How to Buy a Telescope

Four hundred years ago, Galileo Galilei pointed a tiny handmade telescope toward the stars and the modern Age of Science began.

Galileo was a mathematician and philosopher, but when he published what he had seen with his 33 power device - mountains and craters on the moon, "handles" on Saturn, moons circling Jupiter - the Renaissance world was stunned. Soon hundreds of Europeans were star gazing through telescopes.

Galileo's simple telescope was a Refractor - a large convex (bowed out) glass lens set at one end of a tube, and a small concave (bowed in) lens set in an adjustable piece at the other end of the tube. Moving the small, adjustable Objective lens (the Eyepiece) changes the focus.

There was a problem with this system, though - it makes a "rainbow" around bright objects (chromatic aberration). The more you magnify (the bigger the lens), the worse it gets. This distorts stars and planets, and makes viewing them difficult.

Isaac Newton came up with an answer to this problem. He realized that putting a mirror with a parabolic (bowl-shaped) curve on its reflecting side at the end of a telescope tube would allow him to collect a lot of light without distortion.

Newton placed a small, flat mirror near the other end of the tube to bounce the light to an Eyepiece placed on the side of the telescope. This was the first Reflector telescope. The enormous 200-inch telescope on Mt. Palomar in California is a Reflector.

Today's Refractors use multiple lenses, each carefully ground on both sides, to overcome chromatic aberration and give greater magnification with a shorter overall length. Each of these lenses must be very precisely ground to exact shapes to work properly. Consider this: each piece of glass in a telescope - and the grinding it requires - increases its price.

Reflectors have a large mirror that is only ground on one side, and a small flat mirror. Each type uses the same Eyepiece, which usually has multiple lenses carefully ground on both sides (which is why tiny Eyepieces seem so expensive compared to the much larger primary lens or mirror).

WHO?

Most likely you're reading this because you're about to purchase your first telescope, or give someone their first. Let's speculate that you're giving a first telescope. Think about these questions:

• How old and how mature is the person you're giving it too?

• How long and how heavy a telescope can they handle alone?

• Will they be able to handle it alone, or will they need help?

• If they need help, can they expect that help every time they want to use the telescope? Frustration can lead to loss of interest.

• Can they take good care of it? Will they?

• Often, a very young person may have a deep interest in a subject such as astronomy, but moves on to other interests after a short time. Will giving a low-quality telescope discourage them? Will giving an expensive one be a lost cause, or foster a lifelong hobby?

Think along these lines as you consider your purchase. No doubt other questions will occur to you. Of course, If the telescope is for you, you should apply these to yourself, as well!

WHERE?

Most telescope newbies don't think too much about where they will use the instrument. This is actually a crucial consideration.

The most basic question isn't backyard or front, but city/suburb or boondocks? Cities and surrounding areas have what's called "light pollution" - houselights, streetlights, signs, cars, all that light creates a haze throughout the area. You don't notice it when you're in it, but it's there. It acts almost like a mist or thin cloud between you and the stars.

Check the photos below; these examples give you a hint of light pollution's impact. Tonight, or the first cloud-free night, go out where you plan to use your telescope and look up. How many stars can you see? Just a couple of dozen? Or hundreds? In a truly dark sky, you can see thousands.

If you can only see a few, it doesn't mean you should give up the idea of a telescope! You just need to plan your purchase accordingly.

Ironically, a large telescope that gathers lots of light from the sky is more affected by light pollution than a small one. If you're going to be using your telescope mainly in the suburbs or a city, a 10-inch Reflector or 6-inch Refractor may disappoint you by not bringing in as many details as you expected.

On the other hand, if you plan to get out away from the city to do your star gazing, a small telescope may not bring all the richness you hope for.

UNLESS YOU'RE AN ASTRONAUT ON MISSION, wherever you use your telescope, you'll be looking through the earth's atmosphere. Have you ever noticed stars "twinkling?" That's because of the atmospheric conditions at that moment. This isn't the place for a discussion of meteorology, but bear in mind that what's going on with the air above you will affect your viewing that night. If the air is unstable, the viewing will be difficult.

Did you know that many of the world's largest telescopes are located on mountains, where the air is thin and the cities far away? They have more nights with stable air, and less light pollution. This is not to suggest you can only enjoy a telescope on a remote mountaintop, just expect some nights to be better than others.

Answering the Who? and Where? questions helps you figure out what size telescope to consider.

Simply taking the telescope out of the house into the back yard can be easy or a big project, depending on its size and weight. And packing it in the car for a trip to the wild dark yonder can be relatively painless or a challenge.

The 18" Reflector in this picture gives absolutely incredible views, but it's nearly 7 feet long, weighs 245 pounds, and the Eyepiece is more than 6 feet off the ground when it's pointed straight up. Maybe not the one you'd choose for setting up and knocking down every night!

Either backyard or boonies, ease of setup and use must be seriously thought out.

The easier it is to use your telescope, the more often you'll use it and the more you will enjoy it.

Also, what your telescope stands on - its mounting - can affect your enjoyment. A wobbly telescope is difficult to keep aimed on a single planet or star, and may frustrate you so much that you may quickly make it a permanent fixture in a back corner of the basement. More on mounts later.

Telescopes for looking at stars are described by the diameter of the primary lens (Refractors) or the primary mirror (Reflectors). That's because a telescope is really a light-collector. Larger lenses/mirrors collect more light than smaller ones, so the bigger the diameter of the lens/mirror, the fainter the stars you can see. Without getting too technical, the power of a lens is in proportion to the square of its diameter.

