Eye Pieces

The eyepiece is the part of the telescope that focuses the image to your eye. Eyepieces can also be used to magnify the image up to several hundred times. However, it is important to remember that as you magnify an image, you will lose sharpness, detail, and brightness. Beginners are always tempted to magnify that image of Jupiter or Saturn until it is a large as possible in the field of view. The more experienced observer learns that there is often more detail to be seen in a smaller, brighter image. Magnifying the image also reduces your field of view, basically the area you can see through the eyepiece. When observing large objects such as nebulae or star clusters, a wide field of view is needed. Special wide field eyepieces are also available. These eyepieces can provide a nice, bright image full of details on large objects. Eyepieces are labeled by their focal length, in millimeters. Typical sizes are 25mm, 17mm, 10mm, and so on. To determine the magnification of an eyepiece you simply divide the focal length of the telescope by that of the eyepiece. For example, if the focal length of your telescope were 1000mm, then a 25mm eyepiece would give you a magnification of 40 times. The smaller the focal length rating of the eyepiece, the higher the magnification. Again, it is recommended that beginners start out with a few low-power eyepieces at first. Many people are disappointed at how bad the image actually looks with at high magnifications. Low-power eyepieces also make objects easier to find, due to their wider field of view. Work your way up to higher power after you have a little experience under your belt. A low-cost device useful for gaining additional magnification is the barlow lens. This is a long tube with a lens in it that extends the focal length of the eyepiece and increases the magnification by a factor of 2 or 3. For example, a 2x barlow lens used with our 25mm eyepiece from the example above will increase our magnification from 40x to 80x. The image quality is not quite as good as what you would get with a 12.5mm eyepiece by itself. But adding a barlow lens is an affordable way to effectively double the number of eyepieces in your collection.