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3D printed Foucault tester - V2


Chriske

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My first 3D printed Foucault tester did work very well.
Only thing was that all axis were running in PLA'bearings'. It's motion was rather 'heavy'. For measuring a parabola you need a very smooth motion.
So busy designing V2. This time I will use linear bearings to support all these axis.
The tester in the picture is V1
Busy printing V2 now. I'll post some more pictures later on.

image.jpeg.de5f24fba99548212e1d40dd7841e74e.jpeg

image.jpeg.85fe2312a54f64bc247393ea3db2bcdb.jpeg

image.png.fcebc5ab307aea833cfde5ec4077aa11.png

 

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Following with great interest. Focault/Ronchi tester has be on my to do list too but having never used one or the other I don't have the knowledge or experience to design one. 

Is it essentially just a platform that moves along the optical axis and carries a light source and a camera plus another platform that moves across the axis and carries the knife blade or the Ronchi grating in front of the camera? What about height? Does that need to be adjustable too? 

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Nice one Chris. Will you be releasing the plans ? Would be interested to see them having made my own mirrors many moons ago and relied on a razor blade on a stick for the focault test ?. Also would give me more ammo to get myself a 3d printer.

Regards

Steve

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11 hours ago, kbrown said:

Following with great interest. Focault/Ronchi tester has be on my to do list too but having never used one or the other I don't have the knowledge or experience to design one. 

Is it essentially just a platform that moves along the optical axis and carries a light source and a camera plus another platform that moves across the axis and carries the knife blade or the Ronchi grating in front of the camera? What about height? Does that need to be adjustable too? 

It is adjustable in height, it needs to be.
This tester is also equipped with a green laser. Purpose of that laser is to adjust the tester in a matter of seconds.
The very first time the tester is adjusted toward a mirror the old fashion way. It always takes some time for that to do so. Especially for novice ATM it is very difficult to adjust the white lightsource and reflect it onto the knife.
Once the mirror and tester's white light are 'in line', the (adjustable)laser is lit and pointed toward the exact centre of the mirror. The reflection of the laser-beam is visible on a very small screen very near that white light-source.
Now, the next time you need to adjust the Foucault-tester toward a mirror you only need to lit the laser, point that laser beam toward the very centre of your mirror and then adjust the mirror stand so the reflection point to that little screen  located at you're tester and you're done. When the initial setup is performed correct, the reflection of the white light falls exactly on the knife, every time again.
there's only one 'but', I'll explain some more when the new tester is assembled.
Because it's a printed version it is very light, so I added a 2kg steel weight at the bottom of the tester.

Needles to say, when someone do setup a Foucault tester toward his mirror only once, there's no need to use that laser at all.
It's only when the tester needs to be removed from that location and setup again the following day or so, then the laser comes in very handy.
Also in my case when different mirrors needs to be tested (during course) that laser is also a very handy tool. It saves me LOTS of time.
 

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10 hours ago, Gasman said:

Nice one Chris. Will you be releasing the plans ? Would be interested to see them having made my own mirrors many moons ago and relied on a razor blade on a stick for the focault test ?. Also would give me more ammo to get myself a 3d printer.

Regards

Steve

If someone wants to copy this Foucault-tester I'll post the STL files here on the forum.
I could also post the source files. So if someone wants to make some changes feel free to do so.

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14 hours ago, Chriske said:

It is adjustable in height, it needs to be.
This tester is also equipped with a green laser. Purpose of that laser is to adjust the tester in a matter of seconds.
The very first time the tester is adjusted toward a mirror the old fashion way. It always takes some time for that to do so. Especially for novice ATM it is very difficult to adjust the white lightsource and reflect it onto the knife.
Once the mirror and tester's white light are 'in line', the (adjustable)laser is lit and pointed toward the exact centre of the mirror. The reflection of the laser-beam is visible on a very small screen very near that white light-source.
Now, the next time you need to adjust the Foucault-tester toward a mirror you only need to lit the laser, point that laser beam toward the very centre of your mirror and then adjust the mirror stand so the reflection point to that little screen  located at you're tester and you're done. When the initial setup is performed correct, the reflection of the white light falls exactly on the knife, every time again.
there's only one 'but', I'll explain some more when the new tester is assembled.
Because it's a printed version it is very light, so I added a 2kg steel weight at the bottom of the tester.

Needles to say, when someone do setup a Foucault tester toward his mirror only once, there's no need to use that laser at all.
It's only when the tester needs to be removed from that location and setup again the following day or so, then the laser comes in very handy.
Also in my case when different mirrors needs to be tested (during course) that laser is also a very handy tool. It saves me LOTS of time.
 

Am I correct thinking that once everything is aligned properly, you only adjust the distance of the knife from the mirror? Does the light source and camera move with the knife?

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Steve,

All my designing is done in Autodesk Inventor. It is very powerful 3D-software. AI is not free, it is actually rather expensive. But I do have a license from my work in the past. It's already an old version but I can do everything I want to and more. It is in fact so powerful that I only use about (and I'm guessing here) 10% of it's features. But as I said before here on the forum. When you have a son/daughter still in school, you can download a student version for free. Maybe you're a student yourself...? If you download/install you need to give a name of a school.

All my printing is done with a printer I designed myself. UltiPrinter I call them. This very moment a few friends and myself are building five V3 of this printer.

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26 minutes ago, kbrown said:

Am I correct thinking that once everything is aligned properly, you only adjust the distance of the knife from the mirror? Does the light source and camera move with the knife?

