Like Paolo Beradi did last year for example on L4 Panstarrs
http://www.spectro-aras.com/forum/vi....php?f=6&t=567
You can still guide on the coma, offset from the slit on the tail, either using a reflective slit if the field is wide enough or alternatively using an off axis guider or separate guide scope.
Using a slit can sometimes have its downsides though as seen here when trying to distinguish between atmospheric Na emission and that from the comet.
Robin