Hello there, I am trying to improve my understanding regarding spectrometers bandpass, resolution etc. For that, I went through the Ridchardson grating handbook and on Horiba’s website where they have a nice explanation (http://www.horiba.com/us/en/scientific/products/optics-tutorial/monochromators-spectrographs/). I will use this link for reference to explain my issue. In sec. 2.12.4 Determination of the FWHM of the Instrumental Profile, they give an approximation to calculate the FWHM (eq. 2-20) which is a function of -The limiting resolution dλ(resolution) -The bandpass dλ(slits) -The natural line width dλ(line) For the bandpass dλ(slits) I recalculated it from scratch to obtain the eq. 2-21 and I would think that going through the calculations and reading the definition made me understand what it means. However, I am a bit confused by what they mean by the limiting resolution dλ(resolution). I went through sec. 2.12 Bandpass and Resolution and have a hard time understanding the difference between both terms. But then, I read again sec. 2.12.4 where they write “Bandpass (BP) = FWHM”. Thus, is the Bandpass (BP) ≠ bandpass dλ(slits)? Or just that since they write “In general, most spectrometers are not routinely used at the limit of their resolution so the influence of the slits may dominate the line profile.”, it means that dλ(slits) ≫ dλ(resolution) and dλ(slits) ≫ dλ(line) so in the end, their FWHM = BP = dλ(slits)? So I am a bit puzzled there and would appreciate any hints. Furthermore, if someone has a nice book reference on all that, I would be happy to know it to check if they have it in our library.