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Re: Fiber optic cable and artificial stars

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  • a.johnw
    I ve never been there but the gregorian has much to offer really. A simple definitive test for the 2ndry with no shadow guessing at all. All gained by a tube
    Message 1 of 60 , Mar 1, 2008
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      I've never been there but the gregorian has much to offer really. A
      simple definitive test for the 2ndry with no shadow guessing at all.
      All gained by a tube that may be 4 or 5 ins longer in modest sizes
      than it need be. I think the bad press is really down to 200 ins +
      telescopes where that is more important.

      The Dall Kirkam is interesting too. Takahashi's version fetches high
      prices. People keep them too which suggests that they work well. On
      that one it is feasible to make a reference concave sphere for the
      2ndry. Oddly I've never seen any ATM info on newtonian focus
      correctors for these but suspect it's a simple almost zero power meniscus.

      On figuring against an artificial star many professional opticians
      have done that when they must even Texereau.

      The cables I've ordered are 8um by the way. Single mode. I have no
      idea what multi mode means. May be some one could fill me in?

      John


      --- In atm_free@yahoogroups.com, "Tony Gondola" <acgna@...> wrote:
      >
      > If you don't mind a slightly longer tube length there's not a thing
      wrong
      > with the Gregorian..........
      >
      > Tony
      >
      > ----- Original Message -----
      > From: "Arjan te Marvelde" <arjan.te.marvelde@...>
      > To: <atm_free@yahoogroups.com>
      > Sent: Friday, February 29, 2008 12:54 PM
      > Subject: [atm_free] Re: Fiber optic cable and artificial stars
      >
      >
      > >A fiber-end source, maybe even a 62.5/125um multimode one, would
      make a
      > >good
      > > source for null-testing elliptical mirrors. You can put this as a
      source
      > > in
      > > the minor focus, with minmal obstruction of the view on the mirror
      from
      > > the
      > > major focus, where the KE is.
      > >
      > > On the other hand, who wants to make a Gregorian?
      > >
      > > ... Arjan
      > >
      > >> I have lost track of the reasons for your desire to make an
      artificial
      > >> star from fiber optics. If the reason is it is just something
      you like
      > >> to tinker with then continue.
      > >
      > >
      > >
      > >
      > > Yahoo! Groups Links
      > >
      > >
      > >
      > >
      > >
      > > --
      > > Internal Virus Database is out-of-date.
      > > Checked by AVG Free Edition.
      > > Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release Date:
      1/13/2008
      > > 8:23 PM
      > >
      > >
      >
    • a.johnw
      Arjan it reminds me of the speckles a laser pointer produces when it s used close to most surfaces. In this case they are in a circular patern, small and
      Message 60 of 60 , Mar 18, 2008
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        Arjan it reminds me of the speckles a laser pointer produces when it's
        used close to most surfaces. In this case they are in a circular
        patern, small and bright. They have a blue green tinge where as the
        dead centre dot out of core is whiter.
        My thoughts were must be a coherent light effect but had no idea that
        leds gave off any.

        John

        --- In atm_free@yahoogroups.com, "Arjan te Marvelde"
        <arjan.te.marvelde@...> wrote:
        >
        > John,
        >
        > Does it look like light shining on sand?
        > Speckle has a very distinct appearance, and is caused by interference
        > between modes in coherent light, e.g. coming from a laser, under very
        > slightly differing angles.
        > The LED might just have sufficient coherence to get this effect, but
        from
        > your qualification "bright dots" it is not clear to me if what you
        see is as
        > fine-grained as speckle normally is... Bigger dots may as well indicate
        > other modes in the fiber core, but I suppose these dots should be
        fairly
        > large then.
        >
        > Arjan
        >
        > > When I look directly at the end hand held so it's about 500mms or so
        > > away from my eye I can see 2 rings of bright dots arround the core.
        > > The outer ring being more obvious than the inner. They go if I view it
        > > close up with any sort of magnifier. Can also see them wearing a pair
        > > of reading glasses but there is some distortion.
        >
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