The Cambridge Infrared Panoramic Survey Spectrograph is a near-infrared spectrograph with an integral field unit capability which is being built at the Institute of Astronomy in Cambridge, UK. CIRPASS will have a 499 element integral field unit to collect the light from the target object. The light from each lenslet in the IFU then travels down an optical fibre to the spectrograph. At the entrance to the spectrograph the fibres are re-arranged into a linear configuration (the fibre slit) and the light from each fibre is dispersed by a grating. At this point CIRPASS contains the option to block out, or supress, the bright OH sky lines by reflecting the dispersed light of a mask which is non-reflective at the wavelengths of the OH lines. A camera then images the masked spectrum onto a near-infrared detector.