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Re: Michell's foucault tester

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  • Mark Cowan
    Bill, Just a random thought here (in tune with Jerry), but why exactly would you /want/ to use an auxiliary scope with an f/4.6 mirror? All mirrors of the
    Message 1 of 12 , Apr 1, 2007
      Bill,

      Just a random thought here (in tune with Jerry), but why exactly would
      you /want/ to use an auxiliary scope with an f/4.6 mirror? All
      mirrors of the same f/ratio appear the same size on the Foucault
      tester and an f/4.6 appears plenty large enough. The main benefit of
      the auxiliary scope is if you're doing something like an f/18 (I have)
      and there a scope might be helpful (I didn't use one but tested it
      solely by even extinction).

      Small image that but quite distinct what was happening.

      Best,
      Mark Cowan
      Salem, OR



      --- In atm_free@yahoogroups.com, "Mitchell" <funnybone101@...> wrote:
      >
      > --- In atm_free@yahoogroups.com, "hr_stellar_evolution"
      > <mountainclan12@> wrote:
      > >
      > > I just finished building my foucault tester, I purchase the little
      > > opera monocular scope to add to foucault test as Michell did. I
      > have a
      > > question about this little jewell. Will it work for large/fast
      > mirrors?
      > > I have mine mounted right up to the knife edge and can't seem to
      > see
      > > the whole (13"f/4.6 and coated) mirror within the oculars fov. Is
      > there
      > > something I'm doing wrong. I've never used a Foucault tester,so
      > this is
      > > new to me.
      > >
      > > Thanks,
      > >
      > > Bill
      > >
      >
    • hr_stellar_evolution
      In designing my tester,I used several designs as examples as I planned to build mine. I liked Mitchell s monocular, in Berry s book he uses a small scope, and
      Message 2 of 12 , Apr 1, 2007
        In designing my tester,I used several designs as examples as I
        planned to build mine. I liked Mitchell's monocular, in Berry's book
        he uses a small scope, and so I included one in mine. Thinking that
        the little scope made sense by keeping your eye and face away from
        the knife edge. The f/4.6 inch mirror (coated) is one that I bought
        on line and put on the test stand to see how the tester works.
        I think that what I really wanted to know was if the monocular was
        needed for this short a focal length mirror. I liked the idea of the
        little scope, it made sense to me and I added it to my tester. It may
        be an expensive error, but it sure was fun to make, and when I saw
        the mirror's image through it it was like seeing the Orion Nebula for
        the first time in a telescope..ooooooooooooh. I'm learning and this
        is all so new to me. I can read books,ATM digests and hours searching
        web-sights, but until I use my hands I haven't really learned a
        thing. I'm grinding my first mirror, it's learning in progress and
        I've learned from my mistakes,and hopefully they won't be to costly.
        I'm excited, after ten years of waiting, I'm actually putting the
        grind to some glass that has been in storage far too long. And, this
        week I finished making my Foucault tester. Folks,to me- "that is
        progress".

        It's good to be back,

        Bill

        BTW, I'll have some questions on this mirror as to what I saw. I need
        to play around with it a bit first.


        --- In atm_free@yahoogroups.com, "Mark Cowan" <toolontop@...> wrote:
        >
        > Bill,
        >
        > Just a random thought here (in tune with Jerry), but why exactly
        would
        > you /want/ to use an auxiliary scope with an f/4.6 mirror? All
        > mirrors of the same f/ratio appear the same size on the Foucault
        > tester and an f/4.6 appears plenty large enough. The main benefit
        of
        > the auxiliary scope is if you're doing something like an f/18 (I
        have)
        > and there a scope might be helpful (I didn't use one but tested it
        > solely by even extinction).
        >
        > Small image that but quite distinct what was happening.
        >
        > Best,
        > Mark Cowan
        > Salem, OR
        >
        >
        >
        >
      • Mitchell
        The real question is, why not? Many people have told me it s not really necesary. These people usually have a camera setup though :) I have very bad vision
        Message 3 of 12 , Apr 2, 2007
          The real question is, why not? Many people have told me it's not
          really necesary. These people usually have a camera setup though :)

          I have very bad vision (20/100) so I can't test without glasses
          otherwise which is a pain. This is much nicer.

