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Lyot tester question

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#1 Lucullus

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Posted 19 January 2025 - 02:30 PM

I am trying to understand the setup of a Lyot tester and have some basic questions.

So this is a basic article about the Lyot test you probably all have seen who are applying the Lyot test https://adsabs.harva...JBAA..103..184F . And there are several discussions here on CN about how to come up with a good soot plate, probably most prominent see https://www.cloudyni...93-lyot-tester/ .

 

What I am wondering about is the following:

1. One basically uses the setup of a Foucault test and applies the soot plate. Ok. But there are "slitted" (two parallel knife-edges) and "slitless" (one KE creating a virtual slit) Foucault testers - and also pinhole setups. The above hyperlinked adsabs-paper says

Lyot_setup1.jpg

It also says later that

Lyot_setup2.jpg

In combination with the various Foucault test setups of slitted and slitless, I am wondering whether the slitless Foucault setup would not work? Do you really need a real instead of a virtual slit?

Here is an example of a slitted Lyot setup https://www.janvanga...lt/Foucault.htm. On the other hand, in this Lyot setup sketch by Texereau I do not see that the light source is masked besides the soot plate - neither by a real, nor a virtual slit. Do I misread this sketch or is a slit not mandatory? At least in post #37 of the discussion it is said that a slitless tester was tried without results.

Lyot_setup3.jpg

 

 

 

2. I am wondering about the pros and cons of using a soot plate that only shows a distinct cut-off on one side of the soot, and a soot plate having a straight line of soot? #42 of the above discussion mentions a link concerning this question, but which doesn't work anymore. Does anyone have some experiences?

 

 

 

3. About the orientation of the soot pattern (single cut-off or soot line). The last image in #45 is quite confusing.

post-9943-0-63714500-1452759537_thumb.jp

Does the soot have to be orthogonal to the slitted light source?

I rather understand the Lyot test setup to work in such an orientation with the slit and soot being parallel, creating no setup-induced astigmatism at all, just like in the example of the Foucault tester in the link. In the sketch of Texereau above, I would turn the soot line 90°.

Lyot_setup4.jpg

tester_close-up.jpg

 

 

Thank you very much for your insights!

 

 


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#2 Oregon-raybender

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Posted 19 January 2025 - 03:44 PM

Maybe of help. I have never used the Lyot test myself, but here you go.

 

Starry Nightswaytogo.gif

 

 

https://wp.optics.ar...08/LyotTest.pdf

 

https://wp.optics.ar...faceQuality.pdf

 

https://w.astro.berk...fus_Audouin.pdf


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#3 Dale Eason

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Posted 19 January 2025 - 04:08 PM

I have always used the soot knife parallel to the slit.  I also use a wide line that blocks one side just like a knife.  But I have heard of some using a thin line.


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#4 hamishbarker

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Posted 19 January 2025 - 04:12 PM

Hi Lucullus,

 

you wrote "On the other hand, in this Lyot setup sketch by Texereau I do not see that the light source is masked besides the soot plate - neither by a real, nor a virtual slit. Do I misread this sketch or is a slit not mandatory?"

 

I see in that fig 5, that the light (which is annotated as a mercury lamp, presumably to give monochromatic light) is in an opaque chimney (the can structure around the bulb) with a small pipe leading to  right angle prism to the right of the lamp, and a slit indicated with an S on the side of the prism facing the test mirror.



#5 MKV

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Posted 19 January 2025 - 04:28 PM

 Does anyone have some experiences?

The test is analogous to the knife-edge test. The knife edge is parallel to the soot edge. The only difference is that the Lyot test is recorded in almost complete darkness (extended exposure).

 

ke vs lyot.jpg

 

Edit: closup

 

magnified_a_labeled (2023_10_30 21_12_06 UTC).jpg


Edited by MKV, 19 January 2025 - 04:31 PM.

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#6 Lucullus

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Posted 19 January 2025 - 04:39 PM

The test is analogous to the knife-edge test. The knife edge is parallel to the soot edge. The only difference is that the Lyot test is recorded in almost complete darkness (extended exposure).

