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Lyot tester

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  • Jan van Gastel
    I am trying to build a Lyot tester and have some questions. 1. I have a slit somewhere between 0.05 and 0.1 millimeter wide. I read it should be around 0.05
    Message 1 of 6 , Apr 1 6:31 AM
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      I am trying to build a Lyot tester and have some questions.
      1. I have a slit somewhere between 0.05 and 0.1 millimeter wide. I read
      it should be around 0.05 mm. Is this critical?
      2. I also read I need a bright light. Will 20 watts (halogen) be enough
      or should it be (much) brighter?
      3. Do I need a camera or webcam, or is it possible to look naked eye,
      like the Foucault test (for some reason I hate using camera's but if
      really necessary I will use one).
      4. Do I need a micrometer to position the phase plate relative to the
      mirror like in the Foucault test?

      --
      Jan
      http://members.ziggo.nl/jhm.vangastel/Astronomy/
    • John
      Texereau shows an example of the lyot test in how to make a telescope. On an 125mm dia mirror with 2m radius of curvature. He mentions using an 450um slit but
      Message 2 of 6 , Apr 1 8:34 AM
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        Texereau shows an example of the lyot test in how to make a telescope. On an 125mm dia mirror with 2m radius of curvature. He mentions using an 450um slit but give no details other than the slit width range is usually 100 to 200um and intensely illuminated - due to the light loss in the phase plate I would suspect. I would guess 20w halogen would be plenty for visual use and probably modern cameras as well.

        Sounds like the slit width will have to be determined by trial.

        It's possible to make a phase plate with candle soot on a microscope slide or thin piece of glass. Just let it build up until it's fairly opaque.

        His book also describes how to make an adjustable slit. Set by eye against the light. A bit fiddly but easier than it sounds. It needs some basic hand tools plus bits of brass. Junior hacksaw, small file - say 150mm x 12mm flat, and worst of all a tap for cutting screw threads for the screws to fasten into. Also some scrap glass and a little abrasive to matte that. Some fine wet and dry emery paper can be used to tidy the brass bits up. Use some soapy water on it and lay it on a flat surface.

        He also shows a Foucault tester that uses a micrometer feed. The table slides on small brass V's. The main thing about the micrometer feed is the big knob. A large knob on the tilts screw is a good idea too. Any type of tester can be mounted on the sliding table including just a knife with a separate lights source as he shows.

        The book can be downloaded off the internet archive.

        My version has sadly spent several years in a damp place - this is what I mounted on mine, a smaller version of the main table - all made of aluminium but most of it can be made of timber etc. The real micrometer head could do with a large knob on it really. The tilt screw really does need one.

        http://groups.yahoo.com/group/atm_free/photos/album/27923289/pic/1141533014/view

        I tilted the light source to align with the knife as it was convenient. Texereau's version allows the knife edge to be made the same way as the slit sides are. I'm going to make compact moving source tester Texereau style as it's so easy. Just make one of the sides of the slit 1/2 as high as the other that also forms the knife edge and cut a piece of the backing plate away to clear the knife edge.

        John



        --- In atm_free@yahoogroups.com, Jan van Gastel <jhm.vangastel@...> wrote:
        >
        > I am trying to build a Lyot tester and have some questions.
        > 1. I have a slit somewhere between 0.05 and 0.1 millimeter wide. I read
        > it should be around 0.05 mm. Is this critical?
        > 2. I also read I need a bright light. Will 20 watts (halogen) be enough
        > or should it be (much) brighter?
        > 3. Do I need a camera or webcam, or is it possible to look naked eye,
        > like the Foucault test (for some reason I hate using camera's but if
        > really necessary I will use one).
        > 4. Do I need a micrometer to position the phase plate relative to the
        > mirror like in the Foucault test?
        >
        > --
        > Jan
        > http://members.ziggo.nl/jhm.vangastel/Astronomy/
        >
      • Stephen Koehler
        Jan, Dale Eason did some phase contrast testing, and posted some information in the interferometry wiki:
        Message 3 of 6 , Apr 1 8:36 AM
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          Jan,

          Dale Eason did some phase contrast testing, and posted some information in the interferometry wiki:

          http://starryridge.com/mediawiki-1.9.1/index.php?title=Phase_Contrast


          On Mon, Apr 1, 2013 at 8:31 AM, Jan van Gastel <jhm.vangastel@...> wrote:
          I am trying to build a Lyot tester and have some questions.
          1. I have a slit somewhere between 0.05 and 0.1 millimeter wide. I read
          it should be around 0.05 mm. Is this critical?
          2. I also read I need a bright light. Will 20 watts (halogen) be enough
          or should it be (much) brighter?
          3. Do I need a camera or webcam, or is it possible to look naked eye,
          like the Foucault test (for some  reason I hate using camera's but if
          really necessary I will use one).
          4. Do I need a micrometer to position the phase plate relative to the
          mirror like in the Foucault test?

