined by the size of the aperture. This is registered in "magnitudes"--a 1st magnitude star is the brightest, while a 15th magnitude star can only be seen with a powerful telescope. Often, the limiting visual magnitude figures stated by manufacturers refer to the generally accepted figures for any telescope at that aperture.

 

 

Altazimuth

A mount allowing a telescope to move freely both horizontally and vertically. This is a common mounting, though tracking objects requires moving the scope on both axes, which can be difficult. The freedom of this mount makes the design ideal for viewing ground objects though, and a solid altazimuth mount with slow-motion controls (or electronic controls) works well for beginners.

 

 

Equatorial

A telescope mount that allows tracking by merely moving the telescope right or left once a celestial object is centered on. Equatorial mounts are calibrated to follow the polar axis, eliminating the need for vertical adjustments when tracking objects. They must be pointed at Polaris (the North Star) to work, but even beginners should have no trouble becoming familiar with the design after a few hours of use.

 

 

Dobsonian

Dobsonian mounts sit flat on the ground upon a swiveling base, with two "arms" extending upward. Pads on the side of the telescope fit into half-circle cutouts at the top of the arms, and the telescope is then free to rotate up, down, left, and right. Dobsonian mounts are incredibly stable, have few moving parts, and can really cut down on the overall cost of a new telescope.

 

-- Author unknown

 



 

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