spectro-aras.com

And this is my first spectrum of Saturn!
C11 + Lhires III, 2400 l/mm, 15 um slit + KAF-403
I averaged the shifts of 6 lines, measuring between 1.3 and 1.4 Angstrom shifts. This suggests to a shift of 62.3 km/s (maybe I should have just measured the H-alpha line?)
Since Saturn is very close to opposition (little more than 1º), and it is turning around an axis tilted 21.9º to the viewing angle and the illumination, is it safe to divide 1/4 of this shift by cos(21.9) squared ?
If so, can I roughly guess the rings turn at 62.3/4/cos(21.9)^2 = 18 km/s ?
The correct value is between 16.3 and 23.6 km/s depending on distance, so I suspect the result is OK..

(You need to click on the second image to see the 2D spectrum.)

Congratulation Filipe, very nice observation and didactic presentation!
The spectrum show very clearly the different Doppler shift of various surfaces. I think you had a good seeing (and mount!) during observation. You should be able also to measure the different velocity along rings width (distance from planet). On your excellent spectrum, spectral lines along rings section should be slightly tilted. I think it is better visible if you tilt and slant precisely the spectrum.

I like this old diagram (Keeler 1895) that shows doppler shift of spectral lines due to rings particles moving around the planet in accordance with Kepler's third law:


(from: http://www.gutenberg.org/files/27378/27 ... 7378-h.htm)

It seems to me that for simplified formula (v/4), you should divide by cos(21.9) not squared. Else you can use the more precise formula below whereby results should be very similar because, as you pointed out, Saturn was almost in opposition.

With angles (from ephemeris):

dec_e (Earth declination on Saturn, tilt angle you found equal to 21.9 degree)
dec_s (Sun declination on Saturn)
delta_es (angle between Sun and Earth on Saturn)



Where total velocity in your case is 62.3 Km/s.
Note that if all angle values are zero (perfect opposition, edge-on rings), v = total velocity / 4.

See you soon!
Paolo