The Hopkins Ultraviolet Telescope (HUT) on the Astro-2 observatory uses the technique of spectroscopy to study objects of interest to astronomers. A mirror in the HUT telescope focuses ultraviolet light from astronomical objects into a spectrograph. This instrument "spreads" the ultraviolet light into a spectrum in much the same way a prism breaks visible light into a rainbow of colors. It then measures the brightness of the light at each wavelength. By analyzing how the brightness varies across the wavelengths, scientists can determine the elements present in the object, the relative amounts of each element, and the temperature and density of the object. From this, astronomers can gain a better understanding of the physical processes occuring in or near the target being studied.
HUT observes objects like stars, planets and quasars in a portion of the electromagnetic spectrum which has not been well-studied to date. Most satellites and telescopes with ultraviolet instruments, such as the International Ultraviolet Explorer satellite and the Hubble Space Telescope, make observations at longer wavelengths (the "near ultraviolet" or "near-UV", so-called because it is closest to the visible wavelengths we can see with our eye). The Hopkins Ultraviolet Telescope was designed to fill this gap, observing in the "far ultraviolet", allowing objects ranging from planets in our solar system to distant quasars to be studied.
The Hopkins Ultraviolet Telescope was improved for the second Astro mission. The primary mirror and the grating in the spectrograph have been coated with a new compound of silicon and carbon that makes the telescope 3 to 4 times more sensitive than during the first mission. This will allow scientists to obtain higher quality data and make it possible to observe fainter objects.
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HUT Home Page at Johns Hopkins University