The following figure is taken from the article by Mohr et al¹, and shows the schematic of the DIY Raman spectrometer:
A laser (LP) illuminates a sample (S) through a microscope objective (MO), which also collects the Raman scattered radiation. The authors of the original article used a standard 20x objective with a price tag of about 200usd.
So maybe you don’t need a super high quality microscope objective for this application, but I really don’t think I have to justify my fetich for optics, so I’ll just go ahead and tell you what you’re looking at – or rather what you’re looking through:
This microscope objective was taken from a GAIIx DNA-sequencer, which employs a similar setup as Mohr’s Raman spectrometer, but of course recording fluorescence and not Raman. To the best of my knowledge it’s an un-branded Nikon CFI Plan Apo VC 20X Air 0.75 NA and I got it from *b@y for only slighty more than a new standard objective would have cost me.
Of course it’s not all about flashy letters and numbers there are significant benefits. The numerical aperture affects the S/N-ratio by a power of 4 (if I remember correctly, I can’t find the reference). The objective is infinity corrected ..it really shouldn’t matter, but it means I can take pictures like the fly-one above and in theory I don’t have to care about the exact distance from the objective to the slit – but I will need a focusing lens.
Anyway I figured that if it’s good enough for 250k$ DNA-sequencer it’s good enough for me.
¹ J. Chem. Educ., 2010, 87 (3), pp 326–330