From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


Hi
 I use the same source for my tester. I have a slitless
setup with half the LED blocked by the edge. I just put the Roche infront
of the LED and look at the return of to the side a little.
Dwight

>From: Arthur 
>
>Thanks Dwight,
>
>So I'm two understand that I should have two light sources:one for the
>slitless test and one for the ronchi test? I started this project some years
>ago and may have had it all figured out back then.But for now, I'm starting
>all over. From the few pieces I've been able to find in my junk boxes. I
>have a mini-mag- lite bulb and a orange jumbo LED (12000 mcd). I don't know
>if these are currently concidered good for a foucault tester or not (at one
>time I think they were) I believe my plan back then was to use the mag lite
>for the slitless tester and wire test, while using the LED for the ranchi. I
>think my design was to have the mag lite just bellow the LED with both
>immediately above the sight hole. Doe's this sound like I'm going in the
>right direction?
>
>
>I need all the advice and idea's everyone came give me.
>
>Thanks much,
>Bill Morga
>
>-------Original Message-------
>
>From: Dwight K. Elvey
>Date: Monday, July 07, 2003 06:06:40 PM
>To: atm{at}shore.net
>Subject: Re: ATM Basic Foucault tester
>
>>From: Arthur 
>>
>>
>>I'm putting together the testing devices for my Foucault tester. I have the
>>sliding base, which follows the normal design for this device. I would like
>>to make a multi-tester,i.e., slitless, ronchi, and possibly the wire test.
>I
>>have looked at several web pages with testers, which use mini mag-lite
>bulbs
>>and various colored LED's. All seemed good,however, one developer said that
>>some colors of LED's have an affect on reading certain zones when measuring
>>(?). What size of LED (mcd) should I use. I have a orange LED that is 1200
>>mcd. if I can use this, what amount of resistance is need with 3 volt.
>
>Hi
> The LED's towards the blue end produce sharper shadows but I know
>several that use red LED's on their testers. Orange should work
>OK. The main issue is to have a uniform light source over the angle
>that the source edge of slit forms as a cone of light, across
>the face of the mirror. This is just a simple f/ratio thing. Many
>cut the end from the LED and then place frosted glass or magic
>mending tape ( 3M name ) to get a uniform source. The source needs
>to be close enough to the source edge to get uniform light but
>not so close that surface detail is also focused by the resolving
>edge. You only want the source slit or edge to be the defining image
>and no interfered with by source issues.
> You need to know the running voltage and current of your LED to
>determine the size of the resistor. It is just Ohms law:
>
> 3v
>
> ( 3 - Vf ) / MaxCur = R
>
> You'll get better consistency light if you use 3 cells with
>4.5V instead of just two. This give a little more drop across
>the resistor. This tends to regulate better. It also allows
>my method of making a variable source easier. Rather than
>putting a variable resistor in series with the current limit
>resistor, I like to add another fixed resistor and variable
>resistor in parallel to the LED. This does draw a little
>more current but I get better light control. One needs to experiment
>with the particular LED, that they have, to select the best combination.
>
>>
>What type of light source should I use that will satisfy the requirements of
>>each test method? I know every little about electronics, how do I figure
>the
>>amount of resistance to use a LED powered with a 3v battery. (can someone
>>send me a diagram of what they use?) I want to be able to test several
>>blanks sizes, does this mean that I will need a brighter light source for
>>the larger glass?
>
> The best source for the Ronchi is something with a large surface
>area. I use a jumbo green LED from RS. Edge and wire test don't
>require as large a source. For most typical mirrors, the F/ratio
>is high enough the there is no need for different intensities
>( unless you expect to do f/2's and also f/30's at with the
>same setup ).
> The Ronchi should technically have the glass side towards you for
>a minimum effect of the glass. The gratings from Edmond have good
>quality glass and I've not seen enough difference that I feel the
>need to check each time I set it up.
>
>>I have a ronchi screen that I bought some years ago, and can't remember who
>I
>bought it from.(could have been WillBell, or Edmonds?of) It's is made of
>glass
>(almost looks tinted) proximately 1.5 x 1.5x .25 thick. Will the thickness
>of
>this ronchi screen and then changing to knife edge have an effect on the
>test?
>
> Yes and no. It does cause the distance from the tester edge to the mirror
>to be slightly different. As for measuring, we are looking at delta
>value and not absolutes. The thickens has no effect.
>Dwight
>
>>
>>
>
>
>
>.

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