Note though that the instrument response for a slit spectrograph is only smooth provided a flat has been used otherwise it will have ripples from the camera response.(A good reason for always using a flat). Even then I find that my ALPY instrument response has a kink in it which I believe is quite common. (a combination of the flat lamp and atmospheric extinction curves) You can see it in my MILES star report here for example.
from this thread
The acid test to see if your instrument response is correct is to rerun the reduction on the reference star using the instrument response. The result (except for any resolution differences) should perfectly match the reference spectrum (other than the telluric bands)