Elena Rossi
Silver coatings on telescope reflectors have been a topic of interest for many years, with the process being developed as early as 1857. Silver coatings can provide higher reflectivity in the 400-500 nm spectral range, resulting in better performance from visible to near IR. However, silver is susceptible to tarnishing and degradation over time, which has led to the exploration of alternative metals such as aluminium, rhodium, and enhanced aluminium coatings. While aluminium is the most cost-effective option, silver coatings may offer slightly improved reflectivity and visual performance. The use of protective overcoats and adhesion layers can also enhance the durability and performance of silver coatings. In recent years, there has been a resurgence of interest in spray silver coatings for telescope mirrors, with advancements in anti-tarnishing compounds and protective layers.
Characteristics and Values
| Characteristics | Values |
|---|---|
| Silver coatings for telescopes | Silver coatings have higher reflectivity in the 400–500 nm spectral range, providing better performance from visible to near IR |
| Silver coatings are subject to degradation from molecules of O, O-, H2O2, SO2, and Cl- in the air | |
| Silver coatings require protective overcoats to enhance durability and avoid oxidation | |
| Silver coatings can be applied through spraying, dipping, or vacuum deposition | |
| Silver coatings can be removed using silver remover | |
| Alternatives | Aluminum coatings are more durable and cost-effective, with a wider reflectivity range |
| Rhodium coatings are highly reflective, resistant to corrosion, and easy to clean |
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What You'll Learn
- Spray silver coatings are an attractive option for telescopes with mirrors 12-inches or larger in diameter
- Silver coatings are more reflective than aluminium but are prone to tarnishing
- Aluminium is the most cost-effective coating material for telescopes
- Silver coatings require a protective overcoat to enhance reflectivity under 500nm
- Silver coatings can be applied using a dual-nozzle sprayer
Spray silver coatings are an attractive option for telescopes with mirrors 12-inches or larger in diameter
Spray silver coatings are a good option for telescopes with mirrors 12 inches or larger in diameter. The cost of aluminizing has increased, making spray silvering an attractive alternative. The technique was developed at Tinsley Labs and the University of Arizona in the 1970s, and it can achieve a coating with very high initial reflectivity.
The process of spray silvering involves removing the old coating, which can be done with a kit that includes a silver remover. The mirror should be placed in a large plastic tub, and the old coating stripped with ferric chloride. This may take an overnight soak. The mirror can then be cleaned with precipitated calcium carbonate and distilled water.
Once the mirror is clean, the spray silvering can begin. A dual-nozzle sprayer can be used to apply the silver coating. It is important to tilt the mirror horizontally and to wear vinyl gloves to protect the mirror from fingerprints. The anti-tarnish liquid Angel Guard can be applied to protect the silver coating from sulfur in the air. This is done by pouring a small puddle of the liquid onto the mirror and spreading it with cotton balls.
Spray silver coatings can provide high reflectivity in the 400-500 nm spectral range, with an average reflective rate of up to 98% in the 400-1000 nm range. This makes them a suitable substitution for aluminium coatings, which have lower reflectivity above 400 nm wavelengths.
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Silver coatings are more reflective than aluminium but are prone to tarnishing
Silver coatings have a higher reflectivity than aluminium, with bare silver achieving around 98% reflectivity compared to 92% for aluminium. Silver also has an advantage over aluminium for reflectivity of infrared wavelengths. However, silver is prone to tarnishing, which significantly diminishes its reflective qualities.
The tarnishing of silver coatings is caused by exposure to sulfur, humidity, and handling. The thin nature of the coating means it can deteriorate and wear away over time, exposing the base metal underneath. The base metal, often copper, nickel, or brass, can react with sulfur and other corrosive agents, accelerating the tarnishing process.
To prevent tarnishing, silver coatings can be protected by dielectric overcoats, which can also improve reflectivity. The use of an anti-tarnishing compound such as Angel Guard can also protect silver coatings without changing their reflectivity.
In recent years, there has been a renewed interest in using silver coatings for telescope mirrors due to the increased cost of aluminizing. Researchers at the Gemini Observatory have found that silver mirror coatings are effective for observing infrared and visible spectrum wavelengths, with the potential to dramatically increase the efficiency and reflectiveness of existing telescopes.
While silver coatings offer higher reflectivity, aluminium is more durable and has a more mature commercial technology. Aluminium does not tarnish like silver, and its coatings are easier to remove and reapply. Therefore, most sources still recommend protected aluminium coatings for telescope mirrors.
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Aluminium is the most cost-effective coating material for telescopes
Aluminium has been the standard coating for telescope mirrors for almost a century. In the 1930s, John Strong developed a method of depositing aluminium on glass in a vacuum chamber, and since then, aluminium coatings have been widely used for telescopes. Aluminium coatings are highly durable and do not tarnish like silver coatings.
