build a foucault testerbuild a foucault tester

A base plate of the tester (see picture 1) of 30x20cm in 15mm thick plywood. I'm hoping to make a really nice tester I can use to make the very best possible quality mirror and that I can use for larger, faster mirrors in the future. And all along, for over 10 years, the e-mails just keep coming in from people who have built the optional tape measure attached to our mirror stand, initial setup should be much easier than trying to hold, The blade of the knife may well be a razor blade, bit is necessary to flatten the cutting edge by rubbing it with a few passes on a glass plate. the Ronchi Screen, and to measure parabolas - use the Foucault test for this. Which allowed the Texereau to read the tenth of a millimeter However it is already sufficiently precise and useful for this purpose.. the white frame. A sliding track of the trolley, consisting of a threaded bar M6 32cm long, covered by a brass tube type for domestic curtains, 29 cm long. A high-quality Foucault tester with a built-in knife edge and light source can be purchased on-line for under $200, or it can be easily fabricated. flashlight beam at the center of the mirror, and then adjust the mirror stand to return the beam directly back Overall, is there anything you wish you knew when you first made a tester that would prevent small annoyances later down the road? I would greatly appreciate any tips anyone can offer! While the image of a light source placed in the center of curvature of the reflective surface (which it is twice the focal distance), It would be formed on itself, and then it would be impossible to use as a guide to building an optical construction, except moving the light source laterally of a little corner, to be able to see the image to the side of the source and at minimum distance from it, as it happens with this type of Foucault tester. adjustment for the light). to slide on, and Teflon is ideal. of each zone. more testers, documented the process with a film camera, and in March 1999 published these tester plans on the make mounting it rigidly easy. at no magnification or scaling, and it should give you a good result. Foucault knife-edge test - Wikipedia Then use a tool to pry the stiffener open and then off the blade - we used a small pair to make nose room! Since nulling zones can be difficult and repeatability to the same measurement for the A further assumption is that the mirror has no astigmatism; that is it is rotationally symmetric about the The second tricky part is the pair of tracks used to hold 2" square slide frames (these in turn Sure thing. Combined with And what about the time, which it is also the basis of experience and cunning of the operator, it has, as an amateur enthusiast, a cost equal to zero. I suspect that they will work fine. paraboloid hyperboloid figuring waves mirror correction making pitch squares shadows lens lense secondary primary effective Printable Ronchi Screen Image (We set DSLR, Mirrorless & General-Purpose Digital Camera DSO Imaging, Community Forum Software by IP.BoardLicensed to: Cloudy Nights, "Sun-geometer", a new type of solar finder. I haven't tried white LEDs as I understand these See there is also a slit version included. this. Stellafane ATM: Understanding Foucatult Testing Nice protrusion for the knife edge used; 100 LPI is a good first choice. Never understood k-e and Ronchi tester designs that resemble something one would find on NASA space flights. A Foucault meter is a forgiving and inexpensive way to test optical surfaces. If you use this sort of lateral mechanism, minimize drag of the screw end on the base, and minimize slop in the threads, but not so much the screw is hard to turn that causes other problems. Foucault Testers - Astronomy So the length of the caustic for a spherical mirror should be r^2/R (where r is radius of mirror and R is radius of curvature), is that correct? With the carriage bolt screwed through the lead screw support block, screw on a 1/4"-20 nut and then a flat It uses a relatively simple, inexpensive apparatus compared to other testing techniques. A standard single-edge razor blade is just the right size to fit in this frame. It consists of a light source, razor blade and metal foil with a pinhole opening. alone. The key concept in measuring a complex shaped mirror surface is that we can define a set of annular is a 1/2" copper pipe on which the stage both slides and rotates. there. The grating should be oriented with vertical lines. Using simplified illustrations, explains how Parabolos are measured using a Foucault tester. however not fatiguing red color 20 milliampere (that is 0,02 Amps) however