Rayleigh tester


			Remodeling the Coulter 17.5": Raleigh Water Test / Fizeau Interferometer

The cad drawing (at right) was inspired from plans by Ed Jones, "Gleaning from ATMs," page 89, July 1990 issue of Sky & Telescope. I was going to build this as shown, but wound up just throwing together some things from around the house - as you can see in the pictures (below). (continued below...)

	The secondary mirror to be tested is placed inside a small cake tin, barely covered with distilled water dyed with food coloring. A piece of cardboard rests on the cake tin. The cardboard has a hole cut out, the circumference of which is just smaller than the magnifying glass it supports, with a thread stretched across its diameter. This apparatus is placed on a leveling base made from two pieces of wood and three spring loaded bolts with wing nuts. Sockets from a wrench set were used as spacers. The neon bulb (Radio Shack cat#272-1102 - important that this be neon, as a monochromatic light source) is in series with a 22k-ohm resistor on a 120v AC line. I put these on a breadboard (don't electrocute yourself!) with the bulb just behind the edge. Hold this edge right in front of your eye (bulb on the other side from your eye) directly over the magnifying glass at its focus (almost like looking past a slitless foucault tester - you can see the donut-shape ala foucault in the photos. This is caused by the magnifying glass, not the flat, and is of no importance here). The trick is getting the flat level to the water. You won't see any fringes until they are almost perfectly aligned. To do so, I bounced a pocket laser pointer off the water-covered mirror and adjusted the leveling screws until the beams coincided at 20 feet. (You could just as easily align an overhead light's reflections, the one on the water surface with the one on the mirror below the water.) Also try adjusting the water depth (a turkey baster is useful here) and amount of food coloring to get good contrast on the fringes. You don't need the food coloring if your flat isn't yet aluminized. Too much water over the mirror (more than 1/8 inch) and the fringes won't appear, while too little causes surface tension effects. Watchout for floating dust - these little specks cause 10 wave depressions on the water surface. I would also recommend building a much larger leveling base with very fine adjustments (10:1 levers under the adjusting screws would help). To get results, the string acts as a reference line across the flat. If the fringe curves from edge to edge so that center is displaced from the string by one fringe spacing, then you have 1/2 wave surface error. You have to play with the leveling screws to get about 8 fringes across the flat and parallel to the string. Then rotate the string (and fringes) and measure along the other axis. The entire apparatus should be on a concrete floor where there are no vibrations. You can see in my pictures that I never snapped a shot with the water completely still. The top right shot of the middle photo series above was taken just as a motorcycle drove by my house.

		lookie here - dagley design/build star stuff - the mac - boise