About the Subaru Telescope
OHS/CISCO (OH-Airglow Suppressor/ Cooled Infrared Spectrograph and Camera for OHS)
Removing Sky Glow
OHS helps the detection of faint astronomical objects at near-infrared wavelength by removing background light caused by OH molecules in the Earth’s upper atmosphere. It is permanently installed at one of the two Nasmyth foci. It is particularly useful for obtaining low-resolution spectra for studying the stellar or chemical composition of objects such as distant galaxies and brown dwarfs. CISCO is the camera and spectrograph for OHS, but it can also be used without OHS.
►Topics (May 2000) OHS First Light !
Light from the telescope enters OHS through a slit (on the left). A huge dispersion spectrograph spreads the light into many wavelengths (color) and a night line mask removes light corresponding to the wavelengths of OH airglow. The remaining light is recombined and sent to CICSO. Before the removal of OH airglow lines, the sky is very bright in this wavelength range (lower-left panel). OHS can remove the majority of these lines (lower-middle panel), making it possible to obtain spectra of very faint objects (lower-right panel).
S106 is a star forming region approximately 2000 light years from the Earth. There is a large massive star called IRS4 (Infrared Source 4) at the center of S106. The star is approximately one hundred thousand years old, and it is 20 times more massive than the Sun. The hourglass shape of S106 is a result of the way material is flowing outward from the central star. A huge disk of gas and dust surrounding IRS4 produces the construction at the center.
This CISCO near-infrared image of an area of the sky called the Subaru Deep Field (SDF) shows some of the faintest galaxies ever observed, down to a magnitude of 24.5 in K’-band. Models of galaxy evolution predicting haw many galaxies would be messed in deep images suggest that the galaxies in the SDF image account for more than 90% of the total near-infrared light from all the galaxies in the universe along this line of sight.
At the near-infrared wavelength range, the night sky of the Earth is very bright due to emission lines of OH molecules. OHS has a unique capability in that it creates a high dispersion spectrum, removes unwanted light with wavelengths corresponding to OH molecular lines, and then recreates a low-dispersion spectrum. The OH suppression increases the effective sensitivity of the spectrograph by a factor of 2.5.
UV and optical light from distant galaxies are red-shifted into near-infrared wavelengths because of the expansion of the Universe. Near-infrared spectroscopy of distant galaxies has been extremely difficult not only because distant galaxies are faint, but also because the sky is bright in this wavelength range. OHS makes it possible to detect these objects.
The instrument is in constant stand-by at the Infrared-Nasmyth focus of Subaru Telescope, and can be ready for observation anytime. CISCO can be used alone as a near-infrared camera, and was one of the cameras used at First Light.
(From a late 2002 interview with OHS/CISCO support scientist Kentaro Aoki.)