eyepiece "field stop" what is it?

[sunshine185]

is there a simple way of working out an ep`s field stop?

[John]

Some companies. eg: Tele Vue, give the diameter of their field stops in the specs of the eyepiece ranges on their web site. Others don't. With simple types (eg: plossls, orthoscopics) you can physically measure the field stop with calipers as it's usually accessible in the barrel of the eyepiece. With more complex types the field stop is hidden within the body of the eyepiece so it's not accessible. Trawling around the web can sometimes throw up the specs or, if all else fails, you can time a star drifting across the field of view, work out the true field of view and work out the field stop diameter from that - I can't recall the maths on that offhand.

[DirkSteele]

I will add in the last step following on what John above said. Once you have the true field by timing a star from one edge of the eyepiece to the other, take that number, say 1 degree, multiply it by the focal length of your telescope, and divide the result by 57.3. The focal length should be in mm and this will give you the field stop diameter in mm.