Building My Focal Length tester

(Day 3, Pat 1)

We last left off, I had two mirrors of a 64-inch focal length, ground to 25 micron.

Yesterday I realized that It was difficult to hold the mirror, a tape measure, focus the sun on the fence, AND read the tape measure all at the same time. Plus the dang sun kept moving causing me to at the end to hold the mirror over my head to align the focused spot with the fence. Needless to say, I needed to do something different. Here’s what I hacked together out of scraps I found in the garage.

Its ugly, but fully functional. You put the wet mirror in the holder, point the big long stick at the sun, and use a piece of paper to find the focus, then read the focal length off the marks I made on the long stick. Pretty simple and worked great until the Sun went behind the clouds for 5 straight days… GEEEZZZ

Being the tester didn’t work on cloudy days, or during the night, I was fortunate that the measuring stick was long enough to also measure the Radius of curvature of the mirror, which is twice the focal length. To do this I placed an LED at the assumed ROC of 128 inches and again using a piece of paper found the best focus. To adjust the focal length we either grind with the Tool on Top of the mirror to Make the Focal length longer, or Grind with the Mirror on Top to make the focal length shorter. Sounds easy, but its very simple to get confused and start heading of in the wrong direction, something I did on more than one occasion.