Hobbies And Interests
Considerations
Most eyepieces have either a 1 ¼-inch or a 2-inch barrel diameter, and any can be used with any telescope that has an eyepiece holder of the same diameter. The XT10 is supplied with 10 and 25 mm Plossl eyepieces, for high and medium power observing. Dobsonian scopes generally excel at lower powers, showing wide-field views, so your equipment should include a low-power eyepiece in the focal length range 35 to 40 mm.
Kellner and Ramsden Eyepieces
Among the least expensive of eyepieces, Ramsdens and Kellners are used by amateurs for every type of observing. The three-element Kellner design performs well on f/10 or higher scopes, but rather poorly on f/6 and lower (rich field) scopes like the XT10. Kellners and Ramsdens can produce good images, but only near the field-center, and their simple designs limit performance significantly in other areas. Kellner field sizes, although narrow, are an improvement on the two-element Ramsdens. Short-focal-length Kellners and Ramsdens have short eye relief, which make them almost useless for high-power applications.
Orthoscopic Eyepieces
Orthoscopic eyepieces are four-element designs renowned for their high-power performance, especially on f/10 and higher scopes. Orthoscopics are a good choice for lunar and planetary observers because they produce sharp, high-power images, and are relatively inexpensive. Their main drawback is a narrow apparent field of view of no more than 40 degrees. Plossl eyepieces have wider fields than Orthoscopics and generally outperform them, but at higher cost.
Erfle Eyepieces
Erfles are wide-angle eyepieces of multi-element design. With apparent fields of 55 to 60 degrees, Erfles are capable of sharp imagery, but only near the center of the field of view. Erfles offer impressive low-power views of the universe, but performance suffers noticeably in shorter focal lengths (higher power). Their lack of field flatness causes stellar images to become progressively blurry toward the field edges. Erfles have lost popularity because they are expensive and are outperformed by newer wide-field designs.
Plossl Eyepieces
Plossls are highly popular among amateur astronomers because they represent a good balance between competing factors. Using four and five element designs, with focal lengths between 4 and 40 mm, these eyepieces produce sharp images, fairly flat fields, excellent contrast and apparent fields of view of 50 degrees or more, at a reasonable cost. Plossls are perhaps the best choice for Orion XT10 owners who demand better than Ramsdens or Kellners, but who cannot afford premium wide-angle eyepieces.
Premium Wide-Angle Eyepieces
Using up to eight optical elements, premium wide-angle eyepieces offer apparent fields of view of 80 degrees or more, comfortable eye relief, sharp imagery across the entire field, with almost no discernible optical aberrations. On the drawback side, they are big and heavy, and very expensive. These massive eyepieces can be used only on telescopes with rigid mountings -- usually scopes 5 inches or greater in aperture.