UV-enhanced coating caveat: spectrophotometric response

For ground-based optical-IR telescopes, there are several good sites with reasonably good transparency at 310 nm and longer. For the telescope mirrors, much of the coating improvements in recent years were focused on the infrared range. For extremely large telescopes to be most productive on those UV-friendly sites (harmful for humans though), UV-enhanced coating improvements are an important challenge to address. UV observations can contribute to better understanding of the energetics of heavier stars, hence the overall spectral energy budget of the galaxies. Also, there are important stellar spectral lines that relate to metallicity studies and chemical enhancement.

For the optics encased in temperature and vacuum controlled Dewars, or in space which do not suffer from the air pollution or moisture, durability of the coating is not a major issue. Neither is the strippability of the coating.

These almost contradicting challenges – good reflectivity in the short wavelengths and the durability and the strippability of the coating can be met with a multi-layer coating combined with silver for the broad band reflectivity, with layers serving different functions, such as the adhesion to the substratum, enhancement in UV, and protection from the environment.

Several groups are seeing success in enhancing the overall UV performance. Those coatings are already utilized with instruments and space telescope mirrors and have demonstrated good performance in the continuum or broad-band level. The next step will be to evaluate the spectrophotometric/temporal response of such coatings. The problem being that the shorter wavelength side will be more likely to suffer faster degradation over time and show high variations as a function of wavelength. This means, not only the general decreased response over the UV range, but also the effect on the spectral lines that fall into the bands with faster degradation. The dielectric material used to raise the UV response may thus affect the measured line profiles as well.

This report is part of the preparation to study the problem of temporal degradation of UV performance. Optics teams at TMT and NAOJ are planning exposure tests of TMT mirror coating samples. Simple modeling of the degradation of the reflectivity, convolved with spectral lines like Beryllium-7 line at 313 nm is one good sample. Be-7 as a precursor to Li-7 affects the estimate of Lithium in the galaxy.