Thomas Schneider OPTI 521 Tutorial. Vacuum chamber is 9m by 6m, with a total volume of 150 cubic...
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Transcript of Thomas Schneider OPTI 521 Tutorial. Vacuum chamber is 9m by 6m, with a total volume of 150 cubic...
The use of Reflective Silver on Gemini MirrorsThomas SchneiderOPTI 521Tutorial
The Vacuum Chamber• Vacuum chamber is 9m by 6m, with a total
volume of 150 cubic meters• Roughing pumps begin the pumping process, with
cryopumps taking over and bring the vacuum to 10E-6 Torr
• The mirror is supported on a wiffle tree inside of the vacuum chamber
The Vacuum Chamber and Wiffle Tree Support
Magnetrons• The magnetrons consists of a water cooled cathode
which is sputtered, and a counter electrode as an anode
• A series of neodymium magnets behind the target lengthen the effective ionization area
• The 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 bond
• The three targets (cathodes) are made from 99.999% pure Silver, Nickel Chromium, and Silicon Nitrate
Magnetron Sputtering
Mirror Stripping• After the mirror is lowered into the washcart,
stripping can begin• The mirror is first washed with neutral soap to
remove any debris or contamination• Solutions of potassium hydroxide, hydrochloric
acid, cupric sulfate pentahydrate, nitric acid, and ceric ammonium nitrate are used to strip the coating
Mirror Coating• The mirror is placed inside the vacuum chamber,
which is pumped down to 10E-6 Torr• Using sputtering heads (called magnetrons) the
protected silver coating is sputtered onto the mirror
• The 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.
Quality Control• Several tests are used to determine the quality of
the coating• Measurements of 470nm, 530nm, 650nm, 880nm, and
2.2µm wavelengths with a handheld reflectometer• Pinhole hole test• <5 pinholes of 10µm and <5 pinholes under 5µm per
square inch• Scotch tape test• Need Crystal Clear Scotch tape and witness sample
Performance of 4-layer Silver coating vs. aluminum coating• Silver outperforms traditional aluminum coatings in
most wavelengths, especially in infrared• The coating performs well over longer periods of time
compared to aluminum coatings• 4-layer coating causes increased absorption at
wavelengths smaller than 500nm• 3% at 500nm up to 8% at 400nm from SiNx layer• Additional 2.7% absorption by 5Å NiCr layer
• 4-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 alumumium
Performance Comparison of Al, Ag, 3-layer, and 4-layer recipes
Mirror Recoating
Silver Coating Performance• At Gemini North• ~1% reflectivity loss per year for M1 and M2• Approximately 4 years between re-coatings
• At Gemini South• No reflectivity loss on M1 after In-Situ wash• In-situ wash caused loosening on some coating, may
require recoat sooner than anticipated• No reflectivity loss in M2 after almost 1 year
Mahalos
• Thanks to everyone that showed up• A Special thanks to Tomi and Maxime have done
on this subject