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How To Select Your First Telescope

1. How To Select Your First... 2. Magnification and Using Eye... 3. Using a Barlow Lens 4. Image Orientation
5. Telescope Mounts 6. Star Parties 7. Polar Alignment 8. Adjusting Your Eyes...
9. Eyepiece Formats 10 Electronic GOTO and GPS... 11. Got a Nice View? 12. Observing Our Closest Star
13. Filters Filters Filters 14. Using Binoculars for Astronomy 15. What Can You See... 16. Astrophotography
17. Can you see the Flag or... >> Back to the 101 index <<

Looking for a new telescope can be very confusing. There's a bewildering array of telescopes and telescope accessories out there. In fact, it seems as though you almost need to learn another language to understand the field. You'll need to consider cost, portability, versatility, usability and appearance, and a host of other factors. Truly, the task can seem daunting.

It's important, therefore, to know some important ground rules. The knowledge will help you better understand the different types of telescopes and aid in choosing the scope that's best for you.

First Things First A Word About Magnification. Magnification or "power" is one of the least important factors to be considered when you're choosing a telescope. The single greatest misconception about telescopes is that you need to have lots of magnification to see objects. Not true. The telescope's light-gathering ability, referred to as aperture, plays the most prominent role in determining how much you see. Magnification level has much less to do with it. Be particularly cautious of outrageous claims of 500X or 600X magnification on inexpensive telescopes. It's true that magnification levels can be pushed sky high with certain eyepieces and optical appliances. However, image quality suffers severely to the point where it can be difficult to make out any detail at all. It's far better to view with a telescope of larger aperture and moderate magnification than to push a smaller scope to ultra-high magnification levels. Image size will be a bit smaller with less magnification. But because the image you do see will be of far better quality, you'll actually see a lot more! Some manufacturers of "department-store-brand" telescopes take advantage of this number-one misconception among first-time telescope buyers that magnification is everything. It isn't. So beware!

Celestron NexStar 130 SLT Telescope

Aperture Bigger is Better & Sometimes. The most important attribute of a telescope is its aperture size. This is the diameter of the telescope's light-gathering lens or mirror often referred to as the telescope's objective. It's usually measured in millimeters or inches. Why is aperture size important? Because it's like having a bigger eyeball with which to view the heavens. The human eye is a marvel of genetics and engineering, but it does a relatively poor job of seeing in the dark. The big reason is that our window on the world is so small. Humans have a maximum pupil diameter of only about 7mm at full dilation (less as the eye ages). That means all the light the retina can receive is what's able to squeeze in through a tiny opening. No wonder we're only able to discern the brightest of the deep-space objects! The minuscule aperture of our eyes simply doesn't collect enough light to allow us to see fainter objects. In effect, we see the heavens with built-in telescopes only about one-quarter inch in diameter! Man-made telescopes provide a remedy by improving light-collection ability. A telescope with a 6-inch aperture has a diameter 24 times as great as our fully dilated pupil. But the impact on vision is even greater than that. The area the amount of surface able to admit light of the 6-inch-aperture telescope is about 28.25 square inches. Compare this to the .05-square-inch area of the eye's pupil, and you can see there's a big difference. In fact, a telescope with a 6-inch aperture will admit more than 565 times as much light as the human eye a huge difference. A telescope's optics focus this added light into a beam small enough to get through our dilated pupil, and voila! we're able to see objects much dimmer than those we can see with the unaided eye. That's why aperture otherwise known as a telescope's light grasp is so important.

Ok. So a telescope's ability to gather light is paramount. Should you, then, look for the largest telescope you can possibly afford? Not necessarily. You don't want to forget about portability. After all, what good is a monster telescope if you don't use it? You will need to ask yourself Where will I want to use my new telescope?If the answer is nearby in the backyard, then having a great big telescope will be of great advantage to you. On the other hand, if you'll need to take your telescope away from city lights in order to get good viewing, you need to be concerned about size and weight. You'll have to wrestle your scope into your car or truck, so, beyond a certain point, bigger and heavier is definitely not better. It's also good to remember you have to assemble and set up your telescope in the dark, and that also makes ease-of-use a premium commodity. "Dark-sky commuters" should think, therefore, in terms of the biggest scope they can handle comfortably, rather than the biggest scope they can afford. They'll get little enjoyment of a new telescope if it just gathers dust in the garage because the thought of moving it outdoors makes them cringe. Sometimes more of a good thing is just too much. Beyond aperture: Your first major decision - What Type of Telescope should you buy?

Refractors, reflectors and catadioptrics demystified.

There are three basic types of telescopes refractors, reflectors, and catadioptrics. All three designs have the same purpose, to collect light and bring it to a point of focus so it can be magnified with an eyepiece for examination by the eye. Each does this work differently, though. And each has advantages, as well as disadvantages. We offer a discussion to help you choose among them.

