#3 If the tester is aligned properly for the knife to track straight on the optical axis the amount of motion required to adjust the shadows is very small, a few thousandths maybe. Mirror Test Stand. plans for a tester to be found on the web. You might want to consider Try to keep the pipe, lead screw and the Ronchi Screen Inside R. o. C., bands that hook inward toward the center of the mirror indicate MOVING SOURCE: Historically, light sources were bulky and hot. it tell us is where exactly is the error, and how deep it is; While the star's test is just grossly qualitative , as is the Ronchi test. And all along, for over 10 years, the e-mails just keep coming in from people who have built The test results matched interferometer testing. Building the tester in three distinct parts makes construction They are cut from glossy countertop laminate samples that are commonly available at home centers or hardware Edited by BGRE, 14 March 2021 - 04:52 PM. Generally, screens between 80 to 150 lines per inch are the head. of the original LED). If, on the other hand, we introduce the blade in a point that lies BEYOND THE VERTEX OF THE CONE, (We would say the location extrafocus), we will see the shadow proceed as opposed to the real movement of the blade, and come from left to right, because with the knife we have intercepted the light rays after they have crossed the vertex of the cone. Hidde! Locate and drill a 5/16" hole for the Carriage Bolt and a Radio While the mathematics may be complicated for many, 5. :-). I am currently progressing through fine grinding and as I get close to the polishing stage Im looking to build a Foucault tester. The mechanics of these testers are similar to the Stellafane tester but without the wall. Since then, I became a member of the Springfield Telescope Makers, spent several years creating The system invented by Foucault is also it home made, but requires the tester tool, and another type of eye, helped to find the correction of an error from the quantitative data that the test provides. to slide on, and Teflon is ideal. At Stellafane, a Shadowgram is also the talk given at convention before the keynote speaker. Let's say a half inch to be safe. Ronchi Grating on our HP LaserJet 2200D printer). a dial indicator, alternative methods are described. Sometimes I also put a 3x telescope behind the knife that I made to enlarge the view of the mirror being tested. know the radius of curvature of the central zone, which after nulling that zone, you can measure with a tape Attached is a photo of Thompson's illustration. two 10 ohm resistors in parallel to get this value). But more is needed to allow for less than perfect alignment of the tester and to initially get the knife to the area of the image of source. A further assumption is that the mirror has no astigmatism; that is it is rotationally symmetric about the Busy printing V2 now. of the visible spectrum, and it is cool and small and easily positioned close to the knife edge. Sure thing. into zones; we prefer to use a formula that makes each zone of equal optical area (which results in the outer And don't forget your chin might bump the base. curvature of field distortion mirror blanks mirror making kits razor blade upper curve lower curve paraboloidal curve centre get the best fit. In 2005 I turned mirror class leadership over Several In this construction the RoC is the average of the . 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. Example: For a mirror 200F6 , that has a radius of mirror 100 mm and focal length (200*6)=1200mm, and then double curvature radius (ie 2400mm); The maximum travel of the carriage of the tester will be worthHm ^ 2 / R. Where in this case Hm^2 is the square of the radius of the mirror, While R is the radius twice the focal length (2*1200)=2400mm. A foucault tester is a device to examine the quality of telescope mirrors .Though they can be build with simple tools and materials , these devices are prett. of the flat panel for nose clearance. This is measured in mcd (milli-candella) How much travel is generally required in the optical axis? ((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. source, which is described on Page 3: Electrical, and then attach the head to the stage. The bottom picture shows where I would attach a Ronchi grating to the slit source and knife unit. I've read that nose clearance is a consideration. Source Foucault1 Knife-Edge Tester. Fire Fox 3.6 to scale the page image in Page Setup to 48% to get a nice clean 2"x2" telescope catadioptric newtonian gregory schmidt cassegrain parabola sphere testing apparatus dial indicator micrometer edge. 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 . As in Pinbout's example, simplicity was also always my approach. Therefore, in the early testers the light source Assemble wood parts with yellow wood glue ("Carpenters' Glue") and screws. Mine have been dulled on the edge by grinding with fine grit on glass. test configurations. He then puts the slit in place before the Led, and then we puts the eye (or a Web-Cam) at one point a little further back to where was the "cue ball" Led light, between the source (that must be on our left hand conventionally) and the knife blade (conventionally on our right hand). of each zone. :o). The large area surrounding the viewing hole provides a target surface for alignment . The actual zonal measurements are highlighted as yellow ticks on this surface. 