第10回 可視⾚外線観測装置技術WS
2021年12⽉9⽇

⼤型光学系の多層膜
コーティング性能
林 左絵⼦
国⽴天⽂台 TMTプロジェクト

1All Japan -> All Earth

Subaru Telescope and Thirty Meter Telescope (TMT)

Credit: NAOJ

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何が問題か• 対象 望遠鏡光学系 メートルクラスのサイズ• 波⻑域 ひとこえ 0.3 - 30 micron• 地上でも適地では紫外光の⼤気透過率が⾼い(後出)ので観測したい• コーティング性能の要件 ⾼反射率と⼀様性• 反射率が光学系全⾯にわたって⼀様に⾼いこと
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TMTの場合: M1, M2, M3について• ファーストライト⽤コーティング：Gemini望遠鏡レシピ
銀＋保護膜の多層膜

Boccas et al. 2004, SPIE 5494, 239• ぜひUV⽤オーバーコートでUV側反射率上げたい• が、⾚外性能を損なってはいけない• 状況：M2, M3コーティング装置のCoDR経たところ
https://tmt.nao.ac.jp/blog/15774

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https://www.tmt.org/news/542

⾼性能広帯域コーティング

paragraphs 3.5 ± 3.10, including tests performed per MIL13830B, but details are not presented in thisreport.
2. REFLECTIVITY
The reflectivity was measured for witness samples that were coated with the mirrors. The witness
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same processes and conditions as the flight mirrors during the flight mirror coating runs. The reflectivity
was measured at six separate sites for each mirror. The reported data for the PM is the average for thecoatings of all 18 flight segments; an average of 6 X 18 = 108 reflectivity measurements. The reported
data for the other three mirrors is the average of the six measurements for each mirror. Spectralreflectance measurements were made over a wavelength range from 700 nm to 2500 nm using avisible/Near Infrared (vis/NIR) spectrometer. The measurements were made with a spectral sampling of
1 nm and at an angle of incidence to the sample of 6°. The spectral range extended from 2500 nm to20000 nm was using an FTIR spectrometer. The FTIR measurements were made with a spectral
sampling of 5 nm and at an angle of incidence of 10°. Thin film calculations were made for the protective
overcoat on the gold surface. The difference in reflectivity between 6° and 10° angles of incidence issmaller than the noise in the measurements, so the data for the two spectral ranges were simply combined.
The noise for the vis/NIR spectrometer increases near the upper end of its range and is noticeable at thetransition to the FTIR data as shown in Figure 3. The six sample average for each individual PM segment
is plotted along with the average for all 18 segments (solid green curve). The reflectivity specification is
the stepwise red curve.

⾼性能広帯域コーティング
AGCの例（E-ELTのM1, M2, M3 コーティング装置コントラクター）

Case of JWST: focus on IR range
by QCI (Quantum Coating Incorporated)

Figure 3. Spectral reflectivity for the 18 PM segments after coating

SPIE 8442-119
Lightsey et al. 2012www.agc-plasma.com
リーフレットより

coating maintenance. We are acquiring a handheld 2.2µm reflectometer to monitor this requirement.
1.3

Gemini’s feasibility and demonstration study for low-emissivity and durable coatings
The final report of this study8 from Optical Data Associates (ODA) was delivered in 1995 and the main conclusions
are summarized here. ODA selected two subcontractors for the demonstration phase: Airco Coating Technologies (ACT)
produced silicon nitride (SiNx) protected films whereas Deposition Sciences Inc. (DSI) made hafnium oxide (HfOx)
protected films using a microwave energy supported plasma. Both technologies and recipes consistently reached thesame reflectivity R10µm = 99.1% in production but the SiNx protection proved to be slightly superior for durability test totarnishing environment. Because the deposition of the SiNx film was also made under classical sputtering, it is the paththat Gemini selected for specifying the coating plant hardware.
It is worth noting that although the ε2.2µm = 2% goal is achievable according to evaporated Ag results, the reference
wavelength chosen for the feasibility study was 10µm and the demonstration phase reached ‘only’ R2.2µm = 98.6% (i.e.
ε2.2µm = 1.4% per surface).
A very important phase of the study was obtaining accurate reflectivity measurements in order to reach the goal of
R10µm = 99.2%. ODA and its subcontractors used similar and/or directly comparable spectrophotometers: for the UV-VisNIR range, absolute instruments like the Hitachi 4001 and Cary 5E in V-W configuration which yield the square of R
without comparison to a reference mirror; and relative device like the PE 983G for the MIR range where themeasurements were compared to NIST standards (Al and Au) analyzed with the Cary and with a 10.6µm absolutereflectometer (using a CO2 laser source) built by Helios Inc. The accuracy obtained at 10µm was ±0.01%. Other
consistent measurements were also performed at NOAO with an emissometer (called ‘blue toad’) working at λ = 4µm.
The last critical phase of the study was the environmental testing of the samples to assess their durability. Fourdifferent tests were performed: weathering (cycle through high temperature and humidity), delamination (scotch tapepull), abrasion and tarnishing (exposure to hydrogen sulfide fog). However, no real-life exposure tests were conducted atan observatory.
The final optimal coating recipe was a stack with the following layers:

何が問題か：被膜の均⼀性• 膜厚の薄いもの• 副鏡、第３鏡の中⼼部分

MaterialSiNx
NiCrNxAg
NiCrNxFunction
protector
adhesorreflector
adhesorsubstrate

Thickness
15nm
0.8nm
200nm
5nm

Gemini Telescope’s recipe = TMT’s First Light coating240

Proc. of SPIE Vol. 5494

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何が問題か：耐久性vs.剥離性• 外気／環境にさらされる <-> 観測装置⽤光学系はコントロー

ルされた環境（デュワー内など）、⾶翔体光学系は⼤気汚染を気にする必要がない• 劣化への対処はコーティングをはがして再コート <->観測装置⽤や⾶翔体光学系は再コーティングを考えなくてよい（JWST再コートはそもそも不可能）• in-situ cleaning (運⽤状態での清掃) -> これで表⾯が荒れては困る• 多層膜剥離の複雑さ 薬品が多種類必要になる
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何が問題か：剥離⼯程• 薬品およびその廃液の管理• ⾦属の扱い 分離、回収• 鏡材とその直上のコーティングのボンディングを切るよ
うな物質があると良いのだが10

何が問題か：剥離⼯程
薬品がたくさん
すばる望遠鏡
⾚外副鏡

Gemini望遠鏡
主鏡鏡材

ULE

ULE

コーティング

銀

保護膜付き銀トモリムーブ

塩酸
⽔酸化カリウム
硫酸など

コーティング剥離剤

11ぜひアドバイスお願いいたします