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24268Re: [atm_free] Re: Coaters

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  • Pete
    Nov 19, 2013

      The company I work for uses silver mirrors in extremely harsh environments (ie. high energy density around 70 watts/cm2) and outdoor environments with pollution lots of thermal cycles and humidity and the mirrors last many years.

      Somehow the manufacturer found a means to protect the silver coating so I know it can be done.

      Do you happen to know how one can protect a silver coating for astronomical use?



      Sent from my iPad

      On 2013-11-19, at 12:20 AM, paul valleli <paulvalleli@...> wrote:


      Waldo Kitty,
      It used to be more common to use copper under Silver, as it becomes a sacrificial layer in the presence
      of humidity. Tiny black spots appear in time where there are pinholes, but not wholesale failure of the silver.
      One of the problems with evaporated coatings on metal mirrors is that micro-cracking develops with age, 
      caused by the differential expansion of the metal and dielectrics. It results in an increase in light scatter.
      Silver is used for both glass telescope mirrors and second surface mirrors and has the highest reflectance
      over most of the light spectrum, UV to IR. The penalty is tarnishing and short lifetime.
      Dielectric multi-layer coatings can achieve 0.9999 reflectance at single laser wavelength and in the last
      two decades close to the same for white light. One restriction though. If the angle of incidence changes,
      such as a scanning mirror, then the reflected beam will be significantly modulated in brightness due to 
      polarization effects. This is not a problem for Star Diagonals or Newtonian Folding mirrors, which are fixed.
      Starman Paul 

      On Mon, Nov 18, 2013 at 9:41 AM, waldo kitty <wkitty42@...> wrote:

      On 11/18/2013 7:01 AM, paul valleli wrote:

      > For glass mirrors, Gold can be applied by the evaporative process, very similar
      > to aluminizing and a Mag or Thorium Fluoride overcoat can be applied for scratch
      > resistance. It is very soft like bare aluminum and sleeks quite easily. Only
      > non-contact cleaning can be used.
      > Sometimes an underlayer of Chromium is applied first to improve adherence,
      > otherwise the bare Gold may fail the scotch tape adherence test.
      > Reflectance is nil in the Violet but runs up to 0.98 at the red laser wavelength
      > of 633nm. It gets to 0.99 in the thermal IR.
      > I don't believe there is a chemical deposition process for glass as there is for
      > silver.


      it seems that they apply silver under the gold in one of their processes... the
      main thing to remember is that these guys methods are for second surface mirrors
      so their description of adding a layer of silver over the gold has to be
      remembered as being done on the back of a flat glass mirror you would hang in
      your hallway... since we are doing first surface mirrors, we would put the
      silver down first and then the gold...


      the above link is for their gold chemicals kit...

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