One word of advice: NEVER ask someone for help unless you're ready
to surrender the project! <grin>
The one time I asked for help, the tester came back to me with some
of the bells and whistles I tried to keep out of the design!
Don't look for any spiffy JAVA apps, animations, cool sound bytes or any of the other bells and whistles that eat up memory and bandwidth! The ONE thing you CAN do is click on the pictures to see an enlarged view.
This is just a down-and-dirty page to show my Foucault tester. It's hardly state-of-the-art, but it does what it's supposed to. And the best part is that it didn't cost me a lot.
So without further delay, let's take a look at what you can do with less than fifty bucks....
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This is the "working end" of the tester.
It's the side you're on when you're using it. As you can see, the primary
construction material is wood. Because I don't have an actual workshop,
everything you see, with only one exception, was made in my basement with
a Black&Decker WorkMate bench, saw, hammer, T-square, electric drill,
and a Dremmel rotary tool. We'll talk about the one exception a little
later on. It's not much, and my original design worked just as well, but
it's something that I couldn't have done at home.
One word of advice: NEVER ask someone for help unless you're ready
to surrender the project! <grin>
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| This is the probably the most expensive part of the
whole tester. It's a dial indicator gage that I purchased from Harbor Freight
for ten dollars plus shipping. It has a one inch range, and reads in increments
of one thousandth of an inch.
While there are other options for determining the distance that the tester travels, I wasn't "up" for designing a dial that would approximate what this gage does so well for what is really a meager sum. It's attached to its mounting block with a simple bolt. Since the maximum useful travel is only an inch, exact alignment isn't strictly necessary. Every little bit helps, of course, but the error introduced by not having the gage exactly true is very, very small. |
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Here we can see the hole that you look through when viewing
the mirror (if you look closely, you can see the knife edge in the hole.
Below that is the LED, which is covered by duct tape to keep its light from flooding your eye while trying to see the mirror. The left/right carriage rides on a steel rod. To its right is a 1/4" 20 thread carriage bolt which is used for moving the shadow across the mirror. The "knob" ends were purchased at Radio Shack. Below the left/right carriage is the forward/back carriage. The aluminum angle iron is there to give the indicator a nice even surface to help avoid measurement errors. Then comes the indicator gage and its mounting block. The mounting block looks crooked because it is. Having room for only one screw, it does move a bit. I'll have to glue the two pieces together before I tighten the screw down again. Next to the dial indicator is another carriage bolt that controls the forward/back movement. |
| This is the "front" of the tester. The battery up-ended in the
battery holder is my low-tech answer to a rocker switch or other means
of turning the light off and on.
The knife edge can be seen through the viewing hole. The blue pad on the middle "layer" is a teflon pad that I bought in a package of four at WalMart. It's not something I'd use on the telescope itself, but for this application it's pretty good. We'll see another use for these pads in a few minutes. You may also notice that the middle board is somewhat warped. That's what I get for using scrap wood. Fortunately, since the movement of this carriage is front to back, the warping doesn't affect performance. |
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This is a view of the bottom of the left/right carriage with its "V"
blocks. These blocks ride on the steel rod to provide linear movement.
To provide the smoothest movement possible, the insides of the "V" have pieces of teflon tacked to them with finishing nails. This provided a low friction surface so that movement is as smooth as possible. When assembled, the carriage bolt coming through the bottom center of the left/right carriage rides on the long teflon pad to provide a support for the carriage itself so that movement is as linear as possible. |
| Here's a closeup of the "V" blocks with teflon pads attached.
In general, the angle of the "V" should be deep enough to allow the teflon
to be in good contact with the steel rod.
If the angle is too shallow, the carriage will "twist" on the rod, giving you inaccurate reading. It is best to cut these "V" blocks in pairs. It's important that the angles and orientations of the two block be the same to provide a smooth, uniform movement across the steel rod. |
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This is a view of the left-hand side of the tester. You can
see how the left/right carriage rides on the steel rod, and how the carriage
bolts ride on the long teflon pads to provide support and low friction
movement.
