The best results will come from matching the focal ratio of the scope to that of the spectroscope, possibly using a focal reducer. In the case of the LISA that is F/5. Then it’s a matter of as much aperture as you can afford. With comets you need to be able to guide on the brightest part of the coma around the nucleus. It’s a bit of a catch 22 situation. Comets are often at their brightest when they are closest to the Earth but they will then be moving at their fastest. This will test your system’s ability to guide on a faint fast moving object. 46P was moving at about 8″/min last night which was not too bad. Even so, my system was struggling to keep in on the slit. You also ideally want a comet with a well condensed coma. 46P isn’t ideal in that respect. Although its total mag is around 9, the light is spread over an area greater than the Moon so it is very diffuse. Nevertheless the centre of 46P is till bright enough to guide on at the moment.