ARCoIRIS features a fixed slit assembly of width and length 1.1-arcsecs and 28-arcsecs, respectively. The primary longslit assembly is accompanied by two flanking outriggers, which are 1.1-arcsec square slit boxes ~4-arcseconds offset from the main slit aperture. These outriggers can be used for better defining a spatial-spectral wavelength calibration (in the X-Y plane), thereby allowing a better interpolation of the spectrum background.

The lower schematic shows the dimensions of the as-designed slit assembly of ARCoIRIS sent-out for manufacture, with the two flanking square-pitch outriggers clearly visible. The upper schematic shows the actual as-fabricated dimensions of the assembly for the final-installed slit plate.