How do I use my telescope eyepieces?
October 13, 2008
Your eyepieces are the first accessories you should learn to use with your telescope. Eyepieces have different diameters that allow you to view objects through your telescope at a variety of magnification powers simply by swapping out the eyepiece.
Eyepieces come in multiple powers: the higher the number (found printed in millimeters on the eyepiece, such as 15 mm, 32 mm, and so on), the lower the power. Always start with the lowest-power eyepiece because it is easier to focus and has a wider field of view, making it easier to slew the telescope to your desired object. Most of your observing will be done with lower power eyepieces: the images are much brighter and crisper, providing more enjoyment of the objects you're observing. As eyepiece power increases, this sharpness and detail diminishes, so higher powers are mainly used for lunar, planetary, and binary star observations.
Additionally, a simple way to increase magnification is to use a Barlow lens in addition to your existing eyepiece. A 2x Barlow will double the magnification of any existing eyepiece, a 3x Barlow will triple it, and so on. Since longer-focal-length eyepieces generally have longer eye relief, using a Barlow to increase magnification will allow more comfortable high-power viewing.
Here's how to use an eyepiece:
- Insert your lowest-power eyepiece into the telescope and tighten in place.
- Look through the eyepiece. Place your eye just behind it to take advantage of its eye relief (the spacing between the lens and your eye).
- Do not place your eye directly against the eyepiece; getting too close will prevent you from blinking and also cause a black ring to appear around the field of view. You need a space of at least 15 mm for the best comfort (if you wear eyeglasses, the space may be even longer).
- If you place your eye farther away, you'll lose field of view and may even move your eye out of the beam of light from the eyepiece.
- Turn one of the two knobs to the side or below the eyepiece–first one way, then the other–until the object is in focus.
- If desired, switch eyepieces to a higher power and repeat the steps above. Or, remove the eyepiece, insert a Barlow, reinsert the eyepiece, and repeat the steps above.
A basic set of eyepieces generally includes 32 mm, 20 mm, 9 mm, and a 2x or 3x Barlow.