FUSE Mirror Assembly: The FUSE mirrors were assembled and tested at the Johns Hopkins University Department of Physics and Astronomy and at the Johns Hopkins University Applied Physics Laboratory. This is one of four FUSE telescope mirror assemblies. Each mirror is tilted at about five degrees to focus the light back onto a detector. The clear aperture of the mirror is 387 mm by 352 mm (about 15 inches by 14 inches).
Credit: Johns Hopkins University / Applied Physics Lab

Almost all UV telescopes use mirrors to collect and image the light. However, another technical challenge is that even today we still don’t know how to make coatings for the mirrors that will have excellent reflectivity in the UV. Most materials don’t reflect ultraviolet light. One usually has to put a very thin coating on the optical pieces to get them to reflect. The people who make these coatings are very good, and there’s been a tremendous improvement in the quality of the coatings, but we’re still trying to make them better. They’re good enough that we can now literally see out to very large distances. We can now observe the UV spectra of stars and galaxies in exquisite detail. That’s the story in terms of technology. Finding the best possible coatings has been a struggle, as all technical development is, but I think the field is now relatively stable, although, of course, some progress is still occurring.

The other part of the story is that the technology in other areas can still be improved. There’s room here for people who can figure out how we can make telescopes that don’t have to be so large and, therefore, so expensive to launch into orbit. One always needs more light, and the obvious way to obtain it is to build bigger and bigger telescopes with bigger and bigger mirrors. These larger and larger mirrors in turn require larger and larger, and more expensive, rockets to launch them. However, if we can increase the reflectivity of our mirrors, increase the efficiency of our detectors, and improve the instrument designs, we can collect more light in a certain amount of time without taking the brute force approach of making bigger and bigger mirrors. And in fact that’s what happened with the Hubble Space Telescope in the visible, and also in the ultraviolet. The most recent addition to the Hubble Space Telescope instrument complement, the Cosmic Origins Spectrograph, an ultraviolet spectrograph, was designed for optimal efficiency, and it achieved almost a ten to twentyfold improvement over earlier instruments for some types of observations. That’s the same as having a telescope with ten to twenty times larger collecting area. That’s the kind of thing that needs to be done.