4 April 2007

It was raining tonight so after a very abbreviated HoSARC radio net I set about determining the focal length of the Kellner eyepiece.

I remembered that I have the photography adapter for the binocular, which has a 1/4x20 thread for a tripod. It is just what I need to hold everything together for the experiment. I mounted the lens in the adapter and taped a transparent ruler behind it. That was mounted on a tripod set up in a hallway with a flashlight shining through the lens and projecting an image on a wall about eight feet distant.

Here you can see the barrel of the Kellner lens sticking out of the front of the camera adapter.

The transparent mirror with 1/10” grid is taped to the camera adapter behind the back end of the eyepiece. The box of old slides was taped to the adapter to get the flashlight at the right elevation to shine through the lens.

Here is the camera adapter set up on a tripod in the hall. There is an image of the grid projected on the wall but the flash from the digital camera I used to make this picture overpowered it. I tried to take a picture of just the projected image but it wasn’t bright enough to register on the camera’s CCD.

I ran the calculations several ways and found that it doesn’t matter too much if I measure the distance to the wall from the location of the ruler or from the approximate center of the lens. For these calculations I used the center of the lens.

Distance from center of the lens to the wall	113.285”		99.785
Number of squares measured	3		4
Width of measured squares	32.5”		39”
Magnification	108.3333		97.5
Focal Length	0.95629” 24.28977 mm		0.977101” 24.81836 mm
Average Focal Length		24.5 mm

With my telescope that gives me a magnification of 17.5x or 35x with the 2x Barlow.