Technology Nugget—Mirror Coatings
  Jerry Nelson, University of California Observatories/Lick Observatory at UC Santa Cruz, TMT Project Scientist

A common bathroom mirror gives no hint of the challenges of mirror coatings for a telescope. A bathroom mirror has a thin layer of silver behind a sheet of glass, and the back of the silver is then painted black to protect it. Put your finger on a bathroom mirror and you will see that there is a small gap between your finger and its image. This is twice the thickness of the glass. However, the glass that works so well here actually absorbs radiation in the infrared and in the ultraviolet, and that radiation is important to astronomy.

For TMT we will VERY carefully polish our glass mirrors to the desired optical shape and then we must deposit a reflective coating on the FRONT surface of the piece of glass to make the high reflectivity mirror. The reflective properties of this coating are crucial to gathering the light we want to study. We are interested in all the light that gets through the atmosphere, covering a wavelength range of 0.30 to 30µm. Visible light, light that our eyes can detect, covers only a small fraction of this range, from about 0.40 to 0.70µm. It turns out that finding materials that reflect the broad range of wavelengths that astronomers care about is difficult. Some metals do a pretty good job, such as aluminum, silver, and gold. Figure 1 shows the reflectivities of these materials.

TMT gathers and focuses starlight by using three mirrors, so the net throughput is set by three bounces off of the mirror coatings, making high reflectivity even more important. Also shown in Figure 1 is the throughput of light reflecting from three mirrors.

An additional problem arises beyond that of reflectivity. We want a coating that is durable, and does not need to be stripped and recoated very often. This is particularly important with a segmented primary where there are many mirrors to be recoated. Silver, which is quite reflective in the red- infrared part of the spectrum, tarnishes very quickly. Aluminum is better, but still only lasts a year or two.

Today the search is on for a coating that can protect the metallic layer, perhaps silver, and with the right thickness of the protective layer, enhance the ultraviolet reflectivity where silver begins to fail. Shown in Figure 2 are a couple of candidate coatings that are currently being tested. Will they be durable enough, and easy enough to apply to be practical? Time will tell.