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Any other idea are welcome.

Benji

Thanks again to all.
Very interesting spectra Etienne.
One very big difference between Etienne's spectrum and my own is that his is of the sun. If the sun doesn't fully illuminate the slit, nothing will. So I think there may be merit to Benji's suggestion that assuming a gaussian lineshape may underestimate what one can see at low resolution.

I am talking with the great people at Shelyak and I hope to get a (much) more narrow slit. I liked Robin's explanation of how the position of the star within my slit could lead to slight changes in calibration. I definitely want to avoid that.

Hi James,

You could easily test if the higher than expected resolution is due to the star being smaller than the slit by defocusing slightly or dithering the star on the slit (relaxing the guiding). If the star is filling the slit there should be little change in resolution. If the split at NaD and your calibration error disappears then the star was smaller than the slit.

Cheers
Robin