I made a spectroscope some time back, just a toy really:
At the other end, a cheap 500 l/mm diffraction grating which splits the light from the slit into a spectrum.
Inside, a collimating lens takes the rays from the slit and makes them more-or-less parallel on their way to the grating.
Here's a sample pic (halogen lamp):
AF is out of the question.
Twiddling the MF ring produces no obvious "in-focus" point, so I wonder what the correct setting would be in theory?