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What you need to know before you buy. Updated for Holidays 2012
Updated for Holidays 2012
Get set for amateur astronomy and some cool winter stargazing with this handy guide on how to buy a telescope. Armed with the right down-to-earth info, your view will be unbeatable.
Astronomy images courtesy Bushnell
If you ever thought about getting to know the stars and constellations a little better, now is the perfect time of year to start in astronomy. Winter’s icy cold evenings offer some of the most brilliant and star-studded constellations you’ll ever see. With the right telescope, you’ll be getting up close and personal with Orion’s Belt, Canis Major, and Sirus the Dog. Follow this guide to getting started with telescopes, and you’ll even know if Venus and Mars are alright tonight.
This handy reference guide will help you make sense of the different types of telescope models available. Armed with these telescopic tips, you'll have a good idea about what to look for when scanning the market for your new stargazing device.
Adorama carries one of the largest lineups of telescopes in our Binoculars & Telescopes department, which is staffed by some of the world's leading experts on astronomy and astrophotography. Be sure to check out our Scoop on Scopes for our selected favorite models right now, or browse the department for the telescope that meets your needs.
I’m Looking Through You
First off, the telescope you want to buy must have two essential things: high-quality optics and a rock steady, smoothly working mount. In addition, portability and convenience are things to consider in your purchasing decision.
If you live in the city or a well-lit suburb and seek deep sky viewing, you are going to want to purchase a scope that’s small enough to pack in your car so you can drive to darker locations for optimal viewing. And, if you are viewing from your own backyard, look at models that aren’t too bulky or heavy to transport outdoors.
Aperture is the Key
A telescope's main function is to gather light, and the key characteristic of any telescope is its aperture—the diameter of its light-gathering lens or mirror. It is also referred to as the objective. Aperture is so important that it’s always the first feature stated when talking about a telescope’s specs. You can find the telescope's aperture near its focuser, at the front of the tube of the unit. The aperture's diameter is expressed either in millimeters or in inches (1 inch equals 25.4 mm). The telescope you want to buy should have at least 2.8 inches (70 mm) aperture — and in most cases, preferably more.
The objective's focal length is the key to determining the telescope's magnification or power. This is simply the objective's focal length divided by that of the eyepiece, which you'll find on its barrel. Therefore, if a telescope has a focal length of 500mm with a 25-mm eyepiece, the magnification is then 500/25, or 20x. Most telescopes do come with two eyepieces, so you can simply change the magnification by switching eyepieces with different focal lengths.
A larger aperture lets you see much fainter objects with finer detail than a smaller one can. But a good small scope can still show you a lot too, especially if you live in a rural area that’s far from bright city lights.
Basically, there are no fundamental differences between telescopes and camera lenses. If you decide the take your hobby to the next level of astrophotography; you can use your telescope as a camera lens to shoot the stars. You can also buy adapters to convert camera lenses into astronomical telescopes.
There are three main types of telescopes: refractor, reflector and catadioptric.
The most common type of telescopes is called refractor telescopes, which are available in abundance at Adorama. This name also refers to most binoculars, camera lenses and even eyeglasses. Refractor telescopes go all the way back to the days of Galileo. They work by bending light through a primary convex lens known as the objective lens, and forcing the redirected light rays to converge at a space known as the focal point. Then the light rays are focused – most of the time by a prism or a second lens – into the eyepiece, where they are usually refracted again to form an exact replica of the image. While generally ruggedly constructed, low-maintenance units, they quickly get more expensive as the aperture increases. Refractors make great starter scopes, and are perfect for using in urban or well-lit areas.
The Explore Scientific EDT-0806-CF ($1,149.99) is a full featured model in the refractor family. It has an 80 mm aperture and a 480 mm focal giving it a focal ratio of f/6. It has a retractable dew shield that not only keeps moisture from accumulating on the lens but also helps to shield it from outside light.
Reflector Telescopes operate on an entirely different principle altogether. Instead of using lenses to capture and focus light, reflectors utilize large-aperture “light buckets” or tubes that funnel light down the shaft towards a mirror, which in turn reflects the light into an eyepiece consisting of several different types of lenses. They are the lowest cost per inch of aperture, offer a simple design and are easy to adjust and modify. Reflector telescopes do require a lot of space with little background light. They tend to make ideal scopes for utilizing in rural and darker locations. They are sometimes called Newtonian because a type of reflecting telescope was invented by Sir. Issac Newton.
