1999 Nov 2/3: Spectrum of Saturn shows the planet's axial rotation with the slit aligned across the equatorial zone and ring system as a Doppler shift or 'tilt' of the lines in the reflected solar spectrum relative to the 'static' emission lines added at the spectrograph. The tilt in the lines is wavelength dependent - the lines in the near IR [upper spectrum] at ~850nm show a greater tilt compared with the D1/2 Na sodium lines etc. [lower spectrum].
The image above shows the approximate velocities of the planetary disks and ring system in kilometres per second [km/s]. These velocities are effectively doubled across the whole planet with one limb approaching and one receding.
Ideally the slit should run parallel to the equatorial zone. However to satisfy regular stellar observations the spectrometer slit is aligned east/ west. Jupiter currently has a low 3o inclination to the ecliptic but the ecliptic in Aries has the maximum inclination of ~23o. The slit lays across the planet at a similar angle denoted by the purple line and thus the full limb to limb equatorial velocity of 26km/s is not sampled.
Saturn's axial tilt nullifies its current ecliptic inclination in Aries so the slit conveniently lies along the equatorial zone and ring system again shown by the purple line in the top figure. However because Saturn's axial tilt is greater than shown above, the slit lies across higher latitudes and again the full velocity is not sampled. The slit aligned through the poles would show no Doppler Shift.
Segment of Jupiter's spectrum enlarged by a factor x4 shows the gross 'tilt' due to axial rotation of about half a pixel or a Doppler shift ~20km/s.