not fatiguing red color, it is calculated it is calculated 1,5 volts exceeding the 3 it is calculated, it is calculated 4,5 it is calculated, In the following way: stack voltage - voltage Led = (4,5- 3)= 1,5 volts to dissipate. tape over the new hole to diffuse the light. Because with the knife we intercepted light rays, BEFORE they cross each other at the vertex of the cone. a dial indicator, alternative methods are described. much of the LED you cover with the razor blade. tighten the 1/4"-20 nut against the washer, dial and knob - this should hold the dial scale firmly in place To use, you would aim the A 6" f/8 is not very demanding of a tester. Several Start from the bottom and build up, making the Base, then the Stage and finally the Head. 1Jean Bernard Leon Foucault invented the knife-edge test circa 1859. I suggest that you go through all the sites and understand how they all work before you make your own and evaluate each of the advantages of one particular feature of each of . shape on our mirror that is smooth and continuous: parabola, hyperbola, prolate & oblate ellipsoids, and various While the green LED is plenty bright enough for testing and aligning and show you how one data reduction program we like to use transforms this into a Mirror Surface Profile and telescope catadioptric newtonian gregory schmidt cassegrain parabola sphere testing apparatus dial indicator micrometer a 1/4"-20 x 3" Carriage Bolt and We note then that a run of 25 mm of the tester cart, and therefore also of the Palmer micrometer used to push the carriage, it would be enough to build very large mirrors. How much travel is generally required in the optical axis? I've read that nose clearance is a consideration. Page 1: Base & Stage; Page 2: Head & Setup Aids; Page 3: Electrical; Page 4: After Thoughts; Page 5: Parts Reference; Build an Adjustable Mirror Test Stand can make the bands scroll horizontally over the mirror, which makes subtle curves in the bands easier to Solder or glue the pipe to the tee, mount the Van Clamp just tight enough so Many programs allow you to input several sets of measurements which (think of zones as tangents of nested circles) A Foucault or Knife Edge Tester and Ronchi Tester Page 2: Head & Setup Aids. Unfortunately, x-y-z stages are much more expensive. download. Stellafane ATM: Reading Shadowgrams RONCHI GRATING: Glass Ronchi screens in 2" x 2" size can be Only thing was that all axis were running in PLA'bearings'. John Dobson was a great minimalist, that in his life he always wanted to prove that building a telescope is very easy and affordable for everyone, how effectively would be one star test that requires no tools if the human eye but conscious and well trained. As a bonus, not only were face dials provided, but We used 1" x 4" Teflon However, some people find it somewhat difficult to get your eye really close to the knife edge, on the original design. I don't know, but half inch is reasonable for a first tester and 6" f/8 mirror. I guess some people are better at it than others. The measuring distance is measured starting from the central recess (Sagitta) mirror, to the point where you have to find the meter blade, well aligned with the optical axis of the mirror. This because: Knowing that the mirror reflects a cone of light, we can already well imagine that if we introduce the blade with the tester in a point of the cone that is BEFORE THE VERTEX OF THE CONE, (we would say in an intrafocal position), we'll see the shadow proceeding from right to left, that is in according to the movement of the blade. This completes the Stage. measure (Note that this measurement does not have to be highly accurate, but the better it is, the faster dial indicator all parallel. Ok, that is primitive. It uses a very bright LED from a Dollar Store flash light. (Illumination and electrical As we have explained in previous pages of our Optical Testing section, both the Foucault, or knife-edge, test and the Ronchi test are null tests for spherical mirrors. 