A popular reflector model is the Celestron Advanced VX 6” Newtonian Telescope ($899.00). An entry level telescope that includes a mount and is a 150 mm aperture with a 750 mm focal length and a 6x30 finderscope.
A third variety is the catadioptric telescopes, also known as a compound telescope. They use a combination of mirrors and lenses to increase the scope’s focal range and power while keeping weight, length and bulkiness to a minimum.
The most popular type of catadioptric telescope is the Schmidt-Cassegrain, used by amateur astronomers all around the world. This type is based on inventor Bernhard Schmidt’s original camera lens design. In 1930 he invented the Schmidt telescope which corrected for the optical errors of spherical aberration, coma, and astigmatism, making possible for the first time the construction of very large, wide-angled reflective cameras of short exposure time for astronomical research.
There is second type of design called the Maksutov-Cassegrain or 'Mak’. The Maksutov-Cassegrain design employs a full diameter positive meniscus lens to correct the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting chromatic aberration.
Schmidt-Cassegrain scopes offer a corrector plate as the first optical element, which is then figured by placing a vacuum on the internal side, and grinding the lenses and mirrors to the exact specification and angle required to correct the spherical aberration caused by the primary mirror. There are many variations of the Schmidt-Cassegrain configuration, but for the most part, they can be divided into two principal design forms: compact and non-compact.
In the compact form, the corrector plate is located at or near the focus of the primary mirror. The non-compact version moves the corrector plate back towards the center of the primary mirror. This gives the effect of keeping the corrector at the center of curvature of the primary mirror.
Catadioptric scopes offer the most compact tube per inch of aperture and the compact tube is also easy to mount. They also are more expensive, but can operate well in a host of different environments. They do make good entry-level scopes, but with their bigger price tag, are more for the serious user.
There are also the Dobsonian telescopes which first became popular in the late 1960s. Not really suitable for urban environments they feature a large objective diameter with a relatively short focal length and are designed to observe deep sky objects that are very faint such as nebulas. They tend to be portable so that they can be moved to remote areas where there is less light pollution from cities.
The Dobsonian models start around $400 space with models such as the Orion XT8 8” f/5.9 Classic Dobsonian Telescope for a beginner to the high end models such as the Meade LightBridge 16” Truss-Tube Dobsonian Telescope, Deluxe Version for ( $1,999.00). The telescope, seen below, with its large optics can gather details from the darkest corners of the sky providing users with images of large sections of the night sky.
In most cases, the mount is as every bit as important as the telescope it supports. The main job of the mount is to keep the telescope from shaking, just like a tripod with a camera. But even more so for a scope; at high power every little vibration is magnified so much that the entire object can become blurry and out of focus. The mount must also allow the telescope to move very smoothly and quickly. To keep the celestial object you are viewing centered, you have to move the telescope frequently and at tiny increments.
Many telescopes come complete with tripods or mounts, although the tubes of smaller scopes often just have a mounting block that allows them to be attached to a standard photo tripod with a single screw. Mounts that are designed specifically for telescopes usually omit the single-screw attachment blocks in favor of larger rings or plates.
The simplest type of mount is the Dobsonian mount. They are good at keeping the scope steady and allowing it move smoothly. They are relatively inexpensive, lightweight and are ideal for large reflector scopes. “Dobs” can also be equipped with electronic “Go To” drives and motors that tell the user which way to push the scope to reach your target. Some “Go To” programs let users choose an audio guided tour of the best celestial showpieces, complete with a digital readout describing what's known about each object.
Equatorial mounts, which have one axis parallel to the Earth’s axis of rotation, have been the mount of choice for the serious stargazer. These are the only ones that can track heavenly objects without the need of a computer related device. However, with the recent introduction of the Dobsonian design and computer-controlled tracking, altazimuth mounts have also become the mount of choice of professionals. An altazimuth mount permits the scope to move up-down and left-right. It’s quick and easy to set up and use. An equatorial mount can track celestial objects by turning just one axis and can be more easily motorized — but to work correctly it must be properly aligned with Polaris (North Star).
Purchasing your first telescope will take some time, money, and research. A telescope can literally open your eyes to a new world of star-studded wonders. With a little patience in selecting the right one, you'll be off on a new celestial journey exploring the night skies faster than a speeding comet.