OHS First Light !

May 12, 2000

We feature OHS (the OH-Airglow Suppression Spectrograph) this topic. OHS is designed to work with near-infrared radiation with wavelengths between 1 and 2 microns (0.001-0.002 mm). Due to its size (5.2 m long, 2.8 m wide, and 2.8 m high) and weight (2.5 tons), OHS is installed at one of the Nasmyth foci (the IR Nasmyth focus) which provide stable platforms for large, heavy instruments.

The OHS team in the laboratory with OHS

The OH (hydroxyl) molecule is an abundant chemical in Earth's upper atmosphere. When excited by ultraviolet radiation from the Sun, OH molecules very slowly release radiation that makes the sky glow at near-infrared wavelengths even after the Sun has set. This "airglow" makes it very hard to study faint celestial objects in the near- infrared. OHS works as a filter to suppress the light emitted by OH molecules.

Radiation from OH molecules is emitted at a great many closely spaced but not continuous near-infrared wavelengths. If we could somehow block our view at just those wavelengths where OH is emitting radiation, we could greatly improve the visibility of the faint light from distant celestial objects at all other wavelengths. This is exactly what OHS does. Because the OH molecule is generally not a significant constituent of objects being studied by astronomers, the loss of information at the removed OH wavelengths is quite acceptable.

The OH-suppressed light from OHS is fed into CISCO, which acts as the detector. CISCO was originally attached to the Cassegrain focus where it operated as an ordinary near- infrared imager and spectrograph, much like IRCS. CISCO has been used for many of Subaru's initial observations (see our press releases on the Orion Nebula, Radio Galaxy 3C324, and Subaru Deep Field).

CISCO installed in CIAX3 at the Cassegrain focus (pre-OHS)

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.

OHS installed at the IR Nasmyth focus

OHS saw first light on February 22nd. This picture shows an image of the near-infrared sky seen through OHS. The bright spot near the center of the image is quasar PKS 0859-14. The vertical stripe is the region of sky which has been OH- suppressed and therefore appears much darker than the rest of the image. Observations with OHS proceed by moving the telescope so that the target object falls in this suppressed region and light from the unsuppressed region is blocked using a slit centered on the suppressed region. A grism is then placed in CISCO's light path so that it can obtain spectra of the object free of the contamination produced by OH emission lines. OHS will be especially useful for observing very faint objects such as distant galaxies.

An image of the night sky showing the OH-suppressed (dark) region

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