          With it's low power you lose no contrast, the shadows take on a
          bit more life through one of these. It is advantageous no matter how
          you look at it. Just makes things a little nicer and easier to test.

          Good luck with your mirror, I am working on my second now.

          BTW I am hogged out and working on sphereizing / moving down the
          grit scale now.

          Mitch
        • Jerry
          Well I guess I can understand using a scope so you can see the mirror. I use the old mark 4 eyeball for testing. My eyes are good except at reading distance.
          Message 4 of 12 , Apr 2, 2007

            Well I guess I can understand using a scope so you can see the mirror.

             

            I use the old mark 4 eyeball for testing.  My eyes are good except at reading distance. If I had to focus close to do Foucault I would be using some kind of auxiliary optical device too.

             

            So I guess a Nagler zoom eyepiece would be a better choice for the eyepiece so you can vary power from say from 1 to 4 X or so. Then you can mount the 31mm Nagler type 5 in a Clement focuser in front of the micrometer like I have to make reading the numbers easy.

             

            Jerry

             

             

             

            -----Original Message-----
            From: atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] On Behalf Of Mitchell
            Sent: Monday, April 02, 2007 7:21 AM
            To: atm_free@yahoogroups.com
            Subject: [atm_free] Re: Michell's foucault tester

             

            The real question is, why not? Many people have told me it's not
            really necesary. These people usually have a camera setup though :)

            I have very bad vision (20/100) so I can't test without glasses
            otherwise which is a pain. This is much nicer.

            With it's low power you lose no contrast, the shadows take on a
            bit more life through one of these. It is advantageous no matter how
            you look at it. Just makes things a little nicer and easier to test.

            Good luck with your mirror, I am working on my second now.

            BTW I am hogged out and working on sphereizing / moving down the
            grit scale now.

            Mitch

          • Mark Cowan
            Bill, Nothing wrong with an auxiliary scope on the tester, plenty of people have used them. Didn t mean to be down on what you built - I ve never tried a
            Message 5 of 12 , Apr 2, 2007
              Bill,

              Nothing wrong with an auxiliary scope on the tester, plenty of people
              have used them. Didn't mean to be down on what you built - I've never
              tried a scope on the tester, frankly, but my eyes are plenty good.

              More concerning is "keeping your eye and face away from the knife
              edge". <channeling richard mode> You shouldn't be using a real knife
              edge in the first place! It's just a figure of speech! Who in their
              right mind would stick their eyeball up close to a razor blade in the
              dark? Read Texereau where he talks about blunt facets and how to make
              a proper "knife edge-like object" - the bonus is lower diffraction
              than a razor edge produces. Razor blades are sharp but not
              necessarily uniform at the edge. You can fix them by blunting them on
              ground glass with some 30 micron AlOx or so and a dozen over-center
              strokes. </crm>

              Your store-bought mirror looks like the Orion Nebula? WTF? Something
              is wrong, I just know it....

              Best,
              Mark

              PS If you don't know who richard is somebody will eventually explain.

