a rather oddity

Started by Jim kuhns, 06/27/2013 03:48AM
Posted 06/27/2013 03:48AM Opening Post
I have a Klee 2.8x barlow i bought of "Astromart not very long ago.While using it several nights ago i put a 20mm Meade research grade in to view Saturn in my 10" f/6. I changed to a 16mm Konig University and discovered the 20mm is giving me higher power than the 16mm.Both eyepieces are from the 80s and probably the barlow too. note> when i slide the 16mm out a bit, the magnification increases and the same results with 20mm. i had someone to verify, he agreed. I found some extensions at a local "Ace Hardware" store and cut to different lengths and can get all different magnifications. i have seen advertize variable barlows. Anyway can anyone have explain why the difference in the 16mm and the 20mm?
Posted 06/27/2013 10:42AM #1
The magnification of a Barlow is defined by the formula M = 1 + (x/f) where x is the location of the final image behind the lens and f = Barlow focal length.

The final image must be at the eyepiece field stop when the everything is in focus so it seems you have two eyepieces with considerably different locations of the field stops, one towards the end of the eyepiece barrel, and one much further up inside the barrel. You can easily check this by using the eyepieces without the Barlow and noting the difference in the focuser position with each eyepiece.

Variable Barlows do so by changing the distance where the final image is formed, similar to what you have done using extensions. It is also common to change a Barlow's magnification in a refractor by placing the Barlow in front of the diagonal instead of after the diagonal.

Hope this helps,
dan