• 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).


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.

Integral Field Unit

FOCAS has an integral field unit (IFU) which can be used in combination with the grisms described in the above "Spectroscopy" section to perform integral field spectroscopy.

Details in IFU mode page.

Instrument Citation

Please cite the following paper when you refer FOCAS in your publication.

"FOCAS: The Faint Object Camera and Spectrograph for the Subaru Telescope"
Kashikawa, N., et al. 2002
Publications of the Astronomical Society of Japan, Vol.54, 819

For specific devices, please consider to refer to the following papers.

grism :
"Grisms Developed for FOCAS"
Ebizuka, N. et al. 2011
Publications of the Astronomical Society of Japan, Vol.63, SP2, pp.613--622.
polarimetry :
"Properties of FOCAS Optical Components"
Kawabata, K. S. et al. 2003
Proceedings of the SPIE, Vol. 4841, pp. 1219-1228
"Integral field unit for the existing imaging and spectroscopy instrument, FOCAS"
Ozaki, S. et al. 2020
Publications of the Astronomical Society of Japan, Vol.72, 97

Further information

Questions regarding FOCAS should be directed to Kentaro Aoki (

22 July 2022

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