Image of the day

From the
ATWB Customer Gallery

M13 RASA 11

My Account

New to Astromart?

Register an account...

Need Help?

Measuring EP Focal Length

Started by wpaolini, 11/20/2006 09:33AM
Posted 11/20/2006 09:33AM Opening Post
Is it possible to measure the focal length of an eyepiece to determine if it is what the manufacturer claims??

I understand it's not really possible for those eyepieces with integrated barlows. But for others, no measurement seems to be any where near close when I measure where it comes to focus with a light source.

For instance...I have a 60mm Kellner. But even with this, if I move a paper in it to find where it brings an image into focus, it is way different from 60mm distant from either the lower or upper element. Flipping it and measuring where image focuses in reverse also produces nothing close to 60mm.

Any suggestions?
Posted 11/20/2006 09:41AM #1
>>>Any suggestions?

I have looked into this only a bit, mostly asked David Knisely about it... David has a scheme for measuring magnification I believe.

No easy way that I know of to get a good number. Measuring exit pupil is difficult.