If you don't have a set of calipers to measure the diameter of your eyepiece, here's a guide on how you can use a few simple tools to get it done.
You will need:
- Thin strip of paper
Wrap the paper around the circumference of the eyepiece.
With the pen, mark where the end of the paper strip overlaps. (tip: put a little 'x' on the waste side so you know which end of the strip to measure).
Measure from the end of the strip to the pen mark.
Use the equation for the circumference of a circle (C=pi*d) to calculate the diameter 'd' of the eyepiece.