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Telescope eyepiece barrel sizes: 1.25 inch versus 2 inch
Almost every modern eyepiece has a barrel of one of two diameters, 1.25 inch or 2 inch, and the focuser or diagonal on your telescope decides which will slot in.
The two standard barrels
The barrel is the polished tube of the eyepiece that slides into the focuser or star diagonal. The 1.25 inch barrel, about 31.75 millimetres across, is the mainstream size and fits the great majority of telescopes sold today. The 2 inch barrel, about 50.8 millimetres, is the larger format used for wide-field, low-power eyepieces.
A 2 inch barrel physically cannot show a wider true field than its own aperture allows, which is precisely why long focal length, low power eyepieces are made in the 2 inch size: the extra barrel width lets more of the sky through.
Which telescope takes which
The receiving end is the focuser drawtube on a refractor or Newtonian, or the visual back and diagonal on a Schmidt-Cassegrain. A focuser built for 1.25 inch accepts only 1.25 inch barrels. A 2 inch focuser accepts 2 inch barrels and, with a simple adapter, 1.25 inch barrels as well.
So the rule is one-directional. A 2 inch focuser is flexible and takes both sizes through an adapter. A 1.25 inch focuser is limited to 1.25 inch eyepieces and cannot be made to take a 2 inch barrel.
Adapters and diagonals
A 2 inch to 1.25 inch adapter is a short sleeve that drops into the 2 inch focuser and holds a 1.25 inch eyepiece. Many 2 inch star diagonals ship with one. This lets a single telescope use a low-power 2 inch eyepiece and a high-power 1.25 inch eyepiece from the same focuser.
Filters follow the barrel size: filters thread into the base of the barrel, so a 1.25 inch eyepiece takes 1.25 inch filters and a 2 inch eyepiece takes 2 inch filters.
Choosing which to buy
If your telescope has only a 1.25 inch focuser, buy 1.25 inch eyepieces. If it has a 2 inch focuser, you can build a set with 2 inch eyepieces for your lowest powers and widest fields, and 1.25 inch eyepieces, used through the adapter, for medium and high powers where the wider barrel gives no benefit.
| Barrel | Diameter | Typical use | Fits focuser |
|---|---|---|---|
| 1.25 inch | 31.75 mm | Medium and high power | 1.25 in and 2 in with adapter |
| 2 inch | 50.8 mm | Low power, wide field | 2 in only |
A 2 inch focuser accepts both barrels; a 1.25 inch focuser accepts only 1.25 inch.
Find eyepieces that fit your telescope
Open the Fit My Telescope gear finderPick your telescope to see the eyepieces and barrel sizes it takes.Find 1.25 inch telescope eyepiece on eBayaffiliateRelated
Sources: Celestron eyepiece and accessory documentation ยท Tele Vue eyepiece specifications