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