The calibration polarizers are mounted in one of the A&G main filter slides. This severely limits their use in combination with pre-slit filters; however, post-slit filters (if and when available) are preferable anyway, as they do not contribute to instrumental polarization. The present calibrators are: 2 UV polaroids, a calcite slab and a (plastic, scratched, fixed by sticky tape) circular polarizer. The polaroids can be preset to any orientation you wish, but once the filter slide has been inserted into the A&G, they are fixed. The most useful orientation is that corresponding to the 2 beams of the calcite slab (see the discussion on flat fields); this is the default adjustment and instructions for retrieving this orientation are available on site.
In the long run, it would be better to calibrate at astronomical levels of polarization (of order 5 %). A suitable component would be a pair of oppositely tilted silica plates, as in the Multi-Purpose Fotometer MPF. Space for this is available (and has been claimed) upstream of the A&G. Equally desirable would be a proper achromatic circular polarizer. Future implementation of such components will depend a good deal on pressure from the community.