So, there are two versions for this spectrometer :
1 - The microcontroller version, which is cheaper but less efficient than the FPGA version.
2 - The FPGA version which allows to control timings more accuratly. It also has a more efficient ADC.
For the moment I used the optical bench from an old ocean optics spectrometer.
Well, I'm not an optical engineer so, if someone wants to help to build a cheaper optical bench, he is welcome !