Instrument focus

In principle, filters mounted in ALFOSC require different instrumental focus values, but we commonly use the telescope focus to correct for this. To (also) allow for differences in instrument focus to be applied for different instrument set-ups, this parameters has been added to the optical elements data base. We need to measure the proper instrument focus for each filter such that we can start to apply the correct focus offsets to the instrument (and not only to the telescope focus).