How Telescopes Work

Non-Optical Features

There are other parts of a telescope, besides the optical components, that you'll want to consider:

Eyepiece Holder

Eyepieces come in three diameters -- 0.965 inches (2.45 cm), 1.25 inches (3.18 cm), and 2 inches (5.08 cm). However, the eyepiece holder in your telescope is fixed. The "cheap, department store" telescopes tend to have 0.965-inch holders. Most telescopes have 1.25-inch holders. Some have 2-inch holders. Eyepieces of differing diameters are not usable in all telescopes. Make sure that the eyepiece holder in your telescope can support the eyepieces you wish to purchase. You typically do not have an option on the size (i.e. the manufacturer sets the size with the telescope model), but you should definitely know what it is because it will affect other choices you'll be making.



The focuser moves the eyepiece holder up and down slightly, adjusting the focus of the eyepiece for each individual observer. There are friction focusers or rack-and-pinion focusers. Regardless of the type on your telescope, your focuser should move smoothly without causing the telescope to shake. If it does not move smoothly, then when you have to focus the eyepiece (change eyepieces, or when someone else looks in the telescope), the object is likely to move out of the field of view, especially at high magnification. You may not have the choice of focuser on your telescope, but it is nonetheless helpful to know.