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Re: Non-uniform / un-even vacuum coating of a telescope mirror

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  • Ken Hunter
    ... Without knowing what the chamber geometries are, I d say that there is not a free path to the glass from the aluminum source. There may be something
    Message 1 of 7 , Sep 1, 2006
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      --- In VacuumX@yahoogroups.com, "curiosis2003" <curiosis2003@...> wrote:
      >
      > It seems to us that the thickness of the coating is not uniform and
      > there is a problem very similar to the problem called 'shadowing'.
      >
      >

      Without knowing what the chamber geometries are, I'd say that there is
      not a free path to the glass from the aluminum source. There may be
      something blocking part of the filament (assuming thermo deposition),
      the filament may not be centered on the optical axis of the glass etc.

      One way of avoiding this is to rotate the mirror during deposition or
      use more than one filament.

      This is a problem the shop owner would want to know about.

      Ken Hunter
    • Thomas Janstrom
      Hi Ken and All, I have one quick question (although that said the answer might not be as quick!) about the baffle you sent me. How would it go if I hooked my
      Message 2 of 7 , Sep 3, 2006
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        Hi Ken and All,

         

        I have one quick question (although that said the answer might not be as quick!) about the baffle you sent me.

         

        How would it go if I hooked my helium compressor unit up to it? I have a big (100V 3Ph) helium compressor unit from a MRI machine sitting here doing nothing more than gathering dust and I just had the idea of hooking it up to the vacuum system to get the baffle to act as a real cold trap, and not just as an oil back streaming interceptor.

         

        The unit is a fully stand alone type, and I have the manual here some where, it’s not the best seeing as the manufacturer just did a bablefish translation of the original Japanese one, some of it is actually quite funny really!

         

        So the questions are, how do you make connections that are helium tight, even with a 250K plus temperature swing, and how well does aluminium handle being 4 or so degrees Kelvin above absolute zero?

         

        Well I hope this gets some lively discussions going, the list has been a bit slow of late!

         

        Cheers, Thomas.

         

        -----Original Message-----
        From: VacuumX@yahoogroups.com [mailto:VacuumX@yahoogroups.com] On Behalf Of Ken Hunter
        Sent:
        Friday, 1 September 2006 11:55 PM
        To: VacuumX@yahoogroups.com
        Subject: [VacuumX] Re: Non-uniform / un-even vacuum coating of a telescope mirror

         

        --- In VacuumX@yahoogroups .com, "curiosis2003" <curiosis2003@ ...> wrote:

        >
        > It seems to us that the thickness of the coating is not uniform and
        > there is a problem very similar to the problem called 'shadowing'.
        >
        >

        Without knowing what the chamber geometries are, I'd say that there is
        not a free path to the glass from the aluminum source. There may be
        something blocking part of the filament (assuming thermo deposition),
        the filament may not be centered on the optical axis of the glass etc.

        One way of avoiding this is to rotate the mirror during deposition or
        use more than one filament.

        This is a problem the shop owner would want to know about.

        Ken Hunter


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      • Ken Hunter
        Thomas... Just grab a small A/C type compressor (refrigerator, water fountain, de-humidifier etc) and hook it up, suck it down, fill with freon and turn it on.
        Message 3 of 7 , Sep 4, 2006
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          Thomas...

          Just grab a small A/C type compressor (refrigerator, water fountain,
          de-humidifier etc) and hook it up, suck it down, fill with freon and
          turn it on. Of course you'd want to put an orfice in the input (hi
          pressure) side of the baffle to allow the differential pressure and
          expansion to cool things off for you.

          Dunno about the other.

          Ken
        • curiosis2003
          Greetings, I m living in Istanbul Turkey and there are two coating workshops here. They are not telescope optics specific shops but making vacuum coating for
          Message 4 of 7 , Oct 17, 2006
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            Greetings,

            I'm living in Istanbul Turkey and there are two coating workshops
            here. They are not 'telescope optics' specific shops but making vacuum
            coating for general industrial purposes.

            So we took the mirror to the second shop today. Now, it seems that the
            inhomogeneous / uneven coating problem has gone but this time there is
            some yellowish tint on the mirror. The technician said in a couple of
            days, the mirror will be 'brighter' and look better _which seemed
            strange to us_

            So we wanted to ask an expert but there is nobody around us :=(
            Can you give us some suggestions? Is there some foreign metal in the
            aluminium foil that they were using? What might be the reason of this
            dark color given the fact that the mirror has a good polish with no
            visible pits at all when examined with maginifying glass under bright
            light?