When busy polishing you know the ROC of that mirror, so you first place the tester at the correct distance from the mirror indeed(does not have to be 100% perfect, but as accurate as you possibly can. I always use a tape measure) minor corrections are done afterwards. Then you point the laserbeam to the very centre of the mirror. Pointing that beam toward the mirror's centre is done by pushing and rotating the tester manually(there are also 3 set-screws at the bottom of the tester. These are for kinda 'levelling' the tester.
When that is done, the only thing left to do is 'levelling' and rotating the mirror on it's stand until the laserbeam hits a small plane(I explain about that later.
Needles to say this can only been done after that initial set-up I described higher up here is performed.

Yes, camera, knife and LED all move together on the Y-bed

There are a few holes in the camera'disk'. These holes you need to stick a pin in to fix that disk/camera in a certain position. Here too you need to set-up/adjust the camera only once. When that is done you need to remember what hole to put that pin in. When busy visual work, rotate away the camera. When taking a picture rotate the camera near the knife, put the pin in the correct hole and you're ready to make that next focogram. The reason for these many holes is to be  able to use any camera at all.

it all takes far less time to do all this then me typing this 'how to'. ..!!!

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2 hours ago, Chriske said:

Steve,

All my designing is done in Autodesk Inventor. It is very powerful 3D-software. AI is not free, it is actually rather expensive. But I do have a license from my work in the past. It's already an old version but I can do everything I want to and more. It is in fact so powerful that I only use about (and I'm guessing here) 10% of it's features. But as I said before here on the forum. When you have a son/daughter still in school, you can download a student version for free. Maybe you're a student yourself...? If you download/install you need to give a name of a school.

All my printing is done with a printer I designed myself. UltiPrinter I call them. This very moment a few friends and myself are building five V3 of this printer.

Ah right yes sorry Chris I remember you giving me that info on another thread and I made a note to check that software which I'll do now ?.

Thanks again

Steve

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Y-carriage was already printed, but I decided to add a compartment to hide all the electronics :
1- Switch
2- LM2596 'DC-DC stepdown' for the green laser.
3-Potentiometer for the LED. sometimes I need to check a coated mirror, so I need to dim the LED a fair amount.
4-Space for a small jack to connect to the power unit.

Left the old one, right the new one
All wiring will be hidden inside the unit. I made a few canals to lead the wires from the front to the back of the Y-carriage.

image.thumb.png.4b2eaebb82cc7d9a90d6ff4da9cf2e93.png

 

 

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Interesting and good effort there Chris.

I found a small samsung camera similar to the one you're using worked well when I was doing my testing.

Although I never got on with the foucault I would revisit it when I try another mirror making project after seeing the superb camera images you're getting. 

Keep up the good work.

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Rough...?????
I call that VERY rough...!..^_^
Like I said, this mirror is made while testing our M-o-m's.  In fact we were testing different strokes and also different positions of the tool. And this is the result, se-ve-re zone. But hey, with a MOM, removing these zones is a matter of minutes. In the old days (when we all made mirrors by hand) and would see this on our Foucault stand, the only solution to remove such a disaster is go back to grinding again. Polishing out zone like in that picture higher up would have taken us hours, even days.
My pal and myself have both one MOM btw. Mine is 'loaded' with a 20" mirror right now.

btw... Lucky we made these mistakes. It gives us an insight how such machines work, and most of all how to correct these errors.

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One more thing : that 10" f/3.6 mirror(in that focogram) is completely polished out. There is that strange very fine pattern all over the mirror. That is caused by the polishing-pads. When the pads are replaced by pitch that pattern is  all gone in a matter of minutes. But before switching to pitch these rather huge zones must be removed of course.

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  • 4 weeks later...
On 26/09/2018 at 08:48, Chriske said:

One more thing : that 10" f/3.6 mirror(in that focogram) is completely polished out. There is that strange very fine pattern all over the mirror. That is caused by the polishing-pads. When the pads are replaced by pitch that pattern is  all gone in a matter of minutes. But before switching to pitch these rather huge zones must be removed of course.

When I was doing the 22" I was told the optical polishing pads if used leave a rough surface.

When I do finally get around to doing another mirror I will construct a m.o.m and speed things up. It is still a long process and watching a machine does require concentration.

I did do some foucault images and got some good results.I did prefer the Ronchi test though. Next time I will try both again. 

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Damian

there's a never ending discussion here whether Ronchi or Foucault is best. As a matter of fact I never use Ronchi. Reason :  the only value is that Ronchi will give you only an estimate how the curve of your mirror looks like. In the end when that spherical mirror is ready to 'go' to a parabola you absolutely need a Foucaulttester.
What's more all de very small and 'delicate' roughness visible in a FT is not visible at all in the RT.
Look at the focogram higher up in this thread. That mirror has severe zones and on top of it it has scratches and microrippel. Again, there's no way you'll be able to see all that in a RT. And a another thing, when a mirror is not completely polished out, it'll be clearly visible in the FT, not in the RT.

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  • 2 weeks later...

On hold...
We've decided to (maybe) install a cam combined with a zoom-lens to capture the focograms.
That zoomlens is needed to cover the entire area of the chip. The bigger the image the smaller details can be seen . An example of a very detailed focogram is posted higher up in this thread. Using a regular webcam(even with the lens)the image is WAY to small. So I need another solution.
A disadvantage of using my camera(as I always did in the past) is that I can not go 'live'. Meaning after the focogram is taken I need to connect my camera to the PC to have a look at the images on screen.

I have no experience with cams and other stuff. So my question : is there a good quality webcam(or other USB device) I can use to view 'live' images. I do not need the lens, I would use an old zoom-lens instead. Or maybe there is some other good device with a zoom-lens to do the job...?
Preferably not to expensive...
 

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I thought you could use webcams "live".  I'm sure I have in the past.  Otherwise CCTV for live pictures.  I have one watching my yard and gates.

Edited by Gina
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