FMOS InfraRed Spectrographs

The 400 fibers at the prime focus are optically linked with the two InfraRed Spectrographs (IRS1 and IRS2) on the floor two levels above the Nasmyth platform., each of which makes spectra of 200 fibers. The spectral coverage of the spectrograph is from 0.9 to 1.8 microns with an OH airglow suppression mechanism. The spectrographs are operated with cooled down to -50 ~ -60 degC, to reduce the thermal radiation.

The layout of the optics and light path are shown in Fig. 1: The divergent beam from the fiber slit is collimated by the 1.4m-diameter main mirror, is dispersed by the primary grating (4 pieces are mosaiced), and high resolution (R=2200) spectra are formed by the main mirror onto the curved mirror with masks for OH airglow supression. These spectra are re-collimated by the main mirror, go to the camera section and are re-imaged on the detector. The spectral coverage in this high resolution (HR) mode is ~0.2um between 0.9um and 1.8um. By moving the camera dewar, observers can select one of the four bands ("J-short", "J-long", "H-short", and "H-long").

In the Low Resolution (LR) mode, the Volume Phase Holographic (VPH) grating is inserted at the quasi-pupil position in front of the camera. This VPH grating then works as an anti-dispersing element and low resolution spectra from 0.9um to 1.8um are formed on the detector.

Fig. 1 - (Left) Optics layout and light path in the FMOS spectrograph. (Right) Same as left, but the light path is shown by animation.

Below are some spectra of lamps having been taken in engineering works:

Ar lines in HR spectra. The FWHM is ~4-5 pix as designed.
Halogen lamp spectra in the HR mode (H-short). Black parts running vertically are OH masks.
Same as above, but a part is magnified.
Halogen lamp spectra in the LR mode. The gap in the middle is due to the fiber slit.

Readers can also refer to the papers as listed below and references therein for more details of this system:

"The Fibre Multi-Object Spectrograph (FMOS) for Subaru Telescope": Kimura, M., et al. 2010, Publication of The Astronomical Sciety of Japan, Vol. 62, 1135.
"FMOS: the fiber multiple-object spectrograph: Part VI. Onboard performances and results of the engineering observations": Iwamuro, F. et al. 2008, Proc. of SPIE, Vol. 7014, 26
"Integration, commissioning, and performance of the UK FMOS spectrograph": Dalton, G., et al. 2008, Proc. of SPIE, Vol. 7014, 127

Last updated: January 31, 2012