Finding the best Focus at Cassegrain

At Cassegrain, the telescope is focused by eye, using an unsaturated stellar image on the TV. Run through focus a few times, noting the value for the smallest image each time and averaging the results. Further:

• Check that the TV camera itself is focused on the slit (see section ).
• The telescope focus is altered by the insertion of an ABOVE SLIT filter. For the neutral density filters currently in use with FOS or IDS, the focus is increased by 0.04 mm when the filter is put in. Correct the focus to the by entering the new value (after changing to a filter) via the User Interface.