10 Tips for Buying a Telescope Eyepiece

Buying a telescope eyepiece is the most important decision that anyone owning a telescope can make. The eyepiece is the most integral component of any telescope; a cheap eyepiece can cause blurred images, offset correct color and render the entire astronomy experience useless. A good telescope eyepiece will allow the viewer to see crisp images, have a wide view range and show objects in their correct proportion to one another. Buying a telescope eyepiece takes a little research beforehand to make sure that you invest in an eyepiece that is right for you. Here are ten tips on what you should look for when purchasing a telescope eyepiece.

1. Magnification is the most important thing to consider when buying a telescope eyepiece. The magnification of the eyepiece works in ratio with the magnification of the telescope thus determining the actual power of the telescope. Although the whole point of a telescope is magnify far away objects, choosing the highest magnification combination is not the best thing to do. Too much magnification can make objects blurry and out of focus. On the other hand, low magnification can cause colors to wash out. Most telescope users have a variety of lenses in the mid-range, usually starting around 100x.
2. Barrel Size refers to the width of the lens barrel. The size of the barrel determines how much magnification you will be using, allowing the viewer to increase or decrease images. There are three basic barrel sizes: 24.5 mm, 31.75 mm and 50.8 mm. The smallest, 24.5 mm is usually found on inexpensive telescopes. The largest size, 50.8 mm, are the most expensive and usually come in custom sizes for specific brands of telescopes. The best barrel option is the 31.75 mm. It is the standard amongst most telescope users and offers an acceptable amount of FOV.
3. Field of Vision, or as it is often referred to FOV, is the area that you can view when looking through a telescope that is measured in degrees. There are two types of FOV, actual and apparent. The actual FOV is the area of the sky which can be seen using a specific telescope with a specific magnification. If you buying an eyepiece separate from a telescope, then the manufacturer will give you the apparent FOV. The apparent FOV is a generalized area which depends on the telescope and magnification you will using. A general rule of thumb is that a high apparent FOV is good for searching the skies but a lower degree of apparent FOV is better if you plan on studying a planet or star.
4. Eye Relief is the distance between the eye and the lens that you will be looking through. Choosing the proper eye relief is important because the proper distance not only help you to see objects more clearly it also will determine how comfortable you will be when using the telescope. Most telescope eye reliefs fall into the range of 2 mm to 20 mm. For most people, 5 mm - 6 mm is good eye relief distance. Viewers who wear eyeglasses will need a much higher eye relief to adjust for the thickness of the glasses, usually between 18 mm - 20 mm.
5. Brightness. The further away a object is, the less bright it will be. Thus a good telescope eyepiece will be able to enhance the brightness level of an object to make it more viewable. One of the main elements that has to do with the brightness of an object is the magnification levels which was discussed earlier, mid-range magnification is usually best of most viewing experiences. Another factor to consider is the viewer himself. As people age, their eyes change in their relationship with light. In other words, younger viewers usually will need less magnification in order to pick up on the brightness of an object.
6. Color is another important element when it comes to choosing a telescope eyepiece because of the light refraction caused by the distance that light travels and the way light reacts to magnification. A simple and inexpensive way to avoid color problems is to choose a Ramsden type lens which was designed centuries ago to eliminate color refraction problems.
7. Contrast is key when it comes to viewing objects through a telescope. Too much contrast and details are hard to see, too little contrast and the entire object becomes a blur. The best way to solve the issue of contrast is to invest in a high quality telescope eyepiece. It may cost a bit more but in the long run, the image quality you will have makes it well worth the investment.
8. Coated and Uncoated Lenses are available for telescope eyepieces. Coated lenses increase the amount of light which goes through the telescope and can bring out more details. Coated lenses can also eliminate ghost images which sometimes can pop up in the when using uncoated lenses. The type of lens coating is usually a matter of personal preference. Try out a variety of coatings and go online to see what other telescope users recommend.
9. Barlow Lens is a device used with an eyepiece in order to increase the magnification of the eyepiece. Using a Barlow Lens allows you to double or triple the magnification of a telescope eyepiece which in essence giving you 2 telescope eyepieces for the price of one.
10. Design. There are a variety of telescope eyepiece designs; each design varies according to the eyepiece's intended use and the type of telescope it will be used with. It is wise to become familiar with at least a few of the design names: Ramsden, Achromat, Ortho, Erfle, RKE and Nagler. Each was designed with a specific use in mind, from government use to exploring the depths of space. For a beginner, I would recommend a Ramsden or Achromat lens. Ramsden is good for basic astronomy purposes and an Achromat lens is often less expensive than a Ramsden and of the same magnification quality.