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Captain's Log

January 2013

Astronomy Telescope Buying Guide

Astronomy Telescope Buying Guide

Astronomy Telescope Buying Guide

You don't need to go far to learn more about the universe - with clear skies, you can observe the moon, stars, and planets from home. And you don't need hugely expensive or technical equipment - our entry-level telescopes start at less than £80. To help you understand what to look for in a quality telescope, the editors of Astronomy magazine answer 10 of the most-asked questions:
1 I know telescopes make things appear bigger, but what exactly do they do?
A telescope’s purpose is to collect light. This property lets you observe objects much fainter than you can see with your eyes alone. Italian astronomer Galileo Galilei said it best when he declared that his telescopes “revealed the invisible.”
2 Will my telescope be complete, or will I need additional items to make it work?
Most entry-level telescopes are complete systems, ready for the sky as soon as you unpack and assemble them. A few models are “optical-tube assembly only.” This means all you’re buying is the optics in the tube with no tripod or accessories.
3 I’m interested in observing. What should I do first?
Learn all you can about telescopes: what types are available, the best accessories, and what you’ll see through them.
4 Should I buy binoculars before I buy a telescope?
No. The view through binoculars — especially near a city — won’t be what you expect. They are, however, a valuable accessory at a dark site. Star clusters look great through them, as do the Milky Way, meteor trails, and the Moon.
5 Why are objects through my telescope upside-down?
Because of the way a telescope focuses light, the top of what you’re looking at is at the bottom as it enters the eyepiece, and vice-versa. You can re-flip the image with an accessory called an “image erector,” but you’ll lose a bit of the object’s light. And for faint sky objects, you want the maximum amount of light possible to reach your eye. Besides, there’s no up or down in space, and with most objects, you won’t even know they’re upside-down.
6 Can I use my telescope for views of earthly objects?
Absolutely! Many night-time observers (usually those with smaller telescopes) also use their telescopes for bird-watching or other daytime nature-watching activities. Here’s where the image erector comes in most handy.
7 Is there a way for me to “test-drive” a telescope?
Yes. Look in your area for an astronomy club and visit one of its meetings, which usually occur monthly. There, you’ll find others who enjoy the hobby and are willing to share information and views through their telescopes. At one of the club’s stargazing sessions, you’ll be able to look through many different telescopes in a short period and ask all the questions you like.
8 Apart from quality optics, what’s the most important thing in a telescope system?
The mount, which is what the telescope’s tube sits on. You can buy the finest optics on the planet, but if you put them on a low quality mount, you won’t be happy with your system. No telescope can function in high winds, but a poor mount will transfer vibrations even in a light breeze. So, be sure your scope sits on a high-quality mount.
9 If I use my telescope outside, does it need electricity?
Only if it has a motorized drive. In most cases, telescope drives use direct current, which means you can use batteries (including the one in your car). Adapters available from the manufacturer will let you plug your scope into an electrical outlet.
10 What’s the best telescope for me?
It’s the one you’ll use the most. If it takes an hour to set up a scope, or if your scope is large, heavy, and difficult to move, you might observe only a handful of times each year. If, on the other hand, your scope is quick to set up, you may use it several times each week. A small telescope that’s used a lot beats a big scope collecting dust in a cupboard every time.

All about Refractors
• Refractors use a lens system to produce images.
• Refractors require the least maintenance of all telescopes.
• Many small refractors are light enough to mount on a sturdy camera tripod, making them the ultimate grab-and-go scopes.
What to consider
• Nothing blocks any of the light passing through the lens, which makes image contrast better. Observers of planets and double stars (who need high contrast to resolve small details) say that refractors are best for such objects.
• Refractors are low maintenance. Lenses never require recoating like mirrors eventually do. Also, a lens usually doesn’t need adjustment — what telescope-makers call “collimation.” The lens does not get out of alignment unless the scope encounters a major trauma like falling onto a hard surface.
• Because a refractor has a closed tube, it requires some time to adjust to the outside temperature when moved from a warmer or cooler house. Today’s thin-walled aluminium tubes conduct heat well, so they have reduced the cool-down time a lot. But you may still have to take it into account.

All about Reflectors
• Reflectors use a system of mirrors to produce images.
• A reflecting telescope offers the best “size per pound” ratio.
• The largest amateur telescopes are reflectors.
What to consider
• Reflecting telescopes show no excess colour. That means you won’t see colour fringes around even the brightest objects.
• Inch for inch, reflectors are less expensive than other telescope types. When working with a mirror, manufacturers have to polish only one surface. An apochromatic lens has between four and eight surfaces, plus you’re looking through the lenses so the glass has to be defect-free. All of this makes such lenses more expensive. Telescopes with apertures of more than 6 inches, with few exceptions, are all reflectors or compound telescopes.
• The placement of the secondary mirror creates an obstruction that scatters a tiny amount of light from bright areas into darker ones. Unless you’re looking at a planet or bright nebula under high magnification, you’ll never notice this.
• Newtonian reflectors suffer from “coma,” a defect that causes stars at the very edge of the field of view to look long and thin like a comet. Observers generally compensate for this by placing all targets at the centre of the field.
• Because of how the mirror attaches to the tube, a reflector is sensitive to bumping or jostling when transported. To be sure all is well, many skygazers collimate their telescopes (adjust the mirrors) before each observing session.

All about Compound Telescopes
• Compound telescopes employ a combination of lenses and mirrors to produce images.
• They have the most compact design.
• Manufacturers usually sell them as complete systems.
What to consider
• The number-one advantage of a compound telescope is its compact design. Such instruments are often only one-quarter as long as comparably sized reflectors and much shorter than refractors with half their aperture. This feature makes the compound telescope a great grab-and-go instrument.
• Like refractors, compound telescopes also have a closed tube. Adjusting to the outside temperature, therefore, takes longer than with an open-tube reflector with the same size mirror.


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