Naturally, the eyepiece must fit your telescope. Four barrel sizes (tube sizes) are in common use:
• 24.5 mm (0.965 inch), used on smaller Japanese telescopes;
• 14 inches (32 mm), standard for most telescopes nowadays; and
• 2 inches (50.8 mm), used on larger telescopes and long-focal-length eyepieces.
Four-inch (100-mm) eyepiece tubes are occasionally used at observatories.
Why so many sizes? Partly for historical reasons, and partly because long-focal-length, wide-angle eyepieces require big tubes. The lenses of a 6-mm eyepiece will fit in any size tube, but a 40-mm eyepiece with a decently wide field needs a 2-inch tube. I have a 50-mm eyepiece in a 23.3-mm (microscope-standard) tube, but its field of view is quite narrow; it's almost like looking through a drinking straw.
You can generally use more than one size of eyepiece with the same telescope. On most Schmidt-Cassegrains or Maksutov-Cassegrains, you can use either a 2-inch or a 14-inch eyepiece holder. Many Newtonians have a 2-inch eyepiece tube with a removable 11-inch bushing. With Japanese-size (0.965-inch) refractors, you can use a "hybrid diagonal" that takes 11-inch eyepieces.
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