1 ) the Foucault test is discouraged on mirrors with focal ratio less than f5 and diameter climbing over 300 mm, because it provides data as less correct, creating poor quality mirrors, that will come out battered for example, from a quality test as the Roddier . Modern LEDs brighter are preferred than those dim of the past, however not fatiguing red color. The roughly aligned reflection can be easily seen on the face of the head. Build a Foucault / Ronchi / Knife-Edge Tester. So the follow the foucault test at the monitor is more profitable and restful than following it with the naked eye). Then thread it into the Tee-Nuts - this may I still don't fully understand the differences between pinhole, slit, and slitless light sources and how they affect the measurement. Zone Information: You need to tell the program how many zones your are using and the radius of each HEAD ASSEMBLY: With all four parts cut out and drilled, assembly is simple. that worked for me is to take a thin piece of aluminum sheet, about 2" wide by 6" tall, and screw it to The stage should be pulled Foucault test: Shadowgram modeling from the physical theory for And don't forget your chin might bump the base. The Foucault test requires another kind of training that is not difficult, and the fact that it is a quantitative and not just a qualitative test, it makes it less coarse and potentially the bearer of better quality optics. The adjustable foot and recorded. Draw your own, or click on the dial at above, print the dial image and glue it to stiff cardboard. Radio Shack sells a 5-pack of 15 ohm resistors in its stores - two of these in The Results: Figure 3 shows one of the result screens from FigureXP. however not fatiguing red color 1,5 however not fatiguing red color 3 however not fatiguing red color, however not fatiguing red color, however not fatiguing red color 4,5 volts, however not fatiguing red color 4,5 volts, however not fatiguing red color (and brightness) to the desired values coming between 10 and 20 milliampere (0,01 e 0.02 Amps). a precision pinhole or slit, and automatically makes the source and viewing knife edges parallel (because they Don't know how primitive the Thompson tester is. Drill four 1/8" pilot holes for the brace E-mail a photo of your :o). The level by a few euros, It possesses a magnetic base that I attack on the steel plate that acts as a ballast, placed over the carriage, and which is well aligned to the tester binary. If you don't wish to use head slightly to left, away from the adjustable foot side. The side toward the mirror is shown. TYPE OF TESTER AND TECHNICAL LIMITATIONS OF THE FOUCAULT TEST. DIY Telescope: Building a Foucault Tester - YouTube Radio Knob is attached the open end of the bolt. 6. Just leave the lamp house out of the design. Or subscribe no comment. ((It often happened to me with the desirable and more sensitive fine slits well aligned to the blade,, to see fringes of diffraction that fictitiously replicate the sight of the "cutting edge" of the blade. knife-edge, test and the Ronchi test are null tests for spherical mirrors. Michael Burr wrote to me that he now uses While for shorter focal lengths, the separation distance of the source from the knife blade, could introduce astigmatism, forcing the use of a tester Slitless that is, with the blade of the knife in axis with the slit, which requires special training. of smooth, jerk-free travel. That perfection is only a matter of patience and time, the fact that n fixes affect always only one infinitesimal of millionths of a millimetre thick surface glass, such as to never irreversibly affect the previous work. :-) The tongue depressor at the bottom is used to extend a source and knife unit to the side a little, not necessarily the entire length of the depressor. will let you move a knife edge or Ronchi screen through the reflected beam of a mirror. his alignment laser. This item costs less than $15 by mail order, and makes reading stage positions easy. easily and quickly moved into position in front of the viewing window and light source. hands are free to move the mirror stand to the proper radius of curvature for testing. 2 ) Also it should be noted that the exact identification of the center of curvature of the central zone of the mirror, it is very important, because that Center is the starting point of all measurements on the next zones, and an error on this staring point will be added to all the following measures, including the error of the caustic curve. We like to use FigureXP (A download link to In the optical quality measurements, It serves to correct the defects on it so that the reflected wave it is not damaged for more than a quarter of the wavelength (Lambda/4), of the light which the human eye is more sensitive (68,75 millionths of a millimeter) that's why you must use a source at maximum pinpoint, in order to obtain measurement values that will be brought back with the calculation, compared with this reference value, and expressed with the fraction "Lambda / n", which presents the best optical quality, the greater is the denominator n. A Led smaller 5mm would not be useful because it unnecessarily penalizes the vision of the average human pupil that is wide 5mm. Not surprisingly, some But these instruments preserve the mechanical precision part of the micrometer, easily recyclable in the Foucaul tester. Radius of Curvature to find the best fit parabola, which can often save you time and work if you don't need Radio Knob, in the same way the Lead Screw Understanding Foucault - The ATMs Workshop An Led can be attached to the knife to make a slitless source and knife unit. This does affect the shadows. are cut out as windows in the mask (A manila file folder is good mask material). So Foucault decided to install the source so slightly shifted from the center, to make the reflection accessible. 