What are the purposes of slits inside a spectrometer (Czerny-Turner)? 
I am building a Raman spectrometer with a Czerny Turner configuration (with RamanPi).


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*Why does the light source (A) appear to be converging into the entrance slit (B)? When a sample is excited by a laser, the Raman scattering from the sample should be diverging away from the sample. Is a converging lens typically placed in front of B to focus the light at the slit?   

*In another configuration involving a linear CCD, the exit slit is replaced by the CCD. What is the purpose of the exit slit then (F)?



(source: bwtek.com) 
 A: *

*You should use external optics with a monochromator. Outside the setup you should have a lens that focuses collimated the light on to slit A. This is the reason for the converging beams. Note that, for maximum throughout you should choose a focal length focusing lens that matches the numerical aperture of the monochromator.


*With different wavelengths now dispersed across the exit slit, the width of the slits controls the bandwidth of the "monochromated" light. By slightly rotating the grating different wavelengths will pass through the narrow slits. With that background we can now discuss your second point.
It is usual to have a photodetector at the exit slit. This allows you to convert the light intensity for a specific wavelength to an electrical signal. However an alternative configuration is to have a CCD array at the exit. This allows one shot detection of a whole spectrum because the intensity of light falling across the surface of the array can be mapped backwards to spectrum using an appropriate transfer function.
