For imaging, focusing either the instrument or the instrument will suffice to obtain focussed exposures, whereas for spectroscopy both the telescope and instrument need to be focussed on the slit. Both the long slit and the starplate wheel are mounted in such a way that the focus position of the cameras coincide for all modes, making it unnecessary to refocus when switching modes. The focus of the telescope can be determined using either a through-focus exposure sequence, or the focus wedge. The latter is the fastest method, and can be used for any of the EMMI modes. It is better than a through focus, especially when atmospheric conditions are bad and changing rapidly.
Most filters require a correction to be applied to the instrument focus.
The R-filter is used as reference for the focus in RILD and the focus offsets for other filters are entered in a database. The instrument focus is automatically adjusted when changing filters. Focus offsets for the blue filters are also in the database and the focus will also be updated accordingly when switching filters. Table 4.2 lists the temperature change inside EMMI at which an image degradation of 0.1'' occurs. The temperature of the EMMI room is kept approximately constant by the air-conditioning system.