If required, change the gratings/filters in WYFFOS to suit the observing program. Note that at present (May 1996), the slit A filter doesn't intercept the whole beam (it misses the last third of the staggered fibres in the slit unit), so if you want to be sure use only the B filter. Filters can be changed by pulling out an existing filter and sliding in a new one until locked. Filters can be found in a cardboard box file in the WYFFOS cupboard. **Here** is a description of the filters available at present. The filters are especially useful with the present calibration lamp setup, where some of the lamps have a lot of red light that can be blocked with e.g. a BG39 filter. When a filter is changed, you have to update this using the ICL wchange command; just follow the prompts.