Both GMOSs have a number of 'permanent' masks containing long slits suitable for long-slit spectroscopy. The long slits have various widths ranging from 0.25 arcsec to 5.0 arcsec. The slits have small bridges, approximately 3 arcsec long, to keep the mask flat in the focal plane (see the image in the table below).

Long-slits for both standard and Nod-and-Shuffle observations are also available. The Nod-and-Shuffle slits have lengths of 108 arcsec centered in the field of view. The Nod-and-Shuffle long-slits are available in slit widths of 0.5, 0.75, 1.0, 1.5 and 2.0 arcsec.

If users require slit widths other than the permanent ones listed below, or long-slit masks with holes for alignment objects, such requests will be handled as multi-object spectroscopy and the user should request to use GMOS in MOS mode. The user will have to prepare the masks using the usual mask-making procedure. A total of 18 masks (plus the IFU) can be loaded into GMOS at any given time.