Refractors

Refractor Telescope Diagram Refractor Telescopes

Refractor telescopes are what the average person identifies with the word "telescope". These consist of a long, narrow tube in which light passes in a straight line between the front, objective, lens and a rear-mounted eyepiece.

Advantages
>>Simplicity of design contributes to ease of use and reliability;
>>Require little or no maintenance;
>>Excellent for lunar, planetary and binary star observing, especially in larger apertures;
>>Good for distant terrestrial viewing;
>>Offer high-contrast images with no secondary mirror or diagonal obstruction;
>>Render good color in achromatic designs and excellent in apochromatic, fluorite and ED designs;
>>Sealed optical tube reduces image-degrading air currents and protects optics,
>>Have permanently mounted and aligned objective lenses.

Disadvantages
>>More expensive per inch of aperture than reflector or catadioptrics designs;
>>Heavier, longer and bulkier than equivalent-aperture reflectors and catadioptrics;
>>Cost and bulk factors limit the maximum practical aperture size,
>>Less suited to viewing small and faint deep-sky objects because of practical aperture limitations.

Reflector
Reflector telescopes usually use a concave, parabolic primary mirror to collect and focus incoming light onto a flat secondary mirror that in turn reflects the image out of an opening at the side of the main tube, into an eyepiece for viewing.

Reflector Telescopes Diagram Reflector Telescopes

Advantages
>>Lower cost per inch of aperture than offered by refractors and catadioptrics, since mirrors can be produced at less cost than lenses in refractors in medium to large apertures;
>>Reasonably compact and portable;
>>Excellent for faint, deep-sky objects, such as remote galaxies, nebulae and star clusters, because of their larger apertures,
>>Deliver very bright images with few optical aberrations.

Disadvantages
>>Generally not suited to terrestrial observation,
>>Slight light loss due to obstruction from the secondary mirror.


Catadioptrics
Catadioptric telescopes use a combination of mirrors and lenses to fold the light and form an image. Catadioptrics are the most popular type of instrument, with the most modern design, marketed throughout the world in apertures of 3.5 inches and larger. There are two popular designs of these mirror/lens "hybrids", the Schmidt-Cassegrain and the Maksutov-Cassegrain. In the Schmidt-Cassegrain the light enters through a thin, aspheric Schmidt correcting plate, then strikes the spherical primary mirror and is reflected back up the tube, where it is intercepted by a small, secondary mirror. This reflects the light out an opening in the rear of the instrument to form an image at the eyepiece
Catadioptric Telescopes Diagram Catadioptric Telescopes

Schmidt-Cassegrain Advantages
>>Best all-purpose telescope design, combining the optical advantages of both lenses and mirrors, while canceling their disadvantages;
>>Excellent optics with razor-sharp images over a wide field;
>>Excellent for deep-sky observing or astrophotography with fast films or CCD imagers;
>>Very good for lunar, planetary and binary star observing or photography;
>>Excellent for terrestrial viewing or photography;
>>Closed-tube design reduces image-degrading air currents;
>>Compact and portable;
>>Easy to use;
>>Durable and virtually maintenance-free;
>>Offer large apertures at reasonable prices less expensive than equivalent-aperture refractor designs;
>>Greater accessory availability than is the case with other types,
>>Best near-focus capability of all designs.

Schmidt-Cassegrain Disadvantages
>>More expensive than reflectors of equal aperture;
>>It is not what people expect a telescope to look like,
>>Slight light loss due to obstruction by the secondary mirror.

Maksutov-Cassegrain
The Maksutov design is a catadioptric (using both mirrors and lenses) with basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus-correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt's giving it slightly better resolution for planetary observing.

Maksutov-Cassegrain Telescope Diagram >

The Maksutov is heavier than the Schmidt and, because of the thick correcting lens, takes a long time to reach thermal stability at night in larger apertures.

The Maksutov optical design typically is easier to make, but its corrector lens requires more material than the Schmidt Cassegrain's.

Now that you have read about magnification, aperture and the different types of telescopes, you can browse our site more informed. The choice of what telescope to buy or whether to buy one at all is up to you. Make it based on your individual circumstances and needs. Meanwhile, remember we are here to help! You can always send us an email or contact one of our telescope specialists at 1-800-303-5873. We don't just sell telescopes. We're interested in astronomy, and we're interested in helping others learn about it. Happy viewing from Telescopes.com!

1. How To Select Your First... 2. Magnification and Using Eye... 3. Using a Barlow Lens 4. Image Orientation
5. Telescope Mounts 6. Star Parties 7. Polar Alignment 8. Adjusting Your Eyes...
9. Eyepiece Formats 10 Electronic GOTO and GPS... 11. Got a Nice View? 12. Observing Our Closest Star
13. Filters Filters Filters 14. Using Binoculars for Astronomy 15. What Can You See... 16. Astrophotography
17. Can you see the Flag or... ? ? >> Back to the 101 index <<

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