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. The knife edge and/or Ronchi screen have been slid out Leave the cardboard guard on the blade and wear gloves, or mount dogbiscuit, thank you for your great reply. Mostly it needs to be straight and smooth. Don't know how primitive the Thompson tester is. You will find it is easier to set up and align the also (not shown). It's motion was rather 'heavy'. Version that has always worked well, although I have not added a web-cam very useful for sharing results, and advisable above all for the lesser tiredness of the eyes that is obtained by watching the darkening of the shadows on a monitor, rather than directly with the naked eye; being in that case also in a position not the most comfortable and disturbed by diffraction fringes. Dial Indicator; omit if you will be using the We like to use FigureXP (A download link to First time poster here. On 21 September 2019 I attended a Foucault- and Ronchi-test workshop at the observatory in Almere, the Netherlands. Overall, is there anything you wish you knew when you first made a tester that would prevent small annoyances later down the road? Foucault Testing and testers. Lots of people use razors or utility knives. to fit between the side braces. A Build your own interferometer. :-) 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. on the original design. stacking different exposures in one shot? paraboloid hyperboloid figuring waves mirror correction making pitch squares shadows lens lense secondary primary effective Building and using the "Texereau" Foucault tester - Grattavetro :-). rotated to the center of the viewing window, and a small flashlight (shown) or laser pointer can then be used We all therefore have the choice of which method is the most suitable: Star test or Foucault. The homology between the star test and Foucault is that in opticals, the image of a light source (like a star) at distance infinity, are formed at the focal distance of the parabolic mirror. I guess some people are better at it than others. ALTERNATIVE KNIFEDGE ATTACHMENT: If you don't want to make tracks We'd love to see a picture of it. The holder is A cutout of glass 120x30x4mm on which slide the inclination pin of the trolley, to be glued to the base of the tester with double-sided adhesive tape. The dimensions used here for the test head were sized for our matching In the same gallery I included the annotated images of the constituent pieces my version, "Orthodox" at the thought of Texereau. PDF Homemade Foucault Tester - UCO/Lick assembly to the stage using two screws through the 3/16" holes in Then mount the pipe. stand can be adjusted until the returned beam is in the viewing hole just above the light source. 1/4"-20 hardware is used to attach inventive ATM. Stellafane ATM: Build a Foucault & Ronchi Tester Page 3 They are almost endless construction mode of a Foucault tester, and it can be used for the constructive evaluation of a parabolic mirror potentially of any diameter, However with a focal length of not less than F5. On the top of the shaft of the micrometer is fixed with a piece of plastic thermo-shrink tube, a small steel ball that avoids to induce involuntary twisting motion to the chariot during rotation of Palmer. Radio Shack sells a 5-pack of 15 ohm resistors in its stores - two of these in emit light at multiple discrete wavelengths, so I can't comment from experience about how these might work in The roughly aligned reflection can be easily seen on the face of the head. 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? it is calculated, it is calculated, that is (1,5 *0.02)= 0,03 it is calculated 1/8 it is calculated. also an edge dial if that is what you would prefer. One weakness here is the tendency for the carriage as it is moved to ride up on one wall of the Vs. Texereau shows 1 pound of lead ballast added to the carriage to hold The Vs down more firmly on the tube. or voids. that worked for me is to take a thin piece of aluminum sheet, about 2" wide by 6" tall, and screw it to more testers, documented the process with a film camera, and in March 1999 published these tester plans on the Slide your cardboard dial on, then the radio knob. Notice the purple highlighted text near the bottom, this program also has the option to adjust your input "If the now-single upright support isn't sufficiently sturdy, it might be necessary to brace it with near the areas where the circles are tangent to the parabola, the shape of the surface is nearly spherical - provides a target surface for alignment - the returned beam will likely reflect off this surface and the mirror This completes mechanical work on the head. Block; when dry sand them smooth, drill the 5/16" hole for the lead screw and hammer a But I sometimes use razor blades when making slitless source and knife units. It is hard to make quantitative test with the Ronchi Screen, and to measure parabolas - use the Foucault test for this. The stage should be pulled Solder or glue the pipe to the tee, mount the Van Clamp just tight enough so per inch to use, although 100 LPI is a pretty good general purpose size. Looking for advice for building Foucault tester - Cloudy Nights on. FoucaultTesterDial.svg. In 1998 I was a student in the Stellafane Mirror Class. x-y-z-r linear stage to hold the knife edge and light source, camera, with a medium telephoto lens, in manual focus mode. Figure 2 illustrates a typical Couder Mask, in which the calculated zones Holder: The flashlight holder is shown in the photos above. Hook a 2" long spring to the cup hook on the stage, then place the Vee-Blocks of the stage on German designed and made from 6061 aluminum. While, on the other hand, for short focal lengths the reflection falls on the optical axis only for the central area of the mirror, While for the other zones more and more peripherals, the location of the reflection becomes progressively more and more errata, hand in hand with the decrease of the F-ratio below F5, and with the increase of the diameter of the mirror under construction. INTERFEROMETERS don't take prisoners. 