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24270Re: [atm_free] Re: Coaters

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  • paul valleli
    Nov 19, 2013
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      Yes, Peter.
      A durable silver is available from several companies such as Denton FS999.
      Meade cheats a little by peaking the reflectance in the yellow with a slight dropoff in reflectance 
      in the blue  and red wavelengths. Meade uses such a multilayer. The thickness adjustment of 
      the protective overcoat is how this is usually achieved. Advertising usually reports the peak, not 
      the average value. Photometric correction for this dropoff can be compensated for by using several comparison stars to achieve millimag measurements.

      One coating system seals the glass surface with an application by ion implanting of a hard dielectric like Silicon Dioxide.
      Over this is applied a solid layer of Chromium to eliminate pinholes. Over that is a layer of sacrificial copper, then the silver. Finally, a quarter wavelength of a hard protective overcoat such as mag fluoride. There are many variants of this system and all are created using advanced coating software and LOTS of experience.
      A major improvement in coating durability was achieved about twenty years ago using an Ion Assist process of deposition.
      This is now replaced with Ion Implantation ( if the coater can afford the equipment).with a very significant improvement in quality, including adherence, humidity resistance, temperature influence, and resistance to scratching during cleaning operations. 
      Most manual cleaning operations can result in coating damage, so they should be employed very rarely. I clean my Meade Corrector once a year in August to remove pine pollen, dew spots, and dust accumulation. Never more than that.
      Starman Paul
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