• FOCAS/MOS users are asked to describe explicitly about the preimaging observation with FOCAS in their proposal (Entry 16 'Instrument Requirements') when it is necessary for their MOS mask design. Please check the MOS page for details.


The Faint Object Camera and Spectrograph (FOCAS) is a versatile optical imaging and spectroscopy unit which can be installed at the Cassegrain focus of Subaru Telescope. FOCAS uses refracting optics for both the collimator and camera and uses grisms to achieve high throughput optimized for the wavelength range of 3700-10000 A. It has four operating modes: imaging, spectroscopy (long slit and multi-slit), polarimetry, and spectropolarimetry, and covers a circular 6' field of view. The atmospheric dispersion corrector (ADC) installed in the telescope Cassegrain unit will be used in most FOCAS observation.

Changing filters, grisms, slits, or wave-plate angle, or switching from imaging to spectroscopy or polarimetry mode takes 1-2 minutes per exchange.

Due to mechanical limitation of the slit changing system, changing slits at lower elevation angle (EL < 30 deg.) is prohibited and, therefore, target acquisition on long-slits or MOS masks is only available at EL > 30 deg. Note that imaging (without changing slits) can be performed at even lower EL.

Click here for a copy of the SPIE2000 paper FOCAS: Faint Object Camera and Spectrograph for the Subaru Telescope by Kashikawa et al. (0.8Mb gzipped postscript).

Click here for a copy of the SPIE2000 paper Software Structure and Its Performance on FOCAS Instrument Control, a MOS Design, and an Analyzing Package by Yoshida et al. (0.4Mb gzipped postscript).

When referring to FOCAS in a paper, please use the following citation.


FOCAS has a camera section.


FOCAS can perform moderate-resolution spectroscopy, including multi-object spectroscopy.


FOCAS can perform polarimetry. Both linear and circular polarimetry for longslit spectroscopy and imaging are available. It is noted that the polarimetry with 1200_950nm VPH grism is not available because of 1% instrumental polarization and the "ripple" pattern of 20 Å width and 1% amplitude.

Other observing mode

Further information

Questions regarding FOCAS should be directed to Chien-Hsiu Lee (

30 January 2014

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