Monochromator in fluorescence microscopy I'd like to ask why are there no fluorescence microscopes that use a monochromator instead of filters? If there are any, then I should change my question as, which ones use it and why are they so rare?

Zeiss LSM780 uses essentially monochromator to spread all fluorescence across 34 detectors for multi-spectral acquisition. It is used to detect autofluorescence, unmix closely positioned emission spectra (unmix GFP and YFP for example) and to measure precise spectral peaks with ~1nm resolution.

Monochromator is useful when you need to transform frequency domain information into spatial domain. In fluorescence microscopy we usually collect light from wide bands of well-separated GFP-like proteins, so that filters make more sense. We usually don't detect 1-nm wide lines or plot spectra. LSM780 does allow all that, if you need to.

You can use spectral imaging, but for that you will have to either scan for very long time, or spread signal onto array of detectors.