Huh? That simply means a 4-inch telescope will collect 4 times as much light as a 2-inch instrument (2 times bigger multiplied by 2); a 6-inch collects 9 times as much light as a 2-inch (3 times bigger multiplied by 3); an 8-inch, 16 times as much as a 2-inch (4 times bigger multiplied by 4) and so on.

Telescopes marketed in the U.S. are sometimes described in inches, sometimes in millimeters (mm). 25mm is considered equal to 1 inch. So a 75mm Refractor would also be called a 3 inch. A 6 inch Reflector might also be called a 150mm.

The diameter of the primary lens or mirror dictates the overall size and weight of the telescope, which you'll probably find to be the major factor in ease of use. These affect how well the mounting works for you, which will have a big impact your enjoyment, too.

As described in our opening paragraphs, a Refractor will be longer than a Reflector of the same size - and probably a bit heavier, too. This is because the larger a Refractor's primary lens, the longer the distance to where it focuses light (at the Eyepiece).

A Refractor will usually be mounted on a tripod. If you want to get the greatest enjoyment out of your Refractor, the tripod must be of good quality. A $400 Refractor sitting atop a $15 tripod will be so wobbly that it's almost unusable. Do your best to determine how good a tripod comes with the telescope. If the telescope is made by one of the top manufacturers, the tripod will most likely be very good. If you find that your tripod isn't satisfactory, seriously consider spending the money to buy a good one before giving up!

The shape of a Reflector's mirror allows it to focus light in a shorter distance than a Refractor's primary lens. That's why it's shorter for a comparable diameter.

A Reflector's mounting is important, too, and for the same reason. So take care to learn the quality here, as well. A Reflector may have a sturdy mounting that must be attached to a tripod. See the comments above! But it works vice-versa, too - if the Reflector's mounting is not well made, an expensive tripod won't be able to fix the problem.

Many Reflectors have a mounting that avoids a tripod altogether. More below.

As mentioned above, Refractors have more lenses and those must be carefully ground. The lenses also must be of the highest grade glass. While small Refractors offer better quality lenses than Reflectors of the same size, many serious amateur astronomers say it's difficult to find one of good quality below about $250. Prices - and quality - of Refractors take a jump at sizes above 120mm.

All this care in making the lenses gives the Refractor an edge in the quality of the view you'll see. However, the evolution of the optics in modern Reflectors has narrowed the gap significantly.

Most telescopes sold today are Reflectors. They are easier to handle and cost less than Refractors of the same size primary lens. For the same price, a Reflector will give you a wider field of view, higher magnification, and good quality optics. It's hard to go wrong with a well made, top caliber Reflector.

Whichever type you choose, don't expect to see Planetarium Poster quality views. Mostly, those were made by one of the giant telescopes which only professional astronomers have access to. Maybe one day your passion for astronomy will lead you to build a 40-inch telescope inside a dome, and you'll see what the pro's see. But for now, have more modest expectations. Think, "Better than Galileo saw!"

NOTE: If your budget is under $250 or so, take a serious look at a good pair of binoculars. You can find some in this range with tripod mounts. Most telescopes (Refractors in particular) in this price range, especially those sold in department stores, discount department stores and warehouse stores, have poor-quality lenses, wobbly mountings, and not the best quality construction. I recently saw a pair of Barska zoom binoculars for under $200 with shipping, on eBay. These claimed a magnification of 125 - four times what Galileo had, and he could see the rings of Saturn and Jupiter's moons. A good pair of binoculars will probably give you much more enjoyment than a Refractor in this price range.

Of course, there are exceptions to any rule. Check out a Dobsonian telescope. For our purposes, "Dobsonian" means a Reflector with a specially designed mounting that is very stable and makes the telescope very easy to handle. A Dobsonian telescope under $250 can be set on the table on your backyard deck and give you a sharp view of the stars - if you don't bump the table! Dobsonian telescopes offer the best value per dollar today. The photo to the left is a Reflector with one of several types of Dobsonian mount.

Let's make it simple. If you are mildly interested or want to encourage your child's interest in astronomy, you can get some satisfaction from binoculars or one of a handful of telescopes under $250. If you really want to see the night sky up close and in brilliant detail and accuracy, you need to plan to spend at least $500 and up to $700. Of course, you can spend well into 5 figures on a serious amateur telescope - and you'll probably build a dome to house it, too!

You may want to spend some time building your budget while you learn more about telescopes. If you can stand to jump in at $500 to $700, you'll have an instrument that will keep you fascinated. And with common-sense care, a good quality telescope will last a lifetime, or longer if you pass it on to younger generations.

Sure!

It's a bit difficult to recommend something without knowing your specific requirements, but there are some particular telescopes that I like.

Trusting that you have really thought out our qualifying questions, I recommend the binoculars and telescopes shown below.

If these choices seem too limited, I suggest perusing offerings from these manufacturers:

Celestron. Meade.

These companies have been around for decades and enjoy very good reputations. They make fine telescopes that will bring you many hours of enjoyment.

Celestron's NexStar series of telescopes are especially popular - for good reason - and come in a range of prices. They have satisfied thousands of loyal amateur astronomers.

Some serious amateur astronomers make and sell what are essentially handmade telescopes of extremely high quality. And there are a number of small manufacturers, but the quality can be uneven, so beware!

You might want to look into a prefabricated, build-it-yourself kit, if you are mechanically inclined; these can offer tremendous value, but again, be cautious of quality.