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Foucault Shadows Hard to read : Cause?

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#1 JohnH



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Posted 30 April 2013 - 09:05 AM

I'm getting back to testing 12.5" f/4.5 after a hiatus of about 6 months.

There is a picture of the mirror in my gallery.

I rebuilt the tester and built a mirror test stand but the shadows are much different than before and I'm not sure why.

In the old thread, the parabolic correction stood out clearly.

Now it looks very faint.

My tester uses a moving knife edge coming in from the right and a slit light source.

Do I need a wider slit?

Should I clean up the knife edge and the ones in the slit?

Change the light source from an incandescent bulb to a green LED?

I will try posting a current picture later once Ive heard some opinions.

#2 Mirzam



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Posted 30 April 2013 - 11:41 AM

Did you check the light intensity? Need new batteries perhaps?


#3 stmguy



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Posted 30 April 2013 - 11:50 AM

try a dimmer on the light source and see if that helps


#4 Tom and Beth

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Posted 30 April 2013 - 12:58 PM

Personally, I'd ditch the slit in favor of a single "Knife edge" and use a yellow or green LED. The knife edge, of course, covers 1/2 the LED. I know some prefer the slit, and if you are one of them I mean no offense at all.

The Slit less tester is described in "How to Make a Telescope" by Jean Texereau as well as several others as collected in the ATM 3 volume set.

#5 Mike I. Jones

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Posted 30 April 2013 - 02:21 PM

+1 for the slitless tester. SO much easier to align and use than a slit.

#6 Mark Harry

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Posted 30 April 2013 - 04:40 PM

I use a modified version- a large pinhole; similar to a pinhole, and slitless. Good compromise. Never have to worry about slit alignment.

#7 JohnH



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Posted 30 April 2013 - 06:30 PM

The tester operates on 120V stepped down to 12V DC via a transformer. Its probably over 50 years old. The bulb is a standard auto type with metal caps on each end and a long tungsten filament

The light source is diffused by a ground glass inside a metal shell, and out through a 3/16" hole. The slit is formed by brass "jaws" that can be moved apart or closed.

I take it that the slit AND the knife edge MUST be parallel or the shadows don't form properly?

PS Mark, Mike, perhaps you can post photos of your setup to show me how you knife edges work?

PS The base was replaced and the hole with the light source is uncovered. The brass slit mechanism is in the bottom right of the picture

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#8 StarDusty


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Posted 30 April 2013 - 09:04 PM

My tester is like Tom and Beth's.

It has a single straight edge that extends over center of a yellow led that I filed/sanding flat. It is powered with an old RC airplane receiver battery and has resistors to keep the amps low for the led. I have a small adjustable resistor to adjust its intensity.

I find that I need a really dark room. Any daylight coming in through the windows is enough to kill the contrast. Also, this is something that takes practice. It takes my a few long and quite sessions to get back into testing mode if I have been away from it for a month or two.

#9 Meep_Esq


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Posted 30 April 2013 - 09:50 PM

I'll add to the chorus and say definitely make a slitless tester. I used a 10mm LED which means I don't have to black out my hallway when I'm testing

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#10 Ajohn


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Posted 01 May 2013 - 10:46 AM

My tester is some what like yours but I tilt the light source to align it with the knife.

I think Texereau has a good way of setting up the slit width but I suspect it only works well with a spherical mirror. He places the knife some way in from the radius of curvature and fringes form along the knife edge as it cuts into the beam. Lots if the slit is too narrow and he reckons 2 or 3 is about right for testing. The same fringes can be used to check for a sphere right up to the edge without the usual diffraction ring round the rim of the mirror. Not sure if this will work with a moving source tester.

You could try setting the slit this way with the knife say 1in or so in from the rad of the mirror. It might help to make say a mask with a 6in circular hole in it to make the mirror more like a sphere.

Looks to me like you have more or less a pure Texereau style tester. These will take accurate measurements. I'd stick with it. The slit can be removed so that a larger source is available for setting up. Also so that the slit can be adjusted by holding it up to the light. And yes there should be a diffuser behind the slit but few bother.

It also looks like you have a fine tilt mechanism. That helps a lot when taking readings with a mask as the way the shadows form in adjacent holes in it has to be watched for consistent readings. The only way I can explain that is that they need to form in a symmetrical fashion.

The only unusual thing about your tester is that the knife blade isn't opposite the source. It's above it. I'd guess some one went on to make a larger mirror and increased the height that way. I would probably pack up the source as well.

If you want to clean up the slit blades probably best read the book. That is here


Way out of copyright and the owners of that would like people to download and read it. This also explains how to align the tester with the mirror and set the knife square to the slit. That just needs a simple 5x eye cup loupe or maybe an eyepiece used back to front. It's just a case of setting source and knife level. Focusing the loupe on the blade and bringing the slit image onto it. It should be square when it's 1/2 way through the beam. Having a removable slit makes initial rough alignment very easy. Initially catch the return beam on a piece of paper etc.

:) Looking at pictures of some testers I'm reminded of some one some where else having a feeling of despair when many people come to make a tester. Hours and hours on the mirror and as little time as possible on the tester. Some designs aren't too good really either and still take time to make. If you want to try slitless or moving source just make it and stand it on your existing sliding table. The only real advantage with these really is that the source and knife edge can be very close together. They still need the same movement precision if they are going to produce equally accurate results. Many are put of a Texereau type slit as they need to buy a few metal working tools - junior hacksaw, 6in flat medium cut file and a tap for screw holes. Plus the all important piece of lightly ground scrap flat glass. A few grades of wet and dry emery paper with soapy water on it can help tidy things up too.


#11 planet earth

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Posted 02 May 2013 - 07:50 AM

"The only unusual thing about your tester is that the knife blade isn't opposite the source. It's above it."

Good eye John,it would help if the knife edge was lowered.
I'd also move the transformer the heat from may be rising in front of the light source.

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