Barlow magnification

This is a little off topic but... I made a couple of homemade barlows from a sky and telescope article that have turn out to provide excellent views but now that I want to do some CCD imaging I want to better estimate the magnification. They originally were 2X and 3X teleconverters but when I time the ratio of an objects drift across the FOV in the eyepiece as compared to the eyepiece and barlow I get about 2.6X and 3.6X.

Is that the correct way to estimate magnification?

Thanks Dave

It's the most accurate way. The actual multipling factor depends on the distance of the eyepiece's focal plane (it's field stop) from the Barlow lens itself. Different field stop positions with respect to the shoulder of the eyepiece will yield different multipling factors for given eyepieces. You can vary the "power" of any Barlow simply by changing this distance; with a single Barlow this can be accomplished by sliding the lens inside the tube- thus a variable Barlow. I use felt tube inserts to line the holder tube for baffling, a different one for different powers. Without the eyepiece in place, as for photography, the multiplication factor can also be varied by changing the distance to the film plane. Your 2X and 3X teleconverters could be so labled because they worked with fixed measurement distances with regards to the camera/camera lens. So by reasoning since the 2X teleconverter multiplys less at the same given distance than the 3X one it has a longer focal length, and it could also be used at 3X with a longer mounting tube. And the 3X in this same longer tube will multiply even more. Ron

One other note, if you cut a slit part way down one (or opposite) side(s) of your mounting tube at the eyepiece end, you can obtain a friction fit to accomodate eyepieces. Vertually all focusers, diagonals, and barlows in earlier days used such a method and I prefer this to set screws especially for gloved hands. Ron