Is "monochromatic source" different than "monochromator" in astronomical instrumentation?
"Monochromatic source" is a very general term for any source of radiation of any type that emits a very narrow range of energies. Generally electromagnetic and traditionally optical (vis, near-IR or near-UV), though it can apply from gamma rays all the way to radio.
It could refer to an instrument on Earth (as in your application) or a source of monochromatic radiation in space that one is observing (not your application).
Bottom line: In your context, a "monochromatic source" is a source of radiation.
A "monochromator" on the other hand is a processor of light. It is a subtractive element, which means it passes a narrow band of energies and rejects (reflects or absorbs) the rest.
Generally what comes to mind is a scanning spectrometer - a device with input and output slits, and a dispersive element (usually a diffraction grating or possibly a prism) and one or more collimating lenses to re-image the input slit at the output slit.
But it could be a Fabry-Perot etalon or even a passive narrow-band filter.
Bottom line: In your context, a "monochromator" is a processor of radiation from another source. And that source could be astronomical or a laboratory instrument, but it would be discussed separately