Switch OFF power to the Camac crate and the polarimeter head
Pull the chopper to the outer position using the black knob in the upper part of the polarimeter front panel (where the ocular tube is). Do not rotate the knob to avoid need of major adjustment of the chopper position afterwards.
You will now notice that there is a black box that blocks the lower part of the rear side of the panel. It is fixed with some plastic wires, unplug it and remove the box.
Disconnect the cable connector to the lower half of the polarimeter head rear side. Unscrew the bolts and slide out the lower half unit. If necessary, loosen slightly the bolts of the upper half, too, to reduce friction between the two halves. IMPORTANT: do not tilt the unit much while pulling it out, to avoid contact with the delicate parts in the upper unit.
Remove the fixing rings from the free openings in the filter wheel by lifting them (fingertip should work), put the new filters in the holes, and fix the filters with the rings.
NB: It seems that there are no fixing rings left, a reasonable workaround seems to be to use a part of a spring for fixing the filters. The holes are about 25mm in diameter. It might be a good idea to paint the spring black with a marker as otherwise clear reflections from the spring can be seen. This might affect polarimetry!!! So take some minutes for painting.
Put back the unit to the polarimeter head, avoiding contact to the upper part while sliding the unit in. Fix the screws and the cable connector.
Reattach and plug in the little black box.
Push the chopper back to the inner position (important!)