Okulár

Okulár, in English eyepiece, is the optical component at the end of a telescope through which the observer views the magnified image produced by the objective. It contains lenses that further magnify the image and influence the apparent field of view, eye relief, and the exit pupil. The eyepiece works with the telescope’s objective to determine the final magnification and the portion of the sky that is seen.

Magnification is given by dividing the focal length of the telescope by the focal length of the eyepiece: M = F_t / F_e. The eyepiece also sets the apparent field of view, while the true field of view depends on both the eyepiece and the telescope’s focal length and optical design. The exit pupil size, roughly the telescope’s aperture divided by the magnification, affects brightness and eye placement.

Eyepieces come in standardized barrel sizes, most commonly 1.25 inches and 2 inches, which determine compatibility with focusers and diagonals. The choice of eyepiece affects field of view, eye relief, and image sharpness across the field.

Common eyepiece designs include vintage types such as Huygens, Ramsden, and Kellner, as well as the more widely used Plössl and orthoscopic designs. Modern wide-field and ultra-wide-field designs provide larger apparent fields of view but may require longer eye relief and careful eye positioning. Brightness and contrast are influenced by lens quality and anti-reflective coatings.

Choosing an eyepiece involves balancing magnification needs, field of view, eye relief, and optical quality. Proper care—keeping lenses clean, using a blower to remove dust, and avoiding scratches—helps preserve performance. Okuláry are used in astronomy for object detection, survey viewing, and detailed observation across refracting and reflecting telescopes.