Hi again Robin,
Thanks for querying the response correction. There was indeed a very shallow dip in my response correction curve around 5000Å. So I revisited my response correction by applying the continuum response shaper in BASS from a more ‘distant’ perspective. I would normally smooth the response correction curve by zooming in on 2 sections, from 3600-5500Å and then continuing on to 5500-7700Å. In this second attempt I smoothed the curve in one view from 3600-7700Å. The shallow dip around 5000Å was much less visible in this revisited curve, although there is still an ‘undulation’ in the resulting X Per response corrected spectrum, so it would be interesting to compare with another low res spectrum on the BAA database if one appears. I always review the response corrected reference star spectrum against the library reference for obvious discrepancies but I should have questioned more the bump around the HeI emission. I have replaced my spectrum in the database with this newer v2.
Response correction of low res spectra can be a bit of an art form, especially at the near UV end where it can be like grasping the tail of a thrashing snake!
Thanks as ever for your guidance