 

attachicon.gif ke vs lyot.jpg

 

Edit: closup

 

attachicon.gif magnified_a_labeled (2023_10_30 21_12_06 UTC).jpg

So can you imagine what the picture above is all about showing the soot line orthogonal to the slitted red light source?
 


Edited by Lucullus, 20 January 2025 - 12:56 AM.


#7 Lucullus

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Posted 19 January 2025 - 04:46 PM

Hi Lucullus,

 

you wrote "On the other hand, in this Lyot setup sketch by Texereau I do not see that the light source is masked besides the soot plate - neither by a real, nor a virtual slit. Do I misread this sketch or is a slit not mandatory?"

 

I see in that fig 5, that the light (which is annotated as a mercury lamp, presumably to give monochromatic light) is in an opaque chimney (the can structure around the bulb) with a small pipe leading to  right angle prism to the right of the lamp, and a slit indicated with an S on the side of the prism facing the test mirror.

Ah yes, that must be the slit, albeit not visible from behind the right angle prism. To make sense, the slit also must be vertical as the soot line. But this causes a lateral displacement and small setup induced astigmatism. Therefore, I would orient the slit and soot line horizontal, so that the lateral displacement doesn't play a role anymore, like in the last image of a Foucault tester in my initial post #1. Do you agree?
 



#8 Dale Eason

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Posted 19 January 2025 - 04:46 PM

He is not at that point demonstrating the setup.  He is trying to show the width of the slit compared to the width of the soot line.  If they were parallel one would have been centered over the other and thus hard to compare.  He could have off set it laterally but instead just rotated the soot line.


Edited by Dale Eason, 19 January 2025 - 06:32 PM.

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#9 Dale Eason

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Posted 19 January 2025 - 04:50 PM

You can do either horizontal or vertical slit and knife.  Either will be subject to astig if not aligned correctly.  However the Lyot test is not used to see astig it is used to see small scale roughness of the surface.  So that is really moot.


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#10 Lucullus

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Posted 19 January 2025 - 05:16 PM

You can do either horizontal or vertical slit and knife.  Either will be subject to astig if not aligned correctly.  However the Lyot test is not used to see astig it is used to see small scale roughness of the surface.  So that is really moot.

Of course, perfect parallelism I took for granted in my proposal of aligning both slit and knife. And of course, I hope I nowhere raised the impression that I think the Lyot is intended to measure astigmatism.

 

Ok, so my third question is answered. Concerning question No2. there are setups using soot lines and one-sided soot edges. My biggest question mark now lies on question No1. about the mandatory of slitted light sources... hmm.gif
 



#11 Dale Eason

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Posted 19 January 2025 - 06:20 PM

Yes you use a slit.



#12 Dale Eason

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Posted 19 January 2025 - 10:03 PM

The thing about this test is it is more of a curiosity.  It can show surface roughness that is both too big and is also too small to matter.  It requires experience like most tests to know if it is showing anything that matters but it does not show a lot of errors that do matter.  Thus other test can be more sufficient.  In the past it required a very very bright light and so was difficult to accomplish.  Now there are super bright LEDs that can power it with little trouble.  The next hurdle is creating the smoke streak of the right density.  The test is touchy to that. 


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#13 MKV

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Posted 19 January 2025 - 10:57 PM

So can you imagine what is the picture above all about showing the soot line orthogonal to the slitted red light source?

No.



#14 MKV

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Posted 19 January 2025 - 11:38 PM

My biggest question mark now lies on question No1. about the mandatory of slitted light sources

Second Dale's answer, yes. Illustrations are worth a thousand words. :o)

 

lyot tester_lbl.jpg

 

Making a soot plate is easy, as the picture show here. Slide a clean peace of glass over the candle at a steady pace and you'll get a very uniform soot  deposit. Trial and error will tell you what the correct density is.

 

lyot soot ke maker.jpg


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