            Best regards,
            Curiosis

            --- In VacuumX@yahoogroups.com, "Ken Hunter" <atm_ken_hunter@...> wrote:
            >
            > --- In VacuumX@yahoogroups.com, "curiosis2003" <curiosis2003@> wrote:
            > >
            > > It seems to us that the thickness of the coating is not uniform and
            > > there is a problem very similar to the problem called 'shadowing'.
            > >
            > >
            >
            > Without knowing what the chamber geometries are, I'd say that there is
            > not a free path to the glass from the aluminum source. There may be
            > something blocking part of the filament (assuming thermo deposition),
            > the filament may not be centered on the optical axis of the glass etc.
            >
            > One way of avoiding this is to rotate the mirror during deposition or
            > use more than one filament.
            >
            > This is a problem the shop owner would want to know about.
            >
            > Ken Hunter
            >
          • James Lerch
            ... From: curiosis2003 ... Do you know what they evaporated onto the mirror? Was it pure aluminum, or an alloy? Was a transparent
            Message 5 of 7 , Oct 18, 2006
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              ----- Original Message -----
              From: "curiosis2003" <curiosis2003@...>

              > Now, it seems that the
              > inhomogeneous / uneven coating problem has gone but this time there is
              > some yellowish tint on the mirror. The technician said in a couple of
              > days, the mirror will be 'brighter' and look better _which seemed
              > strange to us_

              Do you know what they evaporated onto the mirror? Was it pure aluminum, or an
              alloy? Was a transparent protective layer evaporated over the aluminum?

              In any case, I'm pretty certain the mirror won't look any "Brighter" as time
              goes by...


              Take Care,
              James Lerch
              http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)
              http://lerch.no-ip.com/ChangFa_Gen (My 15KW generator project)

              "Anything that can happen, will happen" -Stephen Pollock from:
              "Particle Physics for Non-Physicists: A Tour of the Microcosmos"

              " Press on: nothing in the world can take the place of perseverance.
              Talent will not; nothing is more common than unsuccessful men with talent.
              Genius will not; unrewarded genius is almost a proverb.
              Education will not; the world is full of educated derelicts.
              Persistence and determination alone are omnipotent. "
              Calvin Coolidge
            • curiosis2003
              Greetings, After referring to Texereau s book, I ve noticed a diagnosis related to my problems: He says (in page 223) insufficient vacuum during evaporation
              Message 6 of 7 , Oct 19, 2006
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                Greetings,

                After referring to Texereau's book, I've noticed a diagnosis related
                to my problems: He says (in page 223) 'insufficient vacuum during
                evaporation may lead to 'yellowish' even 'blackish' color on the
                coated mirror. This can be verified by checking the reflective image
                of a white paper against the mirror. If the image shows color / tint
                shift, then you may conclude that there is a problem during the
                coating. This may be due to leakage or out-gassing of the parts.

                Regards,

                Curiosis



                --- In VacuumX@yahoogroups.com, "James Lerch" <jlerch1@...> wrote:
                >
                > ----- Original Message -----
                > From: "curiosis2003" <curiosis2003@...>
                >
                > > Now, it seems that the
                > > inhomogeneous / uneven coating problem has gone but this time
                there is
                > > some yellowish tint on the mirror. The technician said in a
                couple of
                > > days, the mirror will be 'brighter' and look better _which seemed
                > > strange to us_
                >
                > Do you know what they evaporated onto the mirror? Was it pure
                aluminum, or an
                > alloy? Was a transparent protective layer evaporated over the
                aluminum?
                >
                > In any case, I'm pretty certain the mirror won't look
                any "Brighter" as time
                > goes by...
                >
                >
                > Take Care,
                > James Lerch
                > http://lerch.no-ip.com/atm (My telescope construction,testing, and
                coating site)
                > http://lerch.no-ip.com/ChangFa_Gen (My 15KW generator project)
                >
                > "Anything that can happen, will happen" -Stephen Pollock from:
                > "Particle Physics for Non-Physicists: A Tour of the Microcosmos"
                >
                > " Press on: nothing in the world can take the place of perseverance.
                > Talent will not; nothing is more common than unsuccessful men with
                talent.
                > Genius will not; unrewarded genius is almost a proverb.
                > Education will not; the world is full of educated derelicts.
                > Persistence and determination alone are omnipotent. "
                > Calvin
                Coolidge
                >
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