Astronomy Page
This page was last modified on 04-12-99, and is under constant contruction!
The Foucault tester (pronounced FOO -COH) is an inexpensive, but essential component in mirror figuring. Basically, the tester consists of two small tables (usually made of wood for ease of fabrication), with one being a 'base' and the other being the 'slide'. The slide table sits on top of the base, and it also has a 'tilt' function. The slide table holds a 'knife edge' (KE), light source, power switch, battery supply, tilt bolt and knob, slide stop block, and teflon (or other plastic bearing surface) 'V' blocks.
The base table holds the slide rod, adjustment bolt block and bolt, and tilt bolt slide pad. You may also wish to include rubber feet and a 'T' nut for a camera tripod to the base.
Usually measurements are taken from a dial scale bolted to the adjustment bolt, and a 0-1" side scale (in 0.1" increments). Using a 1/4-20 carriage bolt, we know u can get 20 turns per inch or 0.05" per revolution. I marked my scale in 0.002 increments, and you can easily read to 0.001" resolution. This degree of accuracy is sufficient for figuring beyond 'diffraction limited" optics.
You may also elect to add an inexpensive micrometer barrel instead of the home-made scale, or add an inexpensive dial indicator (0-1"). Do not worry about how your finished instrument looks, but it should slide smoothly with no bumps or shakes. If you follow these pictures, and take a little care in the 'V' block and slide rod fabrication, you are most of the way there. I have seen some pretty 'rough' looking testers (including mine!), and they all work just fine. I used some 30 year old pine shelving for mine...i figured that this wood would be stable by now. 3/4" plywood is also a good choice.
I recommend your light source be a green LED. You can purchase a standard green LED, a 47 ohm 1/4 watt resistor, power switch, and two 'D' alkaline batteries and holders from radio shack. Then buy some utility knife blades, using safety glasses and gloves, break off the sharp points of one of these blades on both ends (about 1/4"). If you can, now grind any sharp edge near the broken off point with a stone. DO NOT grind or touch the cutting edge, this is your light reference. Using a small block of wood 4-5" by 2", drill a hole near the top center to slide the LED into. Grind the rounded end of the LED flat with a coarse stone. Attach the knife blade so that it will 'cut' the light from the LED in half, but yet about 3/4" of the blade extend past the top center of the block (to sight past). It is helpful if you can grind a small hole in the blade's bottom, so that you can screw it to the wooden block and pivot it either in front of the LED or not cover the LED at all. This is useful when using a Ronchi (ron-key) screen.
You may want to add a 'sighting' LED to the knife block. See the last pic in this section. I used a high brightness LED from radio shack. It puts out a lot of light, and the returning 'dot' can be seen easily. You can hold up a plain sheet of paper just behind the tester to see it. Adjust the side-to-side aim and up-down at the mirror stand (my preference). I wired both LED's to a single pole double throw switch that is ON-OFF-ON (3 position) which will allow you to sight in one position, off in another, or read Foucault in the last position. Another handy addition, which I have not installed yet, is another medium brightness red LED and another switch which can be used to illuminate the scale or dial indicator.
The LED requires a 3 volt supply, and the 47 ohm should limit the current to the light source to approximately 20 milli-amps. At this rate, 'D' cells will last a very long time! Wire the batteries in series, to the switch, resistor, and finally the LED. The LED is polarized, so there is a plus and minus pin. The shorter leg is minus, or there is 'flat' moulded into the LED which is minus. The resistor can go into either side in series. You can use a high brightness green LED, but it is not necessary.
Click here for First Tester pic
Click here for Second Tester pic
Click here for Third Tester pic
Click here for Mirror Stand pic
Click here for Couder Masks pic
Using the Foucault Tester for the first time can be a very frustrating experience. At least it was for me. First you must make a mirror stand to hold your mirror, and it should have a 'tilt' knob on it to adjust the mirror from pure vertical to about +10 degrees past vertical. Next you must know the mirror's focal length.
Set the tester up two times the focal length from the mirror at the mirror's radius of curvature (ROC). 'Aim' the mirror towards the tester a little higher than you think is necessary, and search for the LED light. This takes a while your first time. After finding the light, adjust left and right and up and down until the light is just above the wooden block at the knife edge. You must have your eye 'behind' the knife edge, and look past it with one eye. Move the tester left or right until you can clearly see the knife edge dividing the returning light. Take the the tilt knob and turn it up and down and watch the motion of the knife edge shadow. It will move opposite directions whether you are inside the ROC or outside with similar movements of the knob. Move the tester fore and aft enough to see the shadows move opposite each other. Then try to find the 'null' between those two positions with your fore-aft adjustment knob, all the time using the tilt knob the move the knife shadow. When the mirror appears to darken from both directions simutaneously, you have found the null.
Now, you must make a couder mask. If you have not done so, download a copy of the TEX program from the web. With it, you can generate a mask, or at least learn the 'recommended' dimensions of a mask. Almost all telescope making books have this information in them.
You must now move the fore-aft knob away from the mirror (assuming you have already attempted some parabolizing strokes on the mirror) and keep adjusting the tilt knob to obtain a 'light gray' light in either the left or right zone one area of the mask. If the left side is brighter than the right (with the KE to the left), you need to turn the knob in and visa versa. Turn the fore-aft knob, and the tilt until both side have the same light intensity. Keep from getting the 'grayness' too bright. I keep adjusting the tilt knob to change the grayness intensity to insure an equal left and right balance. Remember: the mirror will only balance at one place for that zone. My mistake was that when I moved the fore-aft knob, the picture changed completely. That's all right, just re-adjust the tilt knob to get the same brightness level again. If you can make your tester parallel to the plane of the mirror (perfectly) you will not need to keep readjusting the tilt knob. I have never achieved this, but have been close where just a little 'tweak' was required.
Now balance each zone in turn, and, hopefully, you will be moving away from the mirror for each successive zone. Plug the data, along with your mirror's specifications into the TEX program. Some may also wish to purchase the ADMIRR program sold by Willman-Bell (W-B). Either program works well.
Back to Microsystems Home PageClick here for Ceramic on Plaster Tools pic
Back to Microsystems Home PageClick here for Small Mirrors pic
Back to Microsystems Home Page Back to Microsystems Home PageClick here for Small Figuring Laps pic
Back to Microsystems Home PageSix Inch DOB
12.5" TRUSS TUBE
Click here for Truss Tube Scope pic
Click here for Truss Tube Scope pic
Click here for Truss Tube Scope pic
Click here for Truss Tube Scope pic
Click here for Truss Tube Scope Cover pic
8" DOB
Click here for Krieg-style 8" Dob pic
Back to Microsystems Home PageHome Made Crayford Focuser
Recently Received BVC Blank
Notice the dark spots (small) in upper left. They are chips. Also notice the long defect on the left edge, almost 1 inch long. Dark spot stayed that way from the water I used to check the focal length.
I know it is not real clear from this pic, but there are several high spots, and low spots which bother me.
Back to Microsystems Home Page
