This guide is aimed at the beginner who is looking to buy his or her first telescope. You might want to buy one for yourself or you may be looking for a Christmas present for someone who's expressed an interest in astronomy. Either way I'm assuming you know little about telescopes. My mission is to give you as much information as I can as quickly as I can so I'll be revising and adding information as I have the opportunity to edit it. Let's get started.
There are two kinds of telescopes: refractors and reflectors. Both come in a number of variations, some of which are quite exotic and expensive.
All telescopes share three key components: The objective, the tube and the eyepiece. And they all have three key measurements: The aperture, the focal length of the tube and the focal length of the eyepiece. And they all have a finder scope. This is a smaller (but wider view) scope attached to the main tube to help in initially locating objects.
The aperture (size) of the OBJECTIVE determines the light gathering power of the telescope. The objective is the big lens on the front end of a refractor and the big mirror at the back end of a reflector.
A REFRACTOR is the "classic" design that probably comes to your mind when you hear the word "telescope". Big lens on one end, (the objective) and a little lens at the other end. This little lens is the eyepiece through which you view what you're looking at. This is the design that is said to have been invented by Gallileo. Here's what you need to know about refractors:
Forget about refractors. It's surprisingly difficult and expensive to make a quality refractor. You would not be satisfied by any refractor you could afford as a first scope. The problem is that the way refractors bend light causes the different frequencies (i.e., colors) to bend at different angles. This causes a problem called "coma", which is a rainbow aura around objects like stars and planets. Achromat refractors use a second lens to partially correct the coma problem, adding to the cost. The apochromat refractor adds yet another corrective lens at even greater expense. The lenses in the best refractors aren't even made of glass. They're made of a mineral called flourite.
Your first scope should probably be a REFLECTOR. The objective in a Newtonian (as in Isaac) relector, the simplest kind, is a mirror at the bottom of a big tube, whose front end is open. The mirror is curved and refects the image back into a secondary mirror suspended near the top of the tube. The secondary mirror reflects the image into an eyepiece that sticks out perpendicular from the main tube near the top of the tube. Yes, the secondary is an obstruction but you don't actually see its shadow when you look into the scope. It merely dims the overall view.
Here are some quick facts:
There are two kinds of telescopes: refractors and reflectors. Both come in a number of variations, some of which are quite exotic and expensive.
All telescopes share three key components: The objective, the tube and the eyepiece. And they all have three key measurements: The aperture, the focal length of the tube and the focal length of the eyepiece. And they all have a finder scope. This is a smaller (but wider view) scope attached to the main tube to help in initially locating objects.
The aperture (size) of the OBJECTIVE determines the light gathering power of the telescope. The objective is the big lens on the front end of a refractor and the big mirror at the back end of a reflector.
A REFRACTOR is the "classic" design that probably comes to your mind when you hear the word "telescope". Big lens on one end, (the objective) and a little lens at the other end. This little lens is the eyepiece through which you view what you're looking at. This is the design that is said to have been invented by Gallileo. Here's what you need to know about refractors:
Forget about refractors. It's surprisingly difficult and expensive to make a quality refractor. You would not be satisfied by any refractor you could afford as a first scope. The problem is that the way refractors bend light causes the different frequencies (i.e., colors) to bend at different angles. This causes a problem called "coma", which is a rainbow aura around objects like stars and planets. Achromat refractors use a second lens to partially correct the coma problem, adding to the cost. The apochromat refractor adds yet another corrective lens at even greater expense. The lenses in the best refractors aren't even made of glass. They're made of a mineral called flourite.
Your first scope should probably be a REFLECTOR. The objective in a Newtonian (as in Isaac) relector, the simplest kind, is a mirror at the bottom of a big tube, whose front end is open. The mirror is curved and refects the image back into a secondary mirror suspended near the top of the tube. The secondary mirror reflects the image into an eyepiece that sticks out perpendicular from the main tube near the top of the tube. Yes, the secondary is an obstruction but you don't actually see its shadow when you look into the scope. It merely dims the overall view.
