Hi all! François, just a test concerning the slit pointing problem. At lower resolution (my current spectra are R ~4200) it's not very useful, anyway I tried to observe the R Aqr region at some declination values with the slit E-W oriented. As expected, there aren't appreciable shifts close to the central star (I should try again with the 2400 l/mm grating).
By increasing the distance from the star, something seems to be measurable.
That is especially the case with the observation at DEC -25 arcsec and DEC + 5 arcsec respect to the star declination. I mean, the [O III] 5007 line appears redshifted in the south region and blueshifted in the north. I think this arises from the NE-SW oriented bipolar jet. The observing conditions was not optimal (target low on the horizon, some turbulence, short time available), so take it with caution.
The next night I tried another experience, taking the 2d spectrum of entire bright emitting region around R-Aqr. To do that I oriented the large slit (300 micron) at position angle 15 degree and put the target inside. With this angle most of the bright jet is included. The resulting 2d image shows the extended nebula above and under the star spectrum at the prominent emissions wavelengths.
The position of the slit superimposed to the optical image by Adam Block (Mt. Lemmon SkyCenter/U. Arizona).
We can roughly discern the morphology of the bright part, also the slightly different shape of the hydrogen nebula respect to the oxygen one. The stretched negative image shows fainter nebular regions.
If the doppler shift is somewhere appreciable in the 2d spectrum (the geometry is here corrected), it should be interesting to compare the nebula image with a classic image taken, for example, through a narrowband OIII filter. We may see what is receding and approaching...
Sorry for the digression!
Paolo