HUT Spectrograph Description

(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. Click here 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 of space.