HDS First Light !

August 11, 2000

We are continuing the adjustment of Subaru Telescope and observational instruments in preparation for the Open Use which will begin at the end of the year. We will continue to release new results obtained during test observations.

We feature HDS (High Dispersion Spectrograph) this topic. HDS observes objects in visible light (300 - 1,000 nanometers) and splits their light into 100,000 separate colors using an "echelle spectrograph."

HDS is a huge instrument, measuring about 6 x 6 x 3 meters and weighing 6 tons. Therefore, it is mounted at a Nasmyth focus, which can provide the necessary room and stability. There are two Nasmyth foci, one on each side of the Subaru Telescope. OHS (CISCO) is mounted at the other Nasmyth focus.

Adjusting HDS in Japan

After adjustment in Japan, HDS was dismantled and transported to the Nasmyth focus of the Subaru Telescope in March. We usually perform additional tests of new instruments in the simulator lab at the base facility before taking them to the summit of Mauna Kea but the size of HDS made this impossible. It took about three weeks to reassemble HDS, and then the final adjustments were made for first light.

With the instrument group looking on, HDS achieved its first light on July 1. The following image shows what the observers at the summit saw on the computer screen at the moment of first light. Test observations continued for a week, with the instrument's operation and performance being monitored.

A computer display shows the moment of first light.

On July 5, HDS made observations of topical Comet LINEAR. This image was taken with the slit viewer camera, which allows the observer to see which region of the object will have its light dispersed.

Image of comet LINEAR

The first dispersed spectrum of Comet LINEAR taken with HDS is shown here. Because HDS observes such a wide wavelength region and disperses it into so many individual colors, it is necessary to fold the spectrum into many rows otherwise a very large number of detectors would be required. This is done by means of a "cross-disperser" (one is also installed in IRCS).

Cross-dispersed spectrum of comet LINEAR with HDS

The horizontal stripes mean that the comet emits light at all visible wavelengths; this is because it reflects light from the Sun. The vertical lines show emission from particular atoms and molecules in the comet; the chemical composition of the cometary material can be determined from the wavelengths of these lines. A small region of the spectrum (indicated by the black box in the above figure) is shown below. The emission features indicate that NH2 and C2 are abundant molecules in Comet LINEAR.

Spectrum of comet LINEAR with HDS

HDS will be used to study the chemical compositions of old stars in the Galaxy and distant clouds of gas to understand the chemical evolution history of the Universe. In addition, HDS' high precision will allow the detection of planets around other stars from the "wobble" they produce in the star.

HDS group and Subaru operator (from left: Tom Winegar, Hiroyasu Ando (front row), George Kosugi, Masahide Hidai, Kunio Noguchi (front row), Kozo Sadakane, Satoshi Kawanomoto, Satoshi Honda, Bunei Sato, Wako Aoki)

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