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Re: [atm_free] new member.. old question..Foucault/Ronchi testing

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  • Bob May
    The statement of the Ronchi test at the COC being equivalent to the Foucault test is true. The lines on the surface get so large that all you end up seeing is
    Message 1 of 7 , Jan 1, 2004
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      The statement of the Ronchi test at the COC being equivalent to the Foucault
      test is true. The lines on the surface get so large that all you end up
      seeing is one edge of one of the lines and at some point, the direction of
      the line as you move it swaps direction on the mirror surface. Finding the
      point between the two directions produces a point at which a spherical
      mirror evenly darkens overall which is the Foucault test condition for the
      sphere.
      One thing that many who start doing the Foucault test don't realize is that
      you need to get your eye close to the KE in order to see the light fill the
      surface under test. Remember that this is a geometric test so after the KE,
      the light is an expanding cone and your eye will limit the size of that cone
      as you move back from the crossing point. The shorter the focal ratio of
      the mirror, the closer your eye needs to be to the KE in order to see all of
      the light from the surface of the mirror.
      I've got an article on using a camera on the Foucault test on my website
      that will do a decent job of explaining what is happening.
      For the testing with the Foucault tester, the images and drawings that you
      have seen is exactly what you will be seeing when you do the test. The
      first time anybody sees the test actually being done tends to produce gasps
      of amazement at what is being shown, including the heat waves from somebody
      standing near the test setup. The whole mirror (assuming a sphere or close
      to one) will darken evenly all across at the same time rather than seeing a
      edge where one side is light and the other is dark.
      Bob May
      http://nav.to/bobmay
      bobmay@...
      NEW! http://bobmay.astronomy.net
    • Craig V
      Thanks... I think I have it working now. I am tring to do zonal measurements with a 5 zone mask. The Ronchi images suggest that I am close to a parabaloid.
      Message 2 of 7 , Jan 1, 2004
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        Thanks... I think I have it working now. I am tring to do zonal
        measurements with a 5 zone mask. The Ronchi images suggest that I am
        close to a parabaloid. There is curvature in the Ronchi lines which
        means that the ROC has to be different in different zones. I am
        having a hard time judging where each zone nulls. It seams to be
        very subjective. I can tell when I am far inside or far outside, but
        within 0.1" of the ROC it is very hard. Any tips?

        Craig

        --- In atm_free@yahoogroups.com, "Bob May" <bobmay@n...> wrote:
        > The statement of the Ronchi test at the COC being equivalent to the
        Foucault
        > test is true. The lines on the surface get so large that all you
        end up
        > seeing is one edge of one of the lines and at some point, the
        direction of
        > the line as you move it swaps direction on the mirror surface.
        Finding the
        > point between the two directions produces a point at which a
        spherical
        > mirror evenly darkens overall which is the Foucault test condition
        for the
        > sphere.
        > One thing that many who start doing the Foucault test don't realize
        is that
        > you need to get your eye close to the KE in order to see the light
        fill the
        > surface under test. Remember that this is a geometric test so
        after the KE,
        > the light is an expanding cone and your eye will limit the size of
        that cone
        > as you move back from the crossing point. The shorter the focal
        ratio of
        > the mirror, the closer your eye needs to be to the KE in order to
        see all of
        > the light from the surface of the mirror.
        > I've got an article on using a camera on the Foucault test on my
        website
        > that will do a decent job of explaining what is happening.
        > For the testing with the Foucault tester, the images and drawings
        that you
        > have seen is exactly what you will be seeing when you do the test.
        The
        > first time anybody sees the test actually being done tends to
        produce gasps
        > of amazement at what is being shown, including the heat waves from
        somebody
        > standing near the test setup. The whole mirror (assuming a sphere
        or close
        > to one) will darken evenly all across at the same time rather than
        seeing a
        > edge where one side is light and the other is dark.
        > Bob May
        > http://nav.to/bobmay
        > bobmay@n...
        > NEW! http://bobmay.astronomy.net
      • Bob May
        Try making the masks the color of the light and then match the shade of the color to the light returns. Make a pair or more of masks so that you don t see all
        Message 3 of 7 , Jan 2, 2004
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          Try making the masks the color of the light and then match the shade of the
          color to the light returns.
          Make a pair or more of masks so that you don't see all of the zones at the
          same time. I use two masks to do my testing so that one mask has the odd
          zones and the other mask has the even zones. This helps isolate the shadow
          and drops the mixup of so many openings in the mask from mixing me up.
          Lastly, I use a security camera (typically about $40 or so at surplus
          places) with a different lens on the camera (I've got an article on my pages
          about this) so that I can see the image in comfort and also have those about
          me kibbitz on the actual graying of the zones. I also make sure that the
          other persons understand what is happening with the test so that they can
          make an educated guess as to when I am at the right place.
          Bob May
          http://nav.to/bobmay
          bobmay@...
          NEW! http://bobmay.astronomy.net
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