Instrument focus

A long standing issue is that when using the filters in ALFOSC it has been the practise to use the telescope focus to correct for the change in focus, while in fact it is the instrument focus that should be changed. In principle this is not a big effect and not a problem for normal observing, but when changing from imaging polarimetry to spectro-polarimetry the internal focus is different as the slit is on top of a calcite and positioned somewhat higher than the normal slits. To (also) allow for differences in instrument focus to be applied for different instrument set-ups, this should be added to the optical elements data base. As a first step the plan is to just add the option for each elements to define the instrument focus, where for most elements this will be simply set to zero, maintaining the current situation. A longer-term objective is to define for each optical the correct settings for the telescope and instrument focus and implement these values.