              --- In atm_free@yahoogroups.com, "hr_stellar_evolution"
              <mountainclan12@...> wrote:
              >
              > In designing my tester,I used several designs as examples as I
              > planned to build mine. I liked Mitchell's monocular, in Berry's book
              > he uses a small scope, and so I included one in mine. Thinking that
              > the little scope made sense by keeping your eye and face away from
              > the knife edge. The f/4.6 inch mirror (coated) is one that I bought
              > on line and put on the test stand to see how the tester works.
              > I think that what I really wanted to know was if the monocular was
              > needed for this short a focal length mirror. I liked the idea of the
              > little scope, it made sense to me and I added it to my tester. It may
              > be an expensive error, but it sure was fun to make, and when I saw
              > the mirror's image through it it was like seeing the Orion Nebula for
              > the first time in a telescope..ooooooooooooh. I'm learning and this
              > is all so new to me. I can read books,ATM digests and hours searching
              > web-sights, but until I use my hands I haven't really learned a
              > thing. I'm grinding my first mirror, it's learning in progress and
              > I've learned from my mistakes,and hopefully they won't be to costly.
              > I'm excited, after ten years of waiting, I'm actually putting the
              > grind to some glass that has been in storage far too long. And, this
              > week I finished making my Foucault tester. Folks,to me- "that is
              > progress".
              >
              > It's good to be back,
              >
              > Bill
              >
              > BTW, I'll have some questions on this mirror as to what I saw. I need
              > to play around with it a bit first.
              >
              >
              > --- In atm_free@yahoogroups.com, "Mark Cowan" <toolontop@> wrote:
              > >
              > > Bill,
              > >
              > > Just a random thought here (in tune with Jerry), but why exactly
              > would
              > > you /want/ to use an auxiliary scope with an f/4.6 mirror? All
              > > mirrors of the same f/ratio appear the same size on the Foucault
              > > tester and an f/4.6 appears plenty large enough. The main benefit
              > of
              > > the auxiliary scope is if you're doing something like an f/18 (I
              > have)
              > > and there a scope might be helpful (I didn't use one but tested it
              > > solely by even extinction).
              > >
              > > Small image that but quite distinct what was happening.
              > >
              > > Best,
              > > Mark Cowan
              > > Salem, OR
              > >
              > >
              > >
              > >
              >
            • Mark Cowan
              Mitch, Even though I m building a high-res video based analysis system, there s something extremely cool about having only one optical surface in the system
              Message 6 of 12 , Apr 2, 2007
                Mitch,

                Even though I'm building a high-res video based analysis system,
                there's something extremely cool about having only one optical surface
                in the system for testing. I've never tried it so I really can't say
                one way or the other. Glad it works for you. :)

                Is your vision 20/100 corrected? In that case I can see why. I'm
                nearsighted and can test without glasses (the cone is very narrow) but
                it's a lot easier to test with glasses on.

                Best,
                Mark

                --- In atm_free@yahoogroups.com, "Mitchell" <funnybone101@...> wrote:
                >
                > The real question is, why not? Many people have told me it's not
                > really necesary. These people usually have a camera setup though :)
                >
                > I have very bad vision (20/100) so I can't test without glasses
                > otherwise which is a pain. This is much nicer.
                >
                > With it's low power you lose no contrast, the shadows take on a
                > bit more life through one of these. It is advantageous no matter how
                > you look at it. Just makes things a little nicer and easier to test.

                <snip>
              • Mitchell
                ... My vision was 20/100 a few months ago without glasses. Things start to get nticably blurry about 3-4 feet in front of me. I notice it even on the computer
                Message 7 of 12 , Apr 3, 2007
                  --- In atm_free@yahoogroups.com, "Mark Cowan" <toolontop@...> wrote:
                  >
                  > Mitch,
                  >
                  > Even though I'm building a high-res video based analysis system,
                  > there's something extremely cool about having only one optical surface
                  > in the system for testing. I've never tried it so I really can't say
                  > one way or the other. Glad it works for you. :)
                  >
                  > Is your vision 20/100 corrected? In that case I can see why. I'm
                  > nearsighted and can test without glasses (the cone is very narrow) but
                  > it's a lot easier to test with glasses on.
                  >
                  > Best,
                  > Mark

                  My vision was 20/100 a few months ago without glasses. Things
                  start to get nticably blurry about 3-4 feet in front of me. I notice it
                  even on the computer screen when there is white text on a black
                  background. With glasses I'm probably pretty close to 20/20. Still
                  notice a slight slight bit of near sightedness when I watch TV and they
                  have white letters on black background, it just sticks out more.

                  BTW I have finished painting my 6" Scope. It came out great. I
                  still have to do the base but this scope should be ready for first
                  light soon.

                  Mitch
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