          --
          Jan
          http://members.ziggo.nl/jhm.vangastel/Astronomy/



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          --
          Steve Koehler
        • atmpob
          Think of the test as being a special version of the Foucault test. The setup is the same. You need fine positioning just like Foucault but like the
          Message 4 of 6 , Apr 1 8:38 AM
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            Think of the test as being a special version of the Foucault test. The setup is the same. You need fine positioning just like Foucault but like the Interferometer you do not need it measured. The amount of light reaching the camera is much less. So you need longer exposures.

            It helps to have a large sensor entrance pupil. Larger than your eye. I don't think I ever could see it well with my eye at all. It usually needs many seconds of exposure. For those shots that I posted in the Wiki they are in the many second range in a very dark room.

            I have used a very bright LED but it requires even more exposure time than the slide projector light source. There are now LED's that are about as bright as the projector bulb so they would work. I think I used exposures in the few second range for the projector bulb and 30 to 40 seconds for the LED. I never used a webcam but only a digital SLR. I think it would be very hard to use a webcam since the exposure time is so long.

            I don't remember the slit size but it would be what I said in the Wiki.

            The wiki is part of the interferometer wiki.

            Some people have been able to tease almost the same level of detail out of a good foucuault image by playing with the contrast and brightness of the image.

            The down side to the test is there is no absolute scale to the surface you see. On a smooth mirror one can not tell what fraction of a wave it may be showing you. I have never seen a correlation between the fine detail in the Bath FFT interferometer analysis and the surface from that test. I have tried several times. That level of tiny detail does not show up in OpenFringe even though it can see detail down to 1/60 wave or smaller. So I believe it is seeing structure that is too small to matter. It can also see structure that is big but you can not tell which is which except by comparing with a known good mirror tested some other way.

            Bottom line is if you can not see a hint of ripple in the shadows of a good Foucault, or in well done interferometry then the surface is good. The Phase contrast test (Lyot) will still show fine detail below that.

            Dale Eason

            --- In atm_free@yahoogroups.com, Jan van Gastel <jhm.vangastel@...> wrote:
            >
            > I am trying to build a Lyot tester and have some questions.
            > 1. I have a slit somewhere between 0.05 and 0.1 millimeter wide. I read
            > it should be around 0.05 mm. Is this critical?
            > 2. I also read I need a bright light. Will 20 watts (halogen) be enough
            > or should it be (much) brighter?
            > 3. Do I need a camera or webcam, or is it possible to look naked eye,
            > like the Foucault test (for some reason I hate using camera's but if
            > really necessary I will use one).
            > 4. Do I need a micrometer to position the phase plate relative to the
            > mirror like in the Foucault test?
            >
            > --
            > Jan
            > http://members.ziggo.nl/jhm.vangastel/Astronomy/
            >
          • John
            Texereau also shows a picture of the same mirror using Foucault but with a rather narrow 10um slit. In real terms it shows the problems with the mirror more
            Message 5 of 6 , Apr 1 1:04 PM
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              Texereau also shows a picture of the same mirror using Foucault but with a rather narrow 10um slit. In real terms it shows the problems with the mirror more clearly than the Lyot test which is probably most effective at showing the differences produced by various polishing materials. Ripple down to 10nm can be seen just using Foucault. Things do get tougher though as the mirror radius decreases.

              With a 10um slit the knife needs to be set approximately 1/2 way through it which gives some idea of the control needed. Truth is an M6 fine bolt with a large knob can achieve this sort of level of control.

              John

              PS I hope the surplus shed beam splitters that they are having made are suitable for Bath Dale. I've ordered one but I have 2 things to try on an F3 mirror before I build one.


