Foucault Tester for Free Loan


After figuring and refiguring many mirrors, my old #5 tester is available for others to borrow for use in their optical endeavors.

I built this tester nearly fifteen years ago, after my #4 disintegrated in an explosion of small parts after falling off of the carriage of my large optical bench. My #4 tester was a "Macintosh" style tester, and worked very well, but I was careless one day and failed to set a counterweight on my optical bench's carriage extension, and down went my #4. But, #5 was ready, so I began to use it, and it has served so well that I never built a #6, nor found it necessary to make any improvements to #5.

It is not likely that I will be doing any more primary mirrors, and so I want to keep old #5 out permanently on "circulating loan", so that it can be of service to others, instead of just gathering dust on the shelf.

There are no rules about borrowing it. Each one of you who requests it will be added to the list of scheduled borrowers, and will receive it in turn, in chronological order of request, after the person currently using it is finished with it. You may keep it as long as you require it to complete the figuring of your mirror. Then, after notifying me that you are done with it, I will direct you to ship it to the next borrower in line on the list. I expect it to be out, circulating, when I croak. I do not want to see it here again.

Let's take a look at old #5 in this picture gallery. All of the thumbnails will enlarge to full screen images that will fit your computer's screen best if you set your screen area at 800 X 600 or slightly larger. It is not my purpose to offend anyone by suggesting that they are short on cyber skills, but I am surprised how many people using a computer, and using Microsoft's Internet Explorer browser, are unaware that they can view a large JPEG image full screen by going to "view" on the menu bar, and dropping down the menu, and clicking on "full screen". My apologies to Netscape Navigator users. I am a Bill Gates slave. Please remember that after enlarging a picture, you may save it to your own hard drive with a right click for a contextual menu to select a "save picture as" dialogue box. Many of you, familiar with the elements of "kinematic" design, will want to skip reading the somewhat lengthy text explaining the fundamentals of old #5's design, and simply enjoy the pictures.


Our first picture shows the base of the unit from above right and slightly behind the unit. A Starrett micrometer screw head may be seen protruding out from the rear of the unit clamped in its little "L" bracket clamp on the base. Note the two slots in the foot of the "L", where it is secured with screws to the base, for adjustment fore and aft. This screw head reads (with its vernier scale) in ten thousandths of an inch (reads to .0001"). This vernier is really unnecessary, as the limits of the Foucault test set its accuracy to only near thousandths. I bought two of these to use in a caustic tester, which requires reading to .0001".

Moving forward along the base plate we can see the four machined "nosecones", that form two pairs of contact points (for four contact points) to carry the Y axis stage's sliding axle and define its motion (restrain it from any side motion, but allow motion along Y only, and precisely). These "nosecones" were each turned at an angle of 45 degrees, to form an included angle of 90 degrees with each one's opposite, the ideal angle for support and restraint for a mechanical movement supported "kinematically". Two simple "L" brackets, with deep 90 degree "V" notches cut into them, would substitute nicely here for these four nosecones, and be much easier to fabricate with only hacksaw and file.

A little brass "guardhouse" shaped component can be seen very near the left edge of the base and midway longitudinally between the two pairs of "nosecone" supports for the Y stage's slide axle. This little "guardhouse" provides the required fifth contact point to restrain motion to linear only for the Y-axis movement; the four contact points of the nosecone supports restrain lateral movement, and the guardhouse support restrains rotational movement. This fifth contact point, to restrain rotational motion about the axis, is usually referred to as the "outrigger" support, as its function is analogous to that of the outrigger of a canoe, preventing "tipping". Six degrees of freedom of motion, minus the number of contact points (5) leaves one degree of freedom of motion- forward and backward along the Y-axis. The three small black objects are the heads of the adjustable machine screw feet supporting the base.


In this next picture we see, again, the base (on the left) and the Y-axis stage to its right, upside down here to expose its underside for inspection. To visualize how it goes onto the base cradle to the left, pretend (mentally visualize) that you are putting your open right hand, palm up, under the Y axis stage, and then, while lifting it up, turning it over to the left as you then place it onto the base cradle. You will see, now, how the five eighths' inch diameter cylinder (drill rod stock) clamped to the underside of the Y axis stage (adjacent its left edge) with its two large brass clamping blocks will cradle and slide in the two sets of two nosecone supports on the base cradle, and how the little short length of half inch diameter drill rod along the right underside of this Y axis stage will ride (slide) on top of the little brass guardhouse support of the base cradle.






In these next pictures we have accomplished what we mentally rehearsed in the paragraph above, and have picked up the Y-axis stage, flipped it over to the left, and set it on its five contact points on the base cradle. Now- a careful inspection of the photographs will reveal that the top of this Y-axis stage is functionally a replica of the base cradle. There is a micrometer screw to drive a movement to be cradled on top of this stage, and there are four contact points for the drill rod axle of the stage it will carry. The four contact points on this stage are little loose ball bearing balls epoxied into countersunk sockets. The outrigger contact point for this stage, however, is defined by the tip of a screw protruding from the underside of the stage it carries with this screw riding, sliding on a little outrigger "runway" seen here on the top of this Y-axis stage. This little runway is just a little short section of Starrett flat ground stock epoxied in place.

Rubber bands, not shown in any of the photographs, are looped behind the L bracket of each stage's micrometer driving screws to provide return motion, keep each stage's drill rod axles' face in contact with the face of its drive screw anvil.






In these last two photographs, we show the unit completely assembled with the last, topmost stage that carries the light source, slit, and knife-edge. The razor blade that forms the knife-edge is contiguous with, and forms one of the jaws of the slit. An arrangement of the light source/slit above or below the knife edge and contiguous with it will prevent any anomalous, "false" indication of astigmatism even when viewing a very short focus mirror, as the knife-edge can "see" a one sided aberration such as coma or astigmatism only if the aberration has its axis lying across the knife-edge a 90 degree angle; inasmuch as the coma in the image of the slit is only parallel to the knife edge when it intersects its off axis image with this arrangement (compared to the traditional side by side arrangement of slit and knife-edge), it cannot see it, even though the coma or astigmatism is still there. (The actual case for astigmatism is slightly different, but the result is the same).

Old #5 showed me the completely evenly illuminated figure of a 12.5" f/2 Schmidt Camera mirror that I ground and figured for a customer when GPI was still a commercial enterprise. I doubt if any of you will have a complaint about how well it performs. The largest mirror I ever figured with it was a 20" f/5.5, and the fastest mirror I ever figured with it (paraboloidal, other than the Schmidt mentioned above) was a 10" f/4.5. It works extraordinarily well.

Now, send me your requests for borrowing old #5!!!


Contact Dave
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