Each of the two imaging filter wheels has 9 positions of which 8 are available for mounting filters and one is kept free. Order-sorting filters are also placed into these filter wheels. Both red and blue filters have a free circular diameter of 80 mm and an outside diameter of 85 mm. All filters are permanently mounted in special cells which make replacement very easy. In RILD, filters are mounted in the parallel beam, and the use of smaller filters reduces the aperture.
Adapters have to be requested a few weeks in advance.
More information about EMMI filters is given in Sect. Grisms EMMI has a grism wheel in the red arm only. It has nine positions of which one is kept free for direct imaging and one is used for the focus wedge. Two of the remaining positions are rotated by 90 degrees to be used as cross-dispersers for the echelle gratings. Thus, for low-resolution spectroscopy, five grisms can be mounted for a single night.