I mentioned that using a bar spreader, I managed to finally stretch the short diameter of the upper tube assembly to within 1/8" of round--which is good enough. I can always adjust small discrepancies with washers. (If only I had--or could afford--a lathe large to turn a 20" ID piece of aluminum!)
Anywhere, here it is.
Click to enlarge
I still have to drill the hole in the side of the tube for the eyepiece focuser. That's a 2.25" diameter hole. Fortunately, I have a 2.25" hole saw. (The saw is actually more like 2.23", but I suspect that it will actually make it 2.25" by the time it gets done--and I can sand or file to make it a few thousandths of an inch larger.)
I have been holding off on this operation until I had the new elliptical mirror that I am going to use--one that has a 3.1" minor axis. It has arrived. I bought it used for $100, including shipping, but I can't tell that it isn't new. Purportedly it is 1/10th wave flat. If you don't know what that means: it means that the across the surface of the mirror, there is no variation from flat that is more than 1/10th of a wavelength of blue-green light.
If this seems like startling accuracy--opticians do this all the time. Or at least they advertise that they do it all the time, and I know the methods used for verification. Newton's Rings is an inteference phenomenon produced by the air gap between two surfaces. The example that Wikipedia gives involves a flat surface and a spherical surface, but the same technique can be used with a mirror of known flatness, and another mirror of unknown flatness. The air gap between them produces the rings--and the number of rings that you can see tells you how many wavelengths of light are involved. The pretty rings that you sometimes see on when oil is floating on water are produced by the same mechanism, apparently, and it was this that led to Newton's work on this.
No comments:
Post a Comment