(High Dispersion Spectrograph)
(High Dispersion Spectrograph)
The Universe in 100,000 Colors
HDS is an optical spectrograph with a spectral resolution of up to 100,000. It can be used to understand the chemical evolution of the Universe through the study of the chemical composition of old stars, and the study of the physical state and chemical composition of intergalactic gas using quasar absorption lines. It weights 6 tons and is permanently installed at one of the two Nasmyth foci.
Where are comets born?
This is an image of Comet LINEAR C/1999 S4 taken by the near-infrared camera CISCO. HDS spectra from this comet reveal the distance between the Sun and the comet when the comet first formed.
Raw data : A part of raw HDS data of comet LINEAR. Light dispersed into many wavelengths (or colors) extends horizontally in this image. The spectrum is "folded" onto the detector array to obtain data over a wide wavelength range efficiently.Each horizontal streak is light from a slightly different wavelength from a single object. Light of longer wavelength (red for human eyes) is at the upper-right side, and that of shorter wavelength (blue) is at the lower-left side. Some bright spots in this image correspond to NH2 emission lines.
Spectrum : Extracting useful information from raw data is called data reduction. Raw HDS data is reduced to a spectrum which shows the strength of light at each wavelength (color). This figure shows the optical spectrum of Comet LINEAR C/1999 S4 obtained with HDS. the strong lines found in this spectrum are emitted by NH2 molecules. For comparison, a computer calculated model spectrum of NH2 molecules is shown in the bottom panel. the match between the spectra shows that the comet formed at a temperature of -245 degrees Celsius which in turn suggests that it formed between orbits of Saturn and Uranus.
The upper panel is an optical spectrum of the quasar HS 1603+3820 obtained with HDS. The spectrum shows absorption features by foreground inter-galactic gas. The lower panel shows an optical spectrum of much lower resolution obtained at another telescope. Broad absorption features in the lower panel are clearly resolved into many narrow features in the HDS spectrum. This structure is caused by slight velocity differences in the absorbing gas, whose detailed study is made possible by HDS.
Nucleosynthesis by supernovae
HDS spectra of two different stars. Absorption features by iron (Fe) and magnesium (Mg) are seen. Since the total amount of heavy elements, such as iron, in the Universe increases gradually through nuclear fusion inside stars, very old stars have only a small amount of heavy elements. These two stars were probably born when the Universe was very young. While the absorption strength of iron is similar in both spectra, that of magnesium is much stronger in the upper spectrum. Based on the analysis of these absorption features, we can obtain information on the formation and dispersion mechanism of elements in the early Universe.
HDS is a spectrograph that can divide optical light into 100,000 colors and observe them simultaneously. This is about 100 times higher in resolution than the typical astronomical spectrograph. It weights 6 tons and is the largest of all instruments of Subaru. Dividing the light into 100,000 colors requires extreme precision. If there is any sift in where the light falls on the detector, we cannot tell which color that light was. For mechanical and thermal stability, HDS remains stationary in a refrigerated container at one of the Nasmyth foci. Since HDS is best suited for the study of objects that are brighter than those observed with with other instruments in Subaru.
HDS is well suited for studying the chemical composition of stars, and the Doppler sifts, or the movement toward or away from us, of various astronomical object. As a planet orbits around a star, its gravitational pull makes the star wobble very slightly. HDS can detect this small wobble.
(From a late 2002 interview with support astronomer Akito Tajitsu.)