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Today there is an incredible amount of misinformation about optical coatings.
Do not cheat yourself out of a top quality, extremely-low scatter, highly uniform and highly
reflective optical coating because some other optics fabricator has problems with their
coating laboratory. The bottom line is…Galaxy Optics’ C-1 and C-2 optical coatings are the
best available. No other optical coating offers so much.
Galaxy Optics Optical Coating Process Information
Stationary and Simple Rotation Coating Systems for Telescope Optics
Stationary Optical Coating Systems:
The optic is suspended up side down in the center of the chamber approximately 12 to 24 inches above the coating material sources centered at the base of the vacuum chamber. In a system of this design the optical coating will be always thicker at the center. The difference in the films thickness from the center to the edge of the optic is easily seen visually by looking at the optical coating in bright light at a slight angle. There will be off center circles of different colors, usually red in the center (the thickest area) then green and finally blue at the edge. The circles are always slightly off center in a stationary system because the sources cannot be exactly centered in the chamber. This type of coating system introduces error into the optical figure. The amount of error depends on the coatings overall thickness and the source to substrate distance ratios.
Simple Rotation Optical Coating Systems:
The “Simple Rotation” coating system is by far the best way to coat large optics. The optic is suspended up side down in the center of the chamber and is rotated. The coating material sources are located on a radius near the outside edge of the vacuum chamber (see website photos). During the coating process the radial velocity of the edge of the optic is greater than the radial velocity at the center. Even though the coating material sources are closer to the edge of the optic the radial velocity is greater hence less exposure. A properly designed simple rotation system can yield coating uniformities of nearly 100%. The system used at Galaxy Optics yields a coating uniformity near 100% on a 22” diameter optic. The worst case is 98% for a 16” optic. A 96% EAL optical coating with a total thickness of 257nm and 98% uniformity yields less than 1/100th wave thickness variation from the center to the edge. 257nm x 2% = 5.14nm. The thickness variation measurement at 530nm yields: 5.14nm/530nm = 0.0096 waves or about 1/100th wave.
Planetary Rotation Optical Coating Systems:
The “Planetary Rotation” coating system adds a second element to the optics' rotation during the coating process. Typically, planetary rotation is used in optical coating systems where many small optics are to be coated at one time and the coating thickness is large, 500nm to more 5000nm. The simple rotation is applied along with three or four optical component fixtures that rotate on a separate axis from the simple rotation. Think of this as the same motion as planets rotating around the Sun. The coating uniformity obtained in systems of this design is very nearly 100%, a very important factor for thick optical coatings.
Ion Assisted Deposition (IAD):
A new Galaxy Optics trademark “ARC” or “Activated Reactive Coating”. This process is the same as “IAD” just a different name. “Ion Assisted Deposition” is the process of ionizing pure oxygen or other reactive gases at very low pressure (typically 2 x 10-4 Torr) into plasma. The increased the molecular energy is used to force a chemical reaction to 100% completion at a lower temperature. The coating process for telescope optics involves the use of metal monoxide dielectric* compounds as starting materials. The two most common for telescope optics are silicon monoxide (SiO) and titanium monoxide (TiO). The metal monoxides must be fully oxidized during the coating process to form silicon dioxide (SiO2) the low refractive index material and titanium dioxide (TiO2) the high refractive index material. Chemical reaction equations are: 2SiO + O2 > 2 SiO2 and 2 TiO + O2 > 2 TiO2. If the starting materials are not fully oxidized the coating will be dark in color and have very low reflectivity. The best way to obtain 100% oxidation is IAD. IAD has two very important qualities: (1) the high-energy ionized oxygen plasma drives the reaction to completion assuring a completely transparent film with an extremely low extinction coefficient. (2) The increased molecular energy is sufficient to force the SiO2 or TiO2 molecules to form a dense packed microcrystalline structure that is extremely hard. Depositing thin films of silicon dioxide (SiO2), titanium dioxide (TiO2) and other metal oxides using IAD yields very low scatter coatings. This is the same optical coating technology used for the manufacture of ultra low scatter super high-energy laser optics.
*Short Story
* Dielectric Compounds: Dielectric materials are nothing more than “Electrical Insulators”. A better name for materials used in optical coatings would have been “Refractory Metal Oxides and Fluorides”. I guess “dielectric” is way shorter.
In the early days, capacitors were made by rolling up aluminum foil and waxed paper. The waxed paper was the “dielectric” or insulator. When vacuum systems were invented scientist found that silicon monoxide and other materials could be easily evaporated on to aluminum foil to make much smaller higher quality capacitors…the rest is history.
I have tried my best to be concise but it is difficult to condense a great amount of technical information into a few sentences…volumes have been written about this technology. The exact coating design formulations for C-1 96% EAL and the C-2 98% EAL are proprietary. The C-1 optical coating is a four layer coating consisting of: L-1: Aluminum, L-2: SiO2, L-3: TiO2, L-4: SiO2. The C-2 optical is a 6 layer optical coating with the additional layers L-5: TiO2 and L-6: SiO2.
The next new topic for “Let's Get The Facts Straight” is super clean cryogenic pumped optical coating systems vs oil-diffusion pumped systems.
John A. Hudek
Galaxy Optics
Check back soon for more discussion of this topic!
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