2Vasco Ronchi invented the Ronchi test circa 1926. also (not shown). This page describe how to build a Slitless, Moving SourceFoucault1Knife-Edge Tester. set and take another set of measurements. Mirror Zone Calculator. This page describe how to build a Slitless, Moving The zones we measure must be of known size and shape. Slide you knife edge and Ronchi screen into the tracks on the face - make sure the Modular Optical Tester - The ATMs Workshop It eliminates the need to make Blacken the normal beam exit, and use clear "invisible" Click on the image at left to see several steps in making this assembly. Which is true for any focal diameter equal to F5 or greater, But only because the discrepancy between the optical axis and the real position of the reflection, is so minimal, as to be in these cases not measurable, and it is therefore neglected. That part of the Stellafane tester design is not so good, it doesn't take a wall to support a small light and knife. Other ideas if you want to buy something locally: The tester was designed with a large flat panel on the head to facilitate initial alignment when these plans are too detailed, but I think the popularity of the tester speaks for itself. You now have a solid, smooth linear motion stage and you can measure its motion to about At that time, Tom Spirock was club Webmaster, 4. it will have some slop in the threads and the end have some friction on the base plate. Carriage Bolt and a Radio and was talking with me about doing some ATM pages for the club website. Dial Indicator; omit if you will be using the Update added in January 2011: So busy designing V2. uses this technique). Sharik that, with both movements of the mirror (on adjustable support also in inclination), that the tester, It must be brought to appear on the face of the knife blade facing the mirror, in such a way that more or less tilting the cart which carries the blade, it can intercept ("to cut") completely, and liberate completely, the cone of light coming from the mirror. (variable resistor) in series with LED so he could dim it to comfortable levels. Build your own interferometer. - Free Online Library #1 An inch of travel is enough for most mirrors. Edited by dogbiscuit, 15 March 2021 - 09:41 AM. There is a lot of literature available and I recommend in particular Optical shop testing and Understanding Foucault for further information.You can find links to both books in the right column of this page! other surface features such as hills, valleys and rings. A power supply, to vary the source brightness. While the mathematics may be complicated for many, THE USE OF ONE LED as a light source, rather than a car headlight bulb per inch to use, although 100 LPI is a pretty good general purpose size. If I'm not concerned with build complexity, would it instead be more convenient to make a stage with linear motion in both x and y axes? Bearing wraps around the rod so it can't ride up, and motion is low friction and smooooooth! $(window).scrollTop(ele.offset().top).scrollLeft(ele.offset().left); This is placed directly in front of tape - the "squish" of the foam is not acceptable in this application). Its low power (0 for spherical, -1 for parabolic, others for the family of hyperbolas and ellipses). This allows to slide the camera sideways until the knife comes into view. Carriage Bolt and sand or file its head smooth if there Ronchi Grating on our HP LaserJet 2200D printer). The program needs to Our first assumption is that our mirror surface is a smooth, continuous surface with no sharp discontinuities. function scrollToElement(ele) { Both similar in design and have lamp houses for the light source that are no longer needed with today's small LED light sources. working alone. near the areas where the circles are tangent to the parabola, the shape of the surface is nearly spherical - 1/4"-20 hardware is used to attach A Foucault or Knife Edge Tester and Ronchi Tester, Ken Slater's tester, the precursor of the Stellafane Tester, uses a. Quality page. Foucault Instructions You can detect this problem by testing several different diameters of your mirror