Measure the opening of the focuser or the diagonal. The two current sizes are 1.25 inches, which is 31.75 mm, and 2 inches, which is 50.8 mm. A much narrower opening of about 24.5 mm is the older fitting, and very little is still made for it.
This is the first thing to establish before buying any eyepiece, filter or diagonal, and it takes a ruler.
Where to measure
The inside of the focuser draw tube, or the opening of the diagonal if one is fitted. That is the hole a barrel has to enter.
The three numbers
About 31.75 mm is the 1.25 inch fitting, which is what most telescopes use. About 50.8 mm is the 2 inch fitting, found on larger and more recent instruments. About 24.5 mm is the old 0.965 inch fitting, still supplied on some inexpensive telescopes sold through general retailers.
Why the larger fitting exists
To pass a wider field stop, which allows a wider true field at long focal lengths. At short focal lengths the field stop is small enough to fit either barrel, and the larger fitting adds only weight.
What the barrel also decides
The filter thread. The smaller barrel carries the smaller thread and the larger barrel the larger one, so establishing the barrel answers the filter question at the same time.
The questions that keep arriving
Can a 2 inch focuser take 1.25 inch eyepieces?
Yes, through an adapter, and most are supplied with one. The reverse is impossible: a 2 inch barrel cannot be made to enter a 1.25 inch fitting.
Is it worth upgrading a telescope with the old fitting?
Rarely. The choice of eyepieces made for it is small and shrinking, and the telescopes it appears on are usually modest enough that the money is better spent on a different instrument.
Last reviewed 17 September 2026