Mirror coatings are applied to the polished and figured mirror to make them much more reflective. This page discusses the following topics related to mirror coatings: Silvering, Aluminizing, Enhanced Reflectivity Coatings, and Removing a Coating.
In the early days of amateur telescope making, before vacuum coating services were widely available, silvering was how mirrors were made reflective. But just like the silver in your hutch, it tarnishes quickly and has to be recoated often. It is also soft and does not clean easily. With the right chemicals, silvering can be done by an amateur, and used to be done frequently. In fact, at many Stellafane conventions, we have mirror silvering demonstrations.
However, the silvering process involves using silver nitrates, which like most other nitrates can become explosive. There are tales of amateurs having explosive mishaps while silvering their mirrors.
Given the current environment of increased national security, and caring about your personal safety, we will not say anything more about silvering, particularly with a superior aluminum coating technology available.
Today, most amateur mirrors are coated in a high vacuum chamber where aluminum is evaporated, then as it flies about the chamber unimpeded by air molecules it evenly coats the mirror (and everything else in the chamber, including the viewport). The vast majority of amateurs cannot afford the equipment to do this, the space to house it, and many would be challenged to develop the technical skill to get good coatings (adhesion to the glass and coating uniformity are particular challenges).
You can check our list of vendors who provide coating services. In 2010, one typical vendor charges $70 to coat an 8 inch mirror (two-way shipping and insurance not included). Price varies by mirror size, materials are negligible, you are paying for space in the vacuum chamber. A few vendors give amateurs a price break if they are not in a rush and can wait for available space in the chamber that would not be used otherwise (most coaters make their living servicing the commercial optics industry - amateur one at a time mirrors are just a sideline. Some vendors will recoat your secondary for free if it comes along with the primary. Do a little shopping around to see who is offering a package that is best for you.
Aluminum coatings are soft and prone to scratching, most vendors offer a Silicon Dioxide overcoat that is transparent and harder, allowing infrequent, gently cleaning of your mirror (harder is a relative term, your coated mirror is still fragile and prone to scratching)
Coatings deteriorate over time, and must be stripped and recoated over the span of years. How many years? It depends on your environment: Salt in the air near the seacoast or acid rain in dew will shorten its life; dry, pollutant free areas can have coating life in the decades.
Standard Aluminum Coatings reflect about 91% of the incident visible light. Many vendors now offer Enhanced Coatings, which can reflect as much as 96% of the incident visible light by adding several dielectric films on top of the aluminum. This can make a dramatic difference in telescope performance:
|Standard, 91%||Standard, 91%||83%||17%||Lowest|
|Standard, 91%||Enhanced, 96%||87%||13%||Lower|
|Enhanced, 96%||Standard, 91%||87%||13%||Higher|
|Enhanced, 96%||Enhanced, 96%||92%||8%||Highest|
Why wouldn't you get enhanced coatings? They can cost significantly more, and some can be difficult to remove when it is time to recoat. Some people argue you should take the money you spend on enhanced coatings and just get the next size bigger mirror, with about the same results (of course, the bigger scope will be heavier and less portable, but will also have higher resolving power).
In the end, like most design issues in telescope making, it is a tradeoff between many things, and you will have to figure out what is best for you. Many generations of amateurs did (and still do) quite well without enhanced coatings; on the other hand enhance coatings offer an easy way to improve light grasp by just spending some additional money. Many people choose the middle ground, and put enhanced coatings on their secondary (this is much less expensive than doing your primary, as coatings are sold by area covered).
Most coaters will remove an old coating at no extra charge before recoating your mirror. They actually do quite a bit of prep work to clean your mirror even if it has never been coated - in order to insure good adhesion of the new coating and to prevent out gassing and contamination of foreign material on your mirror in the high vacuum chamber, your mirror will be thoroughly cleaned before it enters the chamber.
An old coating can be 'polished off' if you are refiguring a mirror, so no need to explicitly remove it in this case.
Removing a coating involves using somewhat dangerous chemicals, requires working with adequate protective equipment, adequate ventilation, and there is some risk of damage to the underlying mirror. The chemicals required vary by the type of coating on the mirror; some enhanced coatings can be difficult for even professionals to remove. If you really want to remove a mirror coating, search the internet, I'm sure you will find some interesting instructions - just be careful and don't get hurt.