Lars, I don't understand if you refer to telluric sodium that, being an emission source over the entire field, is undetectable with slitless spectroscope like SA100.I don't think there's any evidence of the Na emission, but maybe the Sun is to low at that time?
Another topic we are discussing on Staranalyser Y group is the sodium tail image, well visible in Rob Kaufman SA100 spectrum (from Australia) but not appreciable in Lars spectra. For this I suppose is due to much larger comet image and a decreased sodium emission. Concerning the last aspect, Robin suggests to measure EW that I find a -50 value on my Lhires III spectrum of 16 march, normalizing on continuum around peak. I don't know if it's correct, should I normalize on the sky background?
Ciao
Paolo