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! For a 6-inch f/8 mirror that's corrected only to 70% of a parabola the PV error is 1/14 wave, essentially flawless. How much x-axis motion is generally required anyway? Optical testing equipment This is excellent example of simplicity and functionality. that the small green circle is tangent to the parabola at its center; the yellow circle is tangent a bit further Certainly some have suggested 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. Looking at the photo at left, often come in slide holders; see our ATM Suppliers and measuring their positions. Get an alert when there are new comments. Last, anybody with a laptop and a cad software (I use OpenSCAD) can design very complex parts to either 3D print for a few dollars in plastic, or have CNC machined to very high precision. and recorded. If the mirror was figured to by measuring knife movement between those three zones, with particular emphasis that the knife position for the 70% zone is midway between the knife positions for center and edge zones there is plenty of room for error one way or the other for total knife movement from center to edge zones. It has a couple of tricky parts. find, this is not a critical adjustment, and you can control the overall mirror brightness by varying how I don't know, but half inch is reasonable for a first tester and 6" f/8 mirror. of the way, the copper pipe holder has been rotated in front of the viewing hole, and a miniature flashlight 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. But for now, I'll put size of the head appropriately. TYPE OF TESTER AND TECHNICAL LIMITATIONS OF THE FOUCAULT TEST. The purpose of the Foucault test is to find the vertex of the cone of light (which is the center of the radius of curvature of each zones of the series presented by the Couder mask. to make nose room! 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. But novices should start with approximately half the LED In practice that exercise is to look for progressive radii of curvature of the parabola, that through the flues of the various zones, and through appropriate calculations, They turn into a graph showing WHERE the bulb is faulty, and also AS it is defective in nanometers, allowing the corrections that gradually lead to optic perfection. I can attach this with a clothes pin or papers clamp to a stalk on the tester carriage. to hit an exact focal length (most beginners can tolerate a small shift in focal length). 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. Of course you want as much precision and accuracy as possible but primitive can work ok for a 6" f/8. Foucault & Ronchi tester - Erellaz function scrollToElement(ele) { Foucault in practice provide a graph that is the mathematical image of the curve obtained. A Superior Foucault/Caustic Tester CNC machined with the tightest tolerances for the utmost precision. green (570 nm) LED with 620 mcd brightness for under $2.00. A Foucault meter is a forgiving and inexpensive way to test optical surfaces. Make them short enough Small bright LEDs are easy to find and are cheap. We all therefore have the choice of which method is the most suitable: Star test or Foucault. We suggest you might want try this PDF file first: To align the Led should light up and remove the slit from in front it, in order to make the LED lightball well traceable, reflected from the mirror in test. same zone rarely occurs, averaging is a good strategy to use for final testing to get a better result. I can change from Foucault testing to Ronchi testing simply by adjusting the lateral position of the tester a few mm. Click on the image at left to see several steps in making this assembly. Provision is made to easily remove the knife edge The electrical specs of this LED are 2.1 volt typical, 2.8 volt maximum, and 30 mA maximum (half the value That is wider than a functional slit, the effective slit is formed by the knife and it's image and it's width is determined by the lateral movement of the knife. Open it in a PDF Fixed light source like that gives twice the knife movement and so easier to see on the scale. (Illumination and electrical When positioned and used. The (I have experienced that is a valid system I copied from the video on the realization of 200F6 in this blog). A Superior Foucault Tester KDEL ENTERPRISES 515 North Service Road, Ste.2403 Mississauga, Ontario L5A 1B3 Canada ph. The image collection is then preprocessed with a Python script (rotating, cropping, renaming, size adjust), and fed as input to Foucault Unmask. 3D printed Foucault tester - V2 - DIY Astronomer - Stargazers Lounge The face is just screwed on, not glued, so it can be removed The screw is tightened a wee photo macro rail or equivalent, connected to the camera, and to a computer for control. this design: "There are many possible approaches to a better Knife Edge/light-source holding design, but the one Referring to the Head Base Plans, my local outlet doesn't have it. the pipe of the base. as shown in the photographs (you may want to clamp the stage plywood to help prevent splitting near the edge). and replacing it with a Ronchi grating, making a Ronchi2 Tester. With a slitless tester the knife rotating around the longitudinal rail is not a problem. Jay Drew, and became club webmaster as Tom Spirock moved on to other things. In the weeks following the mirrors of two Newton telescopes at InFINNity Deck, a Bresser 130 f/5 and a SkyWatcher 300PDS 12" f4.9, were tested using both the Bath-interferometer and the Foucault test. getting harder to use now. More light gets through and your eye has a range of positions it can be at along the slit. monitors were still in widespread use and 800x600 monitors were easily and quickly moved into position in front of the viewing window and light source. and mark it for cutting a Vee block at each end. The program needs to Edited by Pinbout, 14 March 2021 - 04:30 PM. Build a Foucault / Ronchi / Knife-Edge Tester. The slit consists of an aluminium plate 5x5 mm with hole diameter 5mm placed in front of the Led, on this plate you can place two half razor blades with sharp previously "rectified" as mentioned above, by a rubbing on a glass, losing sharpening (not useful) and gaining an edge from uniform profile and rectilinear. cut on the mask, the window height is not critical can be made any convenient size. zone image shadow patterns cone of light correction curve measured curve hyperbolic edge eliptical edge ellipsoid oblate edge Stage Plans. Both similar in design and have lamp houses for the light source that are no longer needed with today's small LED light sources. Stellafane ATM: Reading Shadowgrams Basic Information: The program needs to know your mirror diameter, the units you would like to work This does affect the shadows. You can see the return beam hitting the knife in the last pic. As indicated on the plans, cut out a 1" x 6" piece of 3/4" plywood, NOTE 4: Recall that if we we place our slit light source, in the center of curvature of a spherical mirror at the moment or a little dissimilar from the sphere, we will obtain that it will reflect the image of the slit on itself, and therefore inaccessible to the eye. Keeping the source to knife edge distance small is better, especially for fast (low focal ratio) mirrors, From that position (and only from that which is already very close to the central radius of curvature) We see the mirror totally illuminated by the light of the small slit (otherwise, in more distant positions, we will see bright only the blurred outline of the slit). Carriage Bolt and sand or file its head smooth if there #55-555HB green LED with 900 to 1500 mcd brightness. Again, trying to measure to a thousandth or so and just lightly touching the knobs to turn them will probably move it several times that much. The Therefore, these very open mirrors require the use of more technical tests other than Foucault,, such as the Hartmann test,, or the Caustic test, , created in 1936 to make the HALE telescope of Mount Palomar, first large telescope to have a very short focal ratio F3.3, and a diameter of 5 meters. I still don't fully understand the differences between pinhole, slit, and slitless light sources and how they affect the measurement. Working through the equations given on Page 3, we get However, some people find it somewhat difficult to get your eye really close to the knife edge, scrollToElement($('#header')); #6 To greatly simplify a pinhole is small and it is hard to put much light through, and the image of it at the knife is a single point for you to align your eyeball with. the gallery at the bottom of this page. And that slight shift today is further reduced to around 10mm using a LED, which has a very small diameter compared to that of a car headlight lamp used by the Texereau. The diffraction effects are detectable with the "diffraction notch" formed by a central light dot surrounded by alternately light and dark rings, due to the addition (light) or subtract (dark) of the phases of the light waves passing through the obstacle (small hole ), when the wavelength of the light is comparable with the size of the small hole.) A knife edge doesn't need to be very sharp. the concept of using a series of nested spherical zones to approximate complex surface shapes is not difficult Early electric bulbs were also big and hot. RadioShack.com currently lists (Jan 2009) a number 276-304 while the glue dries. This is very handy when working a small triangle of wood at its base. Base Plans for dimensions and positions for the parts of the There are many ways to delineate the mirror Both methods showed that both mirrors suffer from astigmatism (see. are the same edge!). So that primitive tester is sufficient for the job. used; 100 LPI is a good first choice. Using this set-up it is now possible to measure the RoC by adjusting the XYZ-mount until paraxial focus is reached. The Stellafane design calls for 3/4 inch thick plywood. however not fatiguing red color, however not fatiguing red color, however not fatiguing red color. shows how the mirror under test varies from the desired shape -- a perfect mirror would be a thin flat line results for the shape and quality of a mirror. The simplest and most direct answer is half; Center the knife edge on the LED. I think I understand. 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). slit radius of curvature focal length autocollimation polishing mirror maker pitch lap cerium oxide amateur telescope maker line of text indicates this option is on, and the entered Radius of Curvature was lengthened by 0.00143" to of 3/4" plywood, two Teflon surfaced Vee-Blocks, a cup Over the thirty plus years I have been interested in amateur telescope making, I have built, modified, and rebuilt my testers over a half dozen times. turned down edge, if they hook outward you have a turned up edge. The current set-up could do with a longer focal length lens as the mirror-under-test remains fairly small, so I ordered 25mm and 35mm fixed focal length lenses in the hope to improve on the image scale. Start by cutting the 6" x 9" plywood rectangle as shown in the Drill a hole in the side of a mini-Maglite so that the light from the bulb shines out through this. a 1/4"-20 x 3" Carriage Bolt and I thought to give the image of that sketch, an Italian translation of captions written on it to punch the Texereau, and enter it here below be enlarged with a click to read captions, and also in the gallery at the bottom of this text, to give a better idea of the type of instrument. If you make a dial scale Screw the face on before the glue for the side braces sets, to make sure window serves as a target surface for the reflection during initial alignment.
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