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Re: Mirror test set ups- - OpFr

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  • Ric Rokosz
    ... I agree with Dale on this.Seems I was pushing the limits with my test setup and it s not good enough to test a f/3 at present. Perhaps Dale could include
    Message 1 of 32 , May 1, 2008
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      --- In atm_free@yahoogroups.com, "Dale Eason" <atmpob@...> wrote:
      >
      > Group parden us while we work through some of these issue. At
      > present I think Ric's Igrams are not good enough to do the analysis
      > that he is trying for. Ric does not have the experience to recognize
      > when the analysis goes bad. It is not Ric's fault. This particular
      > area requires more understanding of the process and what can go
      > wrong. I am trying to add that information to the help file of the
      > program as well as some automatic garbage detection.

      I agree with Dale on this.Seems I was pushing the limits with my test
      setup and it's not good enough to test a f/3 at present.

      Perhaps Dale could include the FFT image of the f/3 mirror in the Help
      section to show what a unacceptable unwrap looks like...nothing like a
      picture to set things straight.

      Ric
      >
    • a.johnw
      Hi Tom I m going the same way with an XY foucault tester so that I can add a Z axis for a bath interferometer. Also because I was impressed by the ease of use
      Message 32 of 32 , May 7, 2008
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        Hi Tom
        I'm going the same way with an XY foucault tester so that I can add a
        Z axis for a bath interferometer. Also because I was impressed by the
        ease of use of the simple caustic test and the full one. I initially
        used normal foucault + mask for all of testing. Often finding that one
        pupil was much larger than the other after prolonged spells. I used
        the caustic test to verify what I had done. It's much much easier.

        I'm not too keen on large source slit less testers. The right sized
        slit is very useful for showing that a mirror is truly spherical right
        up to the edge. No need for a ronchi screen. Also the caustic test
        depends on being able to locate a wire on the image of the slit /
        centring diffraction patterns.

        My main concern with the bath approach is vibrations. I can't even
        mount something on a solid wall where I will be mostly working. 3rd
        floor bedroom, low solid walls, floorboards and lath and plaster. All
        put together in 1911.

        The appeal of the ross is really just a quick look and see.

        John

        --- In atm_free@yahoogroups.com, "tomofreno2000" <tomofreno2000@...>
        wrote:
        >
        > Hi John,
        >
        > I used the Ross Null Test a few years ago during figuring of a 22"
        > f/4.2 mirror. The lens radius was 2 3/4" and I used about 65% of the
        > radius to reduce contributions to wavefront error from the (Anchor
        > Optics) lens. It did give a null for the finished paraboloid (and
        > also straight interference fringes with a grating), but I didn't find
        > it as useful or sensitive as the Foucault or Ronchi tests. That may
        > well be because I had more experience with those. I find it easier to
        > see where I am in figuring with the Foucault test by looking at the
        > relative radius of curvature of different areas on the mirror. Plus it
        > is more intuitive for me - the surface just stands out in relief.
        >
        > I am currently in the final polishing (9 hrs), beginning
        > figuring/testing, phase of a 10" mirror, and starting to put a Bath
        > interferometer together to use Open Fringe for the analysis since from
        > what I've seen from others it will give more information/insight into
        > subtleties of the surface for final figuring, as well as a more
        > quantitative measurement of surface error than judging shadow edge
        > positions with the Foucault test. I'll still use the known Foucault
        > along with it though as a sanity check. In fact I plan to use the
        > Foucault tester for x and z adjustment, and just add a y adjustment
        > plate with optics/laser for the Bath that I can set on the Foucault
        > tester to easily switch back and forth between the two with only minor
        > re-adjustment. I use a simple slitless tester as described at:
        > http://stellafane.org/atm/atm_foucault_tester/atm_tester_main.htm
        > I won't bother with the Ross Null again. I find it much more
        > informative to see qualitatively what the surface looks like with the
        > Foucault, and just as quick.
        >
        > Tom
        >
        > --- In atm_free@yahoogroups.com, "a.johnw" <a.johnw@> wrote:
        > >
        > > I haven't made a mirror for some time. Last time I used a std faucoult
        > > set up and then after it was more or less finished tried the simple
        > > caustic test, the caustic test and the Dall null test.
        > >
        > > Came to the conclusion that the Dall null test is by far the easiest
        > > to use but am wondering about using it for a fairly fast mirror. I
        > > know that higher order aberrations aren't "completely" accounted below
        > > F4 but am concerned about having to use a semi reflective mirror or
        > > beam splitter in the test set up. (Also for the other tests)
        > >
        > > I've read that the accuracy of the semi reflective mirror isn't too
        > > critical. I've seen comments like find a microscope cover glass that
        > > flat to a part of a wave which makes me wonder. Those are very thin
        > > not big enough either. Semi silvered mirrors are generally a lot
        > > thicker. Beam splitters are tooooo expensive so I'm not to keen on
        > > that option.
        > >
        > > John
        > >
        >
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