There is adjustable slit....
My setup goes like this:
Round fiber Ø1mm -> condenser lens (old eyepiece, to form paralell from divergent beam) -> cylindrical lens (part of spectrometer) -> slit.
There are lots of losses in this setup.
I tried to get hold of stacked fiber bundle, O to I... see
here for example.. this would reduce losses and possibly allow eliminating the slit. Still waiting for reply from supplier.
This one is too thick... If I could find a bundle made of 50um fibers, I would try to use that.