Here are some quick facts:
- Magnification is determined by dividing the focal length of the main tube by the focal length of the eyepiece. (Any good ad will give you the focal length of the tube and any eyepieces included. For example: A scope has a focal length of 1,000 mm (1 meter, about 39 inches). If you drop in a 25mm eyepiece, magnification will be 40x (1,000 / 25 = 40). Drop in a 10mm eyepiece and magnification will be 100x. You can always buy more eyepieces.
- So if magnification is dependent on focal length, what does increased aperture get you? Compare a four inch aperture scope with an eight inch aperture scope. If they both have a 1,000 mm focal length with 20 mm eyepieces, they'll both magnify the image by 50 times. However, the 8 inch scope, with twice the diameter of the four inch scope will give you an image four times brighter. Light gathering ability increases by the square of the radius. Objects that are barely visible in a four inch scope will be quite bright in an eight inch scope.
- Most scopes use standard 1.25" (width) eyepieces. Different brands and designs can be used interchangeably. Some scopes use 2" eyepieces. These usually have adapters so they can also use 1.25" eyepieces. (Another, less common size is 0.96 inches.)
- By tradition, the aperture of some kinds of scopes is given in inches while others are measured in millimeters. (One inch = 25.4 mm).
- Despite the outlandish promises that may appear on the scope's box (such outlandish promises being a good indicator that the scope is junk) the maximum usable magnification is about 50x the aperture measured in inches or twice the aperture measured in millimeters. For example, a 5" scope is good for a maximum of about 250x. A 90 mm scope is good for about 180x.
- Maximum magnification does not always provide the best view. For a given aperture you can't magically increase the amount of light the scope gathers. What does this mean? The greater the magnification you use, the dimmer the view will be. Think about it: If you take the same amount of light and use it to make a bigger image, the image has to get dimmer.
- Forget about seeing views like the gorgeous photos that illustrate the boxes of some scopes. They're generally taken by giant scopes and they're made up by overlaying dozens, sometimes hundreds of repeated exposures. And each exposure may have lasted many minutes or even hours. Remember learning about the rods and cones in your eyes in junior high school science? The rods, being much more numerous and more sensitive to light, provide most of your night vision. But they're much less sensitive to color than the cones are. So the images you will see will be pale with only a hint of color.
- If you can't afford to spend at least $200 on a scope consider buying a good pair of 10 x 50 binoculars for as little as half that. (The "10" refers to the magnification, which is usually fixed in binoculars; the "50" refers to the size of the objectives, 50mm.) There are whole books written on naked eye and binocular star gazing and binoculars are a great way to get into the hobby. As a bonus if the interest in astronomy wanes the binos will still be good for nature, camping and scenic use.
More about specific types of scopes:
- A Dobsonian is just a Newtonian that uses a particularly simple and inexpensive mount invented by a guy named John Dobson. They're excellent buys for beginners.
- More exotic reflectors that use a combination of lenses and mirrors are called catadioptrics. Popular variations include the Schmidt-Cassegrain (SCT) and the Makustov-Cassegrain (Mak) designs. They've much shorter than standard Newtonians for a given focal length because they bounce the light path back and forth more times within the tube before it reaches the eyepiece. They're also much more expensive per inch of aperture than classic Newtonians.
- A GOTO scope has a computer controlled motor that allows you to punch in the name of an object into a handheld controller. The scope then "goes to" the object. They can be cool but they're much more expensive and take some set-up time every time you use them.
Buying tips:
- BE PATIENT. Excellent used scopes frequently go for 1/3 to 1/2 of what they sold for when new, even when they've been lightly used.
- A scope that's been used by someone who really understands astronomy might be worth a little more than one from someone who doesn't know astronomy well. Novices might not know what to look for and they might overlook problems due to ignorance, not because they're trying to be dishonest.
- Go to your local library and get your hands on the December 2005 issue of Sky & Telescope magazine. It rates ten beginner scopes and it's filled with great advice.
