AAA Telescope Buyer’s Guide
People who attend our many stargazing events often ask us about our telescopes, the differences between them, what they cost, etc. We have put together this guide to help you understand a little more about how to choose which kind of telescope is right for you. All telescopes, regardless of the style and size, do the same thing: they collect light, and bring that light to a focus, at which point you use an eyepiece to magnify that image. You don’t need to spend a lot of money to get a decent telescope and you certainly don’t want to go overboard on the size. The best telescope to buy is the one you’ll take out most often. If it’s too big or heavy, trust us, it will stay in a closet and collect dust. For beginners, we suggest staying in the 80mm-100mm size range. That means the front lens, or objective lens, will measure around 3 or 4 inches in diameter. A telescope of this size will be easy to store and carry, and offer good views of bright celestial objects.
Types of Telescopes
You may have noticed that telescopes not only come in different sizes, but for the most part they fall into one of two basic designs: refractor or reflector.
The refractor is perhaps the most familiar, having a longer, slender tube. Reflectors tend to be a little shorter and wider. Both are good for most applications, but in general a refractor offers clear, crisp images especially on the moon and planets. Reflectors offer more inch of diameter for the money, and will therefore excel where a larger telescope really counts: on more distant, dim, or fuzzy objects like galaxies, which require more light grasp as opposed to crisp sharp detail.
Both are classic, time-tested designs. Galileo first pointed a refracting style instrument at the sky over 400 years ago. Later Sir Isaac Newton invented the reflector, also called the Newtonian reflector. The difference between these two simple optical designs is that a refractor employs a straight light path which uses one glass lens (called the objective lens) at the front, to collect the light which enters it. The objective lens bends or “refracts” the light rays so as they travel down the length of the telescope tube, they come to a focus point at the bottom, where the eyepiece is located. The eyepiece simply magnifies the image that is collected from the light.
In a basic reflector, the light enters an open tube at the front, and hits a large curved mirror (called the primary mirror) at the base of it. That light bounces off the large mirror, and lands on a smaller secondary mirror near the front of the open tube. The secondary mirror is angled to redirect the light out of the side of the telescope at the focal point, where the eyepiece is located. In this way it is the mirror that collects the light, not a glass lens. There are other types of reflectors, (called compound reflectors) which are shorter length, and therefore more compact. They employ a ‘folded’ light path, which offers the advantage of portability, but tend to get a bit heavier above 5 inches of aperture.
Which Type of Telescope is the Best?
Either is a solid choice. Refractors tend to give sharper images, and work very well with lunar and planetary detail. The downside is that good refractors are more expensive per inch of aperture, and some cheaper ones may suffer from false color, or “chromatic aberration” due to the light rays not all coming to a perfect focus in the same spot along the light path. In most cases this is minimal. Reflectors offer larger apertures at lesser cost, and therefore might be a better choice if you want a bigger telescope, somewhere in the 5 inches and above range. In addition, since the reflector employs a mirror, false color is eliminated.
A Little Bit About Tripods
Most telescopes will come with some type of tripod, also called a mount. The easiest and most basic type of tripod is called altitude-azimuth (or alt-az for short). You can push it up and down, and left and right to manually target an object, and it is relatively lightweight and portable. An equatorial mount works in a similar way, but has two axes which you can align to more accurately follow the path of the sky’s rotation. These mounts often require heavy counterweights for balance, and can be a bit daunting to use properly. No batteries or other power source is required for either of these type of tripods.
Computerized “Go-To” vs. Manual Object Location
A go-to mount is a computerized tripod/mount that contains an object database and GPS. With proper set up, a telescope on this type of mount can locate thousands of celestial objects at the push of a button. The benefit of this is that you’ll probably get to see more objects in a given amount of time since you’ll be spending less of it hunting around the sky manually. This system will also “track” the movement of the sky so that an object can remain centered in your eyepiece without the constant manual correction needed as when using an alt-azimuth style tripod. But you will need batteries or some type of power supply, and it will require some patience as you learn to align the telescope at the beginning of each observing session. Many popular, name brand go-to telescopes are reflectors. Note, you probably won’t learn as much about the positions of stars and constellations since the computer is doing the work for you. In addition, what you gain with technology, you might sacrifice in quality. We tend to see more plastic parts on the small, affordable go- to telescopes, but you can always spend more for a larger one of better quality.
What about Magnification?
Don’t be too concerned with how much magnification you can get. This is not a property of just the telescope alone, but more which eyepiece you couple with it. Most telescopes come with at least one eyepiece to start with. If you do want to know how much magnification you have, there is some very simple math you can do to find out.
Every telescope and eyepiece has something called the “focal length”. It may sound a little technical but really it’s an easy concept. The focal length of the telescope is just the length (in millimeters) from the front lens (where light hits the telescope) to the point where the light comes to a focus, in most cases at the back end of the telescope. If your telescope has a focal length of 900mm, and your eyepiece has a focal length of 25mm, simply divide 900 x 25 and that will give you your magnification of 36x.
Be wary of any brand that promises high magnification in the 300-400x power range. That is simply a bogus selling point, and rarely will you ever need to go above 200x magnification, if at all. The highest practical magnification for any telescope is twice it’s diameter in millimeters, or 50 times its diameter in inches. For example, the highest magnification for a telescope with a diameter of 127mm will be 254 power: 27mm x 2 = 254. For a 5 inch telescope: 5 x 50 = 250 power.
If you would like to see the different types of telescopes in action, come out to the many free stargazing events the AAA holds each year. Details can be found at www.aaa.org. If you need a little help in the beginning on set up and use of your new telescope, feel free to reach out to the club. We have many veteran observers who will be happy to help you start your adventure with the stars.