Eyepieces

Types of Eyepieces

Huygens [HOY-kens] and Ramsden
These were some of the first eyepieces invented, and consequently, they are not very good. They have very small fields of view (it's like looking through a straw), bad chromatic aberation (wierd colors appear), and poor eye-relief (you have to jam your eye up against them to see). Cheap department-store telescopes often come with these. They are usually marked with an "H" or "HR".

Kellner
These are better than Huygens eyepieces, but not by much. Many manufacturers sell them, so they aren't hard to find. Even if you are on a tight budget, we recommend that you save up your money and buy something better. RKE, Modified Achromatic (MA), and Achromatic Ramsden (AR) eyepieces are all about the same as Kellners.

Orthoscopic
Orthos have a fairly narrow, 45 degree field of view, but reasonable color correction, sharpness, and eye-relief. They are just fine for planetary and double-star viewing where a wide field of view is not important. These are getting hard to find as they have been replaced by other types.

Plössl
These are very popular, all-purpose eyepieces. They don't have any serious problems, and have fields of view that are 50 to 52 degrees. It may not sound like it, but that's a lot better than the 45 degrees of Orthoscopics. Plössls give the best performance for the price. We recommend them for beginning astronomers.

Erfle
The nice thing about Erfles is their wide field of view—usually 65 to 70 degrees. The problem with them is their distortion around the edges of that field. Near the center of the field, they are about as good as Plossls, but the outer few degrees show stars stretched into slight arcs. They work best at low power and in long focal length telescopes (f/6 or higher). In those cases, the distortion isn't enough to bother most people. They are a little hard to find, but only a little more expensive than Plössls.

Wide Field
This is a generic name for modified Erfles with their abberation corrected or diminished without affecting the field of view. Many companies offer a something like this, but it's usually fairly expensive. Tele Vue Panoptics and Radians, Orion Stratus eyepieces, Meade Super Wides, and GSO SuperView eyepieces are in this category. The exact properties of these eyepieces vary, but usually you get better performance from the higher priced ones.

Very Wide Field
Tele Vue Naglers and Meade Utlra Wides fall into this category. They each have a field of view more than 80 degrees wide with few or no problems at the edge. This means that you see nothing but space all the way into your peripheral vision. It feels more like sticking you head in the telescope than just putting your eye to it! These eyepieces have premium performance, but it comes at a premium price.

Eyepiece Magnification

The main reason to get several eyepieces is that each one gives you a different magnification. The magnification of a particular eyepiece is calculated by dividing the focal length of the telescope by the focal length of the eypepiece. The focal length of an eyepiece is measured in millimeters and is usually printed on the eyepiece itself.

High power would be anything over about 150x. But high magnification is not as important as you might think. Objects in the sky come in all different sizes—some are so large that no telescope can show the whole thing at once! There are small objects too, of course, so high power is useful. However, there are two limits on how high you can go: the telescope and the atmosphere. As you increase magnification, stars, nebulae, and galaxies get fainter because the light is spread out more, makeing them fainter and harder to see. There is also a point where the telescope can't bring out any more detail no matter how high the magnification. That point is determined by the telescope's aperture. The larger it is, the higher you can go. The highest useful power of a scope is about 50 times per inch of aperture, or 20 times per centimeter. Then there is the atmosphere. Turbulance in the atmosphere usually limits you to below 300 power, and often far less. Still, you should have an eyepiece that gives you something near the maximum for your scope for those rare times when you can use it. High power is often used for planets, globular star clusters, and some planetary nebulae.

Anything below 50x would be considered low power. Astronomers with large telescopes have to go to great lengths (read: spend lots of money) to get low power. It is nice to have though, because many of the most famous and nice looking deep space objects are quite large, like the Andromeda Galaxy and the Pleiades. Low power is also great for finding things in the first place. When you are looking for a new object in the scope, you want to use the lowest power you have because it shows the most sky. It is very hard to aim a telescope perfectly, but if you can see a large section of the sky in the eyepice you will be more likely to get the telescope close enough. Once you have an object in view, you can center it in the field and zoom in for a closer look with a higher power eyepiece. The first eyepiece you get for your telescope should be low power.

You will probably want several medium power eyepieces in you collection for general use. Each one will neatly frame an object of a different size. They are most useful for nebulae, open star clusters, and galaxies. Also, each night there will be a maximum usable power imposed by air turbulance. If you have several medium power eyepieces, you will always have the ability to view at close to that maximum power.