While silver coatings may offer higher reflectivity in the 400-500nm spectral range, they are more susceptible to degradation due to environmental factors. Silver coatings can be affected by molecules of O, O-, H2O2, SO2, and Cl- in the air, resulting in the formation of AgCl, AgSO4, AgNO3, or Ag2S, which degrade the film qualities.
To mitigate this issue, researchers are exploring hybrid coatings that combine silver and aluminium. These hybrid coatings aim to provide maximum reflectivity while improving the lifespan of the silver base. One potential process involves using plasma-enhanced atomic layer deposition with an aluminium-oxide top layer to protect against corrosion and maintain reflectivity.
However, aluminium coatings continue to be a popular choice due to their cost-effectiveness, durability, and broad reflectivity range. The Hubble Space Telescope, for example, utilises aluminium coatings on its mirrors to collect infrared, visible, and ultraviolet light effectively.
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Silver coatings require a protective overcoat to enhance reflectivity under 500nm
Silver coatings are widely used in telescopes to achieve high reflectivity. However, silver is subject to degradation over time due to oxidation and the presence of sulphur in the air. This results in the formation of AgCl, AgSO4, AgNO3, or Ag2S, which degrade the film quality and reduce reflectivity.
To enhance the durability and reflectivity of silver coatings, a protective overcoat is necessary. This overcoat serves as a barrier, protecting the silver from the surrounding environment. The overcoat can be a dielectric layer, enhancing the hardness and oxidation stability of the coating.
One such protective overcoat is Angel Guard, an anti-tarnishing compound that protects the silver coating from sulphur in the air without altering its reflectivity. This overcoat can be applied using a dual-nozzle sprayer, ensuring an even and complete coating.
Additionally, silver coatings can be enhanced with UV enhancement layers, particularly in the 400-500nm range. This brings the reflectivity of silver up to par with that of aluminium, which is the most cost-effective coating material for telescopes.
Overall, by utilising protective overcoats and UV enhancement layers, the reflectivity of silver coatings can be significantly improved, making them a viable option for telescope mirrors.
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Silver coatings can be applied using a dual-nozzle sprayer
Silver coatings are essential for manufacturing reflecting telescope mirrors to achieve the highest reflectivity. Silver coatings can be applied to telescope mirrors using a dual-nozzle sprayer. This method was developed in the 1930s by William Peacock, who simplified the process of depositing an optical quality silver coating on glass. Peacock's technique involves spraying silvering chemicals from two nozzles, creating a fine spray that coats the mirror surface.
The Angel Gilding kit is a popular choice for spray silvering telescope mirrors. It includes a silver remover for stripping old coatings and a dual-nozzle sprayer for applying the new silver coating. The process is straightforward, but it requires careful preparation and a methodical approach. While anyone can successfully spray silver a mirror, it is crucial to follow the instructions closely to achieve a great coating.
The dual-nozzle sprayer allows for precise control over the application of the silver coating. The size of the dispenser's nozzle determines the size and volume of the droplets, with the droplet velocity and volume directly influenced by the driving voltage. Additionally, the dual-nozzle sprayer helps create a uniform coating by ensuring consistent droplet size and spray pattern.
To further enhance the performance of silver coatings, adhesion layers can be added between the mirror substrate and the metallic silver layer. This technique improves reflectivity in the blue spectrum and achieves overall high reflectivity in the applied spectral region. Protective layers can also be applied to prevent the silver coating from degrading due to the presence of sulfur in the environment.
Overall, silver coatings applied with a dual-nozzle sprayer offer a cost-effective and efficient way to achieve high reflectivity in telescope mirrors. With the right tools and preparation, anyone can create an excellent coating, improving the performance of their telescope.
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Frequently asked questions
Yes, silver coatings can be used for telescope reflectors. Silver coatings are highly reflective and can provide higher reflectivity in the 400–500 nm spectral range.
The process of applying a silver coating to a telescope reflector involves spraying silvering chemicals onto the surface of the mirror. This method was developed in the 1930s by William Peacock and simplifies the process of depositing an optical quality silver coating.
Silver coatings are more reflective than aluminium coatings, with a reflectivity of over 90%. However, silver is more susceptible to tarnishing and oxidation, and may require additional protective overcoats. Aluminium is more durable and resistant to degradation, making it a more cost-effective option for telescope reflectors.
Silver coatings offer higher reflectivity than aluminium, particularly in the visible to near-infrared range. This can result in brighter images with less scatter. Additionally, silver coatings can be enhanced with protective layers to improve their hardness, oxidation stability, and resistance to sulfurization.
Yes, rhodium has been suggested as an alternative due to its high reflectivity, extreme hardness, and resistance to corrosion. However, rhodium coatings are difficult to remove, and their longevity is uncertain. Other potential alternatives include iridium, osmium, chromium, and silicon coatings.
