IRCS First Light !

June 16, 2000

May was a busy month at Subaru Telescope, with the resumption of commissioning after repairing the detached fixed point (see our May 12 "Topics" ) and the release of the first Call for Proposals inviting astronomers from Japan and elsewhere to use the Subaru Telescope for their own research.

Two instruments, Suprime-Cam and IRCS, are being offered for Open Use operation, and this month we focus on IRCS, the Infrared Camera and Spectrograph. As its name suggests, IRCS is able to take both images and spectra in the wavelength range of 1-5 microns. It can image at longer wavelengths than OHS, and over a wider field of view than CIAO. It also offers higher resolution spectroscopy than either of these two instruments. Its versatility is expected to make it one of the most popular instruments on Subaru Telescope.

IRCS mounted on the simulator at the Subaru Telescope base facility in Hilo, Hawaii.

IRCS saw first light on the night of 25 February, mounted at the Cassegrain focus of the telescope for a total of 6 nights. Since then, the data gathered has been analyzed to see where adjustments need to be made.

IRCS mounted at the Cassegrain focus with support scientist Hiroshi Terada in the foreground.

Although most of the observations taken by IRCS earlier this year were intended to test the instrument's performance, IRCS has already obtained important scientific results from its observations of the gamma ray burst GRB 000301C.

Many of the most interesting places in the Universe, such as regions where stars are forming or the centres of galaxies, are hidden behind large amounts of dust. They can only be seen in detail at wavelengths longer than 2.5 microns, where the dust is less efficient at absorbing light. IRCS is designed to work efficiently at these long wavelengths, which are known as the "thermal infrared". Objects at the ambient temperature (e.g., the telescope) emit large amounts of heat radiation at these wavelengths.

IRCS image of the Orion KL nebula. This is a three-colour composite composed of J, K, and L' images. Note the red objects which are heavily obscured by dust and can only be seen at the longest wavelength.

When IRCS is used in its spectroscopy mode, it breaks infrared light up into its component infrared "colors". One of the important things we can do with spectroscopy is measure the velocity of objects out in space. CIAO and OHS, which use dispersing elements called "grisms", can only provide a course measure of the motions of objects, unable to sense differences smaller than a few hundred kilometres per second. IRCS also contains grisms to perform course observations; but in addition, it contains an "echelle", which can disperse light much more finely. IRCS can detect differences in relative motions as small as 15 km/s, suitable for detailed studies of the gases within objects ranging from young stars to entire galaxies. [For comparison, note that objects at the equator of the Earth are travelling at 0.46 km/s (1037 mph) due to the Earth's rotation and the Earth is moving at about 30 km/s due to its orbital motion about the Sun.]

IRCS echelle spectrum of a region in the Orion nebula. The emission lines come from warm gas heated by the young stars. Note how some of these lines are separated into more than one component.

IRCS will be mounted on the telescope several more times during the remainder of this year for further testing, including some using the Adaptive Optics unit which arrived in Hilo last month. Among other projects, the IRCS team hopes to use their instrument to search for brown dwarfs (''failed stars'' with masses 10-100 times that of Jupiter), investigate chemical reactions around stars, and study the dynamics of distant galaxies.

After initial testing in Japan, OHS was dismantled and transported to Hawaii. It was reassembled at the IR Nasmyth focus of Subaru Telescope and underwent further tests before receiving first light.

The IRCS team in the control room of Subaru Telescope. Standing (from left) Mark Weber, Hiroshi Terada, Alan Tokunaga, Bob Potter, Miwa Goto. Sitting: Naoto Kobayashi.

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