Author Topic: How to Measure the Focal Length of Barlow (teleconverter, telecentric, Powermate  (Read 27 times)

Jody Mukherjee

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I had a project where I needed to know the focal length of a Televue 2X Powermate. I couldn’t find this information on a web search so I decided to measure it myself. This is how I measured the focal length of a Barlow.

The Powemate is a diverging lens assembly and has a negative focal length.
e.g. Catch sunlight with one and there is no convergence of light into an intense focal spot as with a magnifying glass (convex lens). Instead the light diverges with no apparent focal spot. Thus measuring the focal length of such a ‘concave’ lens is a challenge.

There may be several ways to do this but this is how I measured the focal length of a 2X Powermate.

Here’s my Televue 2X Powemate.Unscrew the extension barrel (black) from the lens assembly (chrome).Here’s a close up of the lens assembly. In use, light enters the bottom and exits at the top (black ring).I needed a laser to perform the measurements and used a Kendrick laser collimator.
Note: Any laser with do; I was going to use my $2 dollar-store laser but couldn’t find it.I mounted the lens assembly in a vice – very gently.I mounted the laser in a cross-slide vice and aligned the components.
Note: The back reflection form the first lens element shows on the front of the laser so I would adjust positions to get the laser reflection onto the center of the laser.I fixed a sheet of graph paper behind the lens assembly so that I could mark where the laser dot hit. I began with the laser striking the center of the front lens element and marked where the laser dot hit the paper.TO BE CONTINUED…