The PFIP filter wheel has seven positions, into each of these positions
a lobe which can hold a 125mm diameter filter is mounted. Adaptors are
available for smaller filters. Currently changing filters in the
PFIP filter wheel is a fairly difficult and time consuming procedure, and it
is not recommended to be carried out during the night. The telescope
must be positioned at the Access Park (AP3) position with the access gates
open. Only experienced technical staff, including Telescope Operators,
should attempt to position the telescope there. The Prime Focus Rotator
should be positioned at a mount position angle of 90 
. It is
only possible to access filters opposite the main beam position. If only
one filter is to be changed, it is necessary to put a filter diametrically
opposed to this into the beam, for instance if you want to change the filter
in position 7, then put the filter wheel into position 3 or 4. If a number
of filters are to be changed, it is easier to withdraw the detent pin, when
the filter wheel can be rotated by hand. To do this, select a negative
filter position, in the range -1 to -7, from the PFIP engineering terminal.
The cover
for the filter wheel is on the left hand side of the instrument platform at
mount position angle 90 ; this can be removed by unscrewing by
hand the six large bolts. Four
cables pass through this filter wheel cover, these need to be disconnected
carefully at the LIMPIT controller. The connectors are keyed LIMO connectors,
and can only be replaced in the correct sockets. The cover can now be removed,
passing the cables and connectors through the hole provided. Once the cover
has been removed the filter wheel cannot be driven either from ICL or
the engineering controller, even if these cables are reconnected.
With the filter wheel cover removed, eight 4mm allen screws which clamp the filter wheel slide can be slackened off (not removed). After this has been done, a spring loaded pin which holds the filter slide in can be lifted by hand, and the filter wheel retracted. When it is withdrawn by about 15cm, it is possible to access the two outermost lobes.
Filters within the lobes can be changed by taking the cover off the lobe with a small flat screwdriver. When handling 125mm filters it is important to wear gloves to avoid fingermarks on the filters. Alternatively the whole lobe can be changed using a 4mm allen key.
When mounting filters it is important to keep the filter wheel in balance. The adaptors for the smaller filters are heavier than the 125mm filters, and if two of these are to be mounted it is advisible to mount them on opposite sides of the filter wheel. Similarly if two positions are to be left empty, these should be on opposite sides of the wheel.
The filter wheel should then be slid back into the assembly, ensuring that the spring loaded pin locates correctly, and the eight allen screws holding the wheel against the slides should then be tightened. The cover can then be replaced, passing the cables through the hole, and the six large bolts inserted. Finally the LIMPIT should be recabled.
After changing filters the wheel should be initialised, by typing PFILT INIT at the ICL interface, or issuing MFW102 from the engineering controller. The ICL procedure PCHANGE must be run to update filter names focus offsets. Once this is done, typing PSETFOCUS ENABLE will enable automatic filter focus offsets during later observing. During observing, filters in the PFIP main filter wheel may be changed by typing PFILTER <filtername>, where <filtername> is one of the available filters.