FOCAS field of view
The field of view of FOCAS is that of the unvignetted Cassegrain focus of Subaru Telescope, i.e., 6.0 arcminutes in diameter. There is an unexposed gap 5'' in width between the two CCDs which runs east-west when the position angle is zero, as indicated in this figure.
The FOCAS longslit runs perpendicular to the long axes of the CCDs. Slit at offset position may be useful to obtain spectra over wider spectral range without losing red-most part of the spctrra when one uses, e.g., 300B grating (1st order) + Y47 order sorting filter and 300R grating (2nd order) + L600 order sorting filter.