![geometer geometer](https://3.bp.blogspot.com/-3vH4TGCBTnQ/U7A9--OMvdI/AAAAAAAABEo/ZCxwqaOPZN0/s1600/mearusring+length+and+distance+GSP.jpg)
Purchasing high-precision, very expensive commercial interferometers to measure the surface of the mirror is out of the question, but it turns out that very inexpensive methods have been developed for doing that – at least on Newtonian telescopes. At the telescope-making workshop here in DC, I want folks to be able to make the best ordinary, parabolized, and coated mirrors possible with the least amount of hassle possible and at the lowest possible cost. Is light a wave, or a particle, or both? Why can nothing go faster than light? We forget that humans have only very recently discovered and made use of the vast majority of the electromagnetic spectrum that is invisible to our eyes.īut enough on that. Optics themselves are amazingly mysterious. Towards the end of this very long post, you can see the corrections, if you like. It involves optical testing for the making of telescope mirrors, which is something I find fascinating, as you may have guessed. Several people have helped me with this applied geometry problem, but the person who actually took the time to check my steps and point out my error was an amazing 7th grade math student I know. I don’t know what algorithm FU uses, but it sure is f***ed up.
![geometer geometer](https://sketch.io/media/sketchpad-4.4.jpg)
![geometer geometer](https://learningcenter.dynamicgeometry.com/Images/tutorial-2_01.gif)
As you can see from these images, FU is unable to find zones that are symmetrically placed on either side of the center of the mirror. There is an app that supposedly does this for you, called Foucault Unmasked, but it doesn’t seem to work well at all. Now I just need to do the same thing for all of the other images, and then correlate the radii of the bright zones with the longitudinal (z-axis) motion of the camera and stand, and I will know how close this mirror is to a perfect paraboloid. This is a somewhat crude measurement of the radii, but it appears that this zone is is at 83% of the diameter (or radius) of the original disk, which is 16 inches across. I then captured and pasted that image into Geometer’s Sketchpad, which I used to draw and measure the radii of two circles, centered at the doughnut marking the center of the mirror. A bright ring appeared, which shows the circular ring or zone where the light from our LED, located just under the camera lens, goes out to the mirror and bounces directly back to the lens and is captured by the sensor as a bright ring. Just now, I finally figured out how to use IrfanView to take one of the images, flip it left-to-right (that is, across the y-axis) and superimpose one onto the other with 50% transparency. I used an old Canon FD film camera lens (FL=28 mm) that I got about 40 years ago and haven’t used in several decades to get a bunch of really nice knife-edge images of a 16″ Meade mirror, located on a stage that can be moved forward and back in whatever steps I like by a smartphone app and a stepper motor setup that Alan Tarica and Pratik Tambe designed and put together. Same mirror, same location as the image directly above, with circles and measurements added in Geometer’s Sketchpad