Thomas Schneider OPTI 521 Tutorial

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 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 % pure Silver, Nickel Chromium, and Silicon Nitrate

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

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.

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

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

Mirror Recoating

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

Thanks to everyone that showed up A Special thanks to Tomi and Maxime have done on this subject