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Question about operating a Stellafane Foucault tester

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#1 hydrox

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Posted 19 September 2021 - 09:02 PM

Hello, I'd like to make a Foucault tester. I read Stellafane instruction and I'm still not sure how to operate the tester, and have some questions about the parts:

AyJDPOV.jpg

  1. Where do I put my eyes? Is it the same as hole (A) where the knife edge is?
  2. The tutorial says turn the knob so the head rotate about the pipe on the left in the picture, but won't that cause the LED light to move as well?
  3. The tutorial uses a razor blade for knife edge assembly. Can I just use a piece of plastic and tape it into hole (A)? The knife edge assembly seems complicated and I don't see the "knife edge" move at all, beside having the whole head moved.
  4. What's the size of the spring(s)? Are there 2 springs? How do I hook the spring into the plywood base and stage?
  5. Do I need laminate sample at dial indicator contact point? Do I need a dial indicator?
  6. Nose issue: can I cut out part (B) to solve it? Will that affect the functionality in any way?

I look at Edmunds Optics Foucault tester and they look way basic compare to this yet the price is almost $200 shocked.gif


Edited by hydrox, 19 September 2021 - 09:03 PM.

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#2 Dale Eason

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Posted 19 September 2021 - 09:27 PM

Yes you put your eye behind the knife.

Yes the LED light will move up and down but that is OK.

I've always used a razor blade and that is just as easy to mount.

No there is just one spring and it looks like they use screw eye's to attach it.   They are there to provide a a force that keeps the sled against the back and forth adjusting screw.  Nothing critical about the spring.   Rubber band will work as well.

You want the dial indicator to touch  something smooth and not to drop into rough spots as the sled is adjusted up and down with the side knob.

 

You can cut a spot for your nose if you want.

 

There is not much special about the usual Foucault tester it just needs to move a knife edge and light smoothly back and forth in small increments and to accurately measure the to/from distance it is moved in relation to the mirror.

 

Here is a pic of mine without the Micrometer attached at the moment.  But in mine it attaches to the sled and touches the aluminum strip at the back.

 

 

Attached Thumbnails

  • stage2.JPG
  • stage1small.jpg

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#3 dogbiscuit

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Posted 19 September 2021 - 10:28 PM

Hello, I'd like to make a Foucault tester. I read Stellafane instruction and I'm still not sure how to operate the tester, and have some questions about the parts:
1. Where do I put my eyes? Is it the same as hole (A) where the knife edge is?

6. Nose issue: can I cut out part (B) to solve it? Will that affect the functionality in any way?

 

I look at Edmunds Optics Foucault tester and they look way basic compare to this yet the price is almost $200 [shocked.gif]

The Edmund Optics tester is very basic and rather expensive.

It has a light source separate from the knife.

 

The Stellafane tester has the light source touching the knife edge and they move together. This is the  most common form of the tester in recent years, maybe a couple of decades.

 

As for eye accessibility to the knife, there is no need for the nose blocking wall supporting the light source and knife.  You could do away with the peepshow wall and incorporate the Edmund type stalk supporting the knife and light source to your Stellafane tester measuring engine carriage. The light source only needs to be a small light attached to and partially obscured by the knife.  A small bright LED with some sort of diffusing material between the knife and LED.  White plastic milk carton or similar material makes a good diffusion layer between LED and knife. It could help to have a 1 or 2 mm or so pinhole mask either between the diffusion layer and the knife or on the other side of the knife. Try to minimize the size  distance the light source extends behind the knife edge because you need to get your eye very close to the knife.

 

 

The knife doesn't need to be razor sharp.  For safety if a razor blade is used I recommend you dull it.  I sometimes use a razor that I have ground with very fine grit, 5u or 9u against a flat plate glass (window pane). Holding the blade at nearly a right angel to the glass pane, it only takes 30 seconds or so to dull the edge. Works just as good, maybe even a little better than a razor sharp edge. It can help reduce reflected light from the knife if you paint it black on the back side. You might want to rub the edge again on the glass to remove any paint that might be on the very edge.

 

Consider which eye you will use and put the knife edge on the side of the support stalk that will allow that eye easiest access to the knife.

 

Make it all as comfortable to use as possible.  To do good testing you will be spending some time with your eye up to the tester.


Edited by dogbiscuit, 19 September 2021 - 10:30 PM.

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#4 hydrox

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Posted 19 September 2021 - 11:11 PM

Thanks all. I have a better idea now. Will start gathering the materials and parts

..The light source only needs to be a small light attached to and partially obscured by the knife.  A small bright LED with some sort of diffusing material between the knife and LED.  White plastic milk carton or similar material makes a good diffusion layer between LED and knife. It could help to have a 1 or 2 mm or so pinhole mask either between the diffusion layer and the knife or on the other side of the knife. Try to minimize the size  distance the light source extends behind the knife edge because you need to get your eye very close to the knife.

 

..

Do you have a picture? I'm bad with visualization. I plan to buy this LED kit from Amazon https://smile.amazon...CS4CTRVLE&psc=1. Will it work?

 

For knife blade, I plan to get this https://www.homedepo...11709/205480266, based on Dale's photos
 


Edited by hydrox, 19 September 2021 - 11:17 PM.


#5 dogbiscuit

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Posted 20 September 2021 - 01:55 AM

Here is a suggestion about how to mod your tester for more comfortable use.

 

HydroxTesterMod.jpg

 

Assuming you use your right eye for testing the source knife holder is a single narrow upright on the left side of the carriage.

If you want you can put a gusset on the front (toward the mirror) of the upright

The source/knife is to the left side of the upright making it easy to get your eye to the knife without bumping the tester with your nose, cheek, chin or other body parts.  With your eye to the knife your right hand can still get to the screws for adjusting the knife position and the dial indicator will not be in the way of your chin.

In the drawing I put a green dot where the LED should go but of course it should shine toward the mirror and no light should come from it to the rear.  On the mirror side the knife covers half  of the light source. The source and knife are in contact, not separated except for a diffuser.  The source can be glued to the knife. Put it in the right place and it will not need to be moved.

 

The LEDs you linked to are suitable.

I haven't used a utility blade for a knife. I mostly use single edge razor blades that are dulled by fine grinding on flat glass.

Maybe Dale will tell you if and how he prepares the utility blade.

 

 


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#6 Dale Eason

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Posted 20 September 2021 - 02:04 AM

One source of cheap bright LED's are the dollar store flash lights.   Many have the bright square LED chips that are easy to take apart and even come with a battery holder and switch.

 

Dale


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