(Click on the above diagram to see a full resolution version.)
This diagram illustrates the main components of the HUT spectrograph,
which is the part of the instrument that breaks incoming ultraviolet
light into its component wavelengths for analysis. The spectrograph is
of the "Rowland circle" variety, named after the famous Hopkins
physicist Henry Rowland. The spectrograph is
housed in a stainless steel casing and is held in position at the prime
focus of the telescope by three Invar spider arms and a support ring.
A converging beam of light from the primary mirror approaches from the
right of the figure and comes to focus precisely at the entrance slit
wheel. This rotating wheel is used alternatively to seal the spectrograph
from the outside world or to place various openings (called
apertures) in position to allow light in. A curved diffraction grating
is used to create the spectrum, which is focussed onto the detector.
This portion of the instrument actually intensifies the UV light,
and then uses an array of light-sensing diodes (known as a Reticon)
to sense and record the spectrum electronically for analysis.
for a photograph of the spectrograph as it appeared during the
construction of the telescope.
The materials used to coat the optical surfaces inside the
spectrograph are what provide the ultraviolet sensitivity of the
instrument. However, these coatings also present a tricky technical
challenge: they must be kept in vacuum or they lose
their ultraviolet sensitivity! During construction these components
were kept briefly in a "dry nitrogen" atmosphere, but after insertion
into the completed spectrograph, the slit wheel was rotated into a
"closed" position and "vacuum pumps" (see diagram above) were turned on
to remove the air from inside. The spectrograph had to remain this
way during the entire assembly and testing phase of the telescope,
during transport from Baltimore to Kennedy Space Center, and even
during its voyage into space! Only then could the commands be sent
to turn the slit wheel and open the spectrograph to the near-vacuum