Transcript The reflective Silver on the Gemini 8 m Primary Mirror

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 99.999% 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