The use of Reflective Silver on Gemini Mirrors - PowerPoint Presentation

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The use of Reflective Silver on Gemini Mirrors

Thomas Schneider

OPTI 521

TutorialSlide2

The Vacuum ChamberVacuum chamber is 9m by 6m, with a total volume of 150 cubic metersRoughing pumps begin the pumping process, with cryopumps taking over and bring the vacuum to 10E-6 TorrThe mirror is supported on a wiffle tree inside of the vacuum chamberSlide3

The Vacuum Chamber and Wiffle Tree SupportSlide4

MagnetronsThe magnetrons consists of a water cooled cathode which is sputtered, and a counter electrode as an anodeA series of neodymium magnets behind the target lengthen the effective ionization areaThe electron trap creates high specific ionization (high plasma density), the plasma accelerates towards the cathode and the impulse energy is transferred to, knocking target particles out of their lattice bondThe three targets (cathodes) are made from 99.999% pure Silver, Nickel Chromium, and Silicon NitrateSlide5

Magnetron SputteringSlide6

Mirror StrippingAfter the mirror is lowered into the washcart, stripping can beginThe mirror is first washed with neutral soap to remove any debris or contaminationSolutions of potassium hydroxide, hydrochloric acid, cupric sulfate pentahydrate, nitric acid, and ceric ammonium nitrate are used to strip the coatingSlide7

Mirror CoatingThe mirror is placed inside the vacuum chamber, which is pumped down to 10E-6 TorrUsing sputtering heads (called magnetrons) the protected silver coating is sputtered onto the mirrorThe layers in order are 65Å NiCr, 1100Å Ag, 6Å NiCr, and 85Å Si

Coating thickness of ±5% is required, and measured with quartz crystal sensors with a repeatability of 1Å.Slide8

Quality ControlSeveral tests are used to determine the quality of the coatingMeasurements of 470nm, 530nm, 650nm, 880nm, and 2.2µm wavelengths with a handheld reflectometerPinhole hole test<5 pinholes of 10µm and <5 pinholes under 5µm per square inchScotch tape test

Need Crystal Clear Scotch tape and witness sampleSlide9

Performance of 4-layer Silver coating vs. aluminum coatingSilver outperforms traditional aluminum coatings in most wavelengths, especially in infraredThe coating performs well over longer periods of time compared to aluminum coatings4-layer coating causes increased absorption at wavelengths smaller than 500nm3% at 500nm up to 8% at 400nm from SiNx layer

Additional 2.7% absorption by 5Å NiCr layer4-layer coating allows for In-Situ wash, which restores mirror coating optical performance but shortens coating life

4-layer silver coating generally lasts 4 years, 2 years for

alumumiumSlide10

Performance Comparison of Al, Ag, 3-layer, and 4-layer recipesSlide11
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Mirror RecoatingSlide13
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Silver Coating PerformanceAt Gemini North~1% reflectivity loss per year for M1 and M2Approximately 4 years between re-coatingsAt Gemini SouthNo reflectivity loss on M1 after In-Situ washIn-situ wash caused loosening on some coating, may require recoat sooner than anticipatedNo reflectivity loss in M2 after

almost 1 yearSlide17

MahalosThanks to everyone that showed upA Special thanks to Tomi and Maxime have done on this subject