The carriage bolts should be held in the holes in the wood with "T" nuts. They provide not only a secure fit, but also a thread, which allows you to adjust the carriage to keep it level. You may notice that the carriage bolt supporting the forward/back carriage (the middle layer) has a nut sunk into the board.... I ran out of T-nuts. This method will work if you use the nut on the bottom side of the board. That way, the weight of the board and everything on top of it will keep the nut secure. Having the nut on the top side of the board (as pictured) will eventually push the nut out of the board. The heads of the carriage bolts usually have raised lettering that can impede the movement across the teflon. In some cases, it's actually enough to scratch the teflon. A few minutes with a file and emery cloth will smooth the surface and keep these bad things from happening. |
| This is the forward/back carriage. The carriage bolt is held
in the block by a T-nut.
The steel rod is held in place by a bracket. This is the part that I mentioned at the beginning of the page. My original design used three wood screws on each end of the rod to secure it. One screw was screwed into the wood, leaving about 1/4" inch from the top of the screw to the board. Then the rod was placed on top of the screw head and a screw tightened on either side of the rod to hold it securely. As long as care is taken to ensure that both ends of the rod are the same height from the board, this method works well. The design pictured here has the ends of the rod drilled and tapped, with a screw holding the rod securely in the bracket. |
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Here is the front of the tester again. You can see another low-tech
mechanism here... the "return spring" which, in the real world, is known
as a rubber band. Look just below the top board. There it is, hooked to
a cup holder on the right side of the picture, and around a wood screw
on the left side.
The carriage bolt can be adjusted to level the carriage. Using a 2-1/2" carriage bolt allows a good amount of movement for this purpose. You can also see the rubber band for the forward/back carriage hooked to a cup holder on the bottom of the middle board. This picture also gives a good view of the V-blocks for the forward/back carriage. |
| Here's a view of the right side of the tester, showing both
movement adjustments, the dial gage, knife edge assembly, battery holder,
etc.
Since we've been over all of the components already, an in-depth decsription really isn't necessary. But a look from a different angle never hurts, right? One thing that you CAN'T see in any of the pictures is that I've used Velcro pads to secure the battery holder to the left/right carriage. When you're testing, the one thing you DON'T want is stuff moving around! |
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Here's a view of the "eye" side of the knife edge assembly with
the duct tape removed.
You can see the knife edge and the lit LED. A double thickness of wood was used here only to provide enough wood to "bury" the LED. Wiring the LED is a fairly easy step. The "+" lead has a 47 ohm resistor soldered to it. That's about the extent of "electronics knowledge" that you need.... Otherwise, I couldn't have done it. <grin> |
| This is a closeup of the side of the lightsource and knife edge that
faces the mirror.
As you can see, low-tech reigns supreme here. The face of the LED was ground down and roughed up with grit to diffuse the light. The knife edge, a blade for a razor knife, is held to the board with duct tape. This will allow me to quickly replace the knife edge with a Ronchi screen. The placement of the light source and viewing hole are not critical, but they must both be cut roughly in half by the knife edge. The flattened "face" of the LED is quite close to the knife edge, but not touching it. The light from the LED strikes the mirror and returns to the viewing hole. This is the part of the tester that always confused me. Having seen a working tester, I was able to build this one. Using duct tape to hold the razor blade to the assembly allows me to easily switch the knife edge for a Ronchi screen. |
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Well, that's the extent of my technical and editorial prowess. The tester ain't pretty, but it works, and works well. The movement is smooth, and the dial indicator gage tracks very nicely.
I would like to thank my friends David Harbour, Dan Fundo, and Dan Cassaro for all of their help and encouragement. Without them, I doubt that I'd be as far along in this project as I am now.
If you're looking to purchase plate glass mirror blanks, Dan
Cassaro's page is the place to go.
If you're as confused as I was about Foucault testing, David
Harbour's Foucault page is a must-read.
If you have any questions or comments about this tester or this page,
please feel free to email me at:
j.mirando@snet.net
ICQ# 37479822
AIM: Joe Atari