              --- In atm_free@yahoogroups.com, "atmpob" <atmpob@...> wrote:
              >
              > Think of the test as being a special version of the Foucault test. The setup is the same. You need fine positioning just like Foucault but like the Interferometer you do not need it measured. The amount of light reaching the camera is much less. So you need longer exposures.
              >
              > It helps to have a large sensor entrance pupil. Larger than your eye. I don't think I ever could see it well with my eye at all. It usually needs many seconds of exposure. For those shots that I posted in the Wiki they are in the many second range in a very dark room.
              >
              > I have used a very bright LED but it requires even more exposure time than the slide projector light source. There are now LED's that are about as bright as the projector bulb so they would work. I think I used exposures in the few second range for the projector bulb and 30 to 40 seconds for the LED. I never used a webcam but only a digital SLR. I think it would be very hard to use a webcam since the exposure time is so long.
              >
              > I don't remember the slit size but it would be what I said in the Wiki.
              >
              > The wiki is part of the interferometer wiki.
              >
              > Some people have been able to tease almost the same level of detail out of a good foucuault image by playing with the contrast and brightness of the image.
              >
              > The down side to the test is there is no absolute scale to the surface you see. On a smooth mirror one can not tell what fraction of a wave it may be showing you. I have never seen a correlation between the fine detail in the Bath FFT interferometer analysis and the surface from that test. I have tried several times. That level of tiny detail does not show up in OpenFringe even though it can see detail down to 1/60 wave or smaller. So I believe it is seeing structure that is too small to matter. It can also see structure that is big but you can not tell which is which except by comparing with a known good mirror tested some other way.
              >
              > Bottom line is if you can not see a hint of ripple in the shadows of a good Foucault, or in well done interferometry then the surface is good. The Phase contrast test (Lyot) will still show fine detail below that.
              >
              > Dale Eason
              >
              > --- In atm_free@yahoogroups.com, Jan van Gastel <jhm.vangastel@> wrote:
              > >
              > > I am trying to build a Lyot tester and have some questions.
              > > 1. I have a slit somewhere between 0.05 and 0.1 millimeter wide. I read
              > > it should be around 0.05 mm. Is this critical?
              > > 2. I also read I need a bright light. Will 20 watts (halogen) be enough
              > > or should it be (much) brighter?
              > > 3. Do I need a camera or webcam, or is it possible to look naked eye,
              > > like the Foucault test (for some reason I hate using camera's but if
              > > really necessary I will use one).
              > > 4. Do I need a micrometer to position the phase plate relative to the
              > > mirror like in the Foucault test?
              > >
              > > --
              > > Jan
              > > http://members.ziggo.nl/jhm.vangastel/Astronomy/
              > >
              >
            • Jan van Gastel
              Thanks for all the answers. I have read the wiki and some other (French) articles and I think I will succeed in buiding one. ... -- Jan
              Message 6 of 6 , Apr 3 12:00 AM
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                Thanks for all the answers. I have read the wiki and some other (French)
                articles and I think I will succeed in buiding one.

                Op 1-4-2013 17:38, atmpob schreef:
                > Think of the test as being a special version of the Foucault test. The setup is the same. You need fine positioning just like Foucault but like the Interferometer you do not need it measured. The amount of light reaching the camera is much less. So you need longer exposures.
                >
                > It helps to have a large sensor entrance pupil. Larger than your eye. I don't think I ever could see it well with my eye at all. It usually needs many seconds of exposure. For those shots that I posted in the Wiki they are in the many second range in a very dark room.
                >
                > I have used a very bright LED but it requires even more exposure time than the slide projector light source. There are now LED's that are about as bright as the projector bulb so they would work. I think I used exposures in the few second range for the projector bulb and 30 to 40 seconds for the LED. I never used a webcam but only a digital SLR. I think it would be very hard to use a webcam since the exposure time is so long.
                >
                > I don't remember the slit size but it would be what I said in the Wiki.
                >
                > The wiki is part of the interferometer wiki.
                >
                > Some people have been able to tease almost the same level of detail out of a good foucuault image by playing with the contrast and brightness of the image.
                >
                > The down side to the test is there is no absolute scale to the surface you see. On a smooth mirror one can not tell what fraction of a wave it may be showing you. I have never seen a correlation between the fine detail in the Bath FFT interferometer analysis and the surface from that test. I have tried several times. That level of tiny detail does not show up in OpenFringe even though it can see detail down to 1/60 wave or smaller. So I believe it is seeing structure that is too small to matter. It can also see structure that is big but you can not tell which is which except by comparing with a known good mirror tested some other way.
                >
                > Bottom line is if you can not see a hint of ripple in the shadows of a good Foucault, or in well done interferometry then the surface is good. The Phase contrast test (Lyot) will still show fine detail below that.
                >
                > Dale Eason
                >
                > --- In atm_free@yahoogroups.com, Jan van Gastel <jhm.vangastel@...> wrote:
                >> I am trying to build a Lyot tester and have some questions.
                >> 1. I have a slit somewhere between 0.05 and 0.1 millimeter wide. I read
                >> it should be around 0.05 mm. Is this critical?
                >> 2. I also read I need a bright light. Will 20 watts (halogen) be enough
                >> or should it be (much) brighter?
                >> 3. Do I need a camera or webcam, or is it possible to look naked eye,
                >> like the Foucault test (for some reason I hate using camera's but if
                >> really necessary I will use one).
                >> 4. Do I need a micrometer to position the phase plate relative to the
                >> mirror like in the Foucault test?
                >>
                >> --
                >> Jan
                >> http://members.ziggo.nl/jhm.vangastel/Astronomy/
                >>
                >
                >
                >
                > ------------------------------------
                >
                > Yahoo! Groups Links
                >
                >
                >
                >


                --
                Jan
                http://members.ziggo.nl/jhm.vangastel/Astronomy/
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