Measuring to rotation of Saturn

[Terry Bohlsen]

Dear All
I took some spectra of Saturn a few nights ago with my LISA.
I placed the slit in multiple places over the disc and rings. These included the middle of the disc as a control and from the east and west limbs. I also took images of the outer aspect of the rings.
After processing the spectra using a narrow 4 pixel area in the middle of the spectra there is an obvious shift in the Ha line as well as other features.
I cropped each spectra between 4700 and 6700A to exclude most of the telluric features and the normalised the spectra to create a relatively flat spectra.
Then using the "radial velocity" tool in ISIS I measured the radial velocity difference between the middle of Saturn and each limb.
The results I measured were -21 km/s for the east limb coming in and 15.7 km/s going out for the west limb. The difference may be that I did not have the slit as close to the limb on the western side.
I found a reference giving Saturns equatorial rotational velocity of 37000km/hr or 10.28km/s
My measurements are about double this measurement. Have I made a mistake or is this measurement just beyond the accuracy of the LISA?
Terry

[Martin Dubs]

Hello Terry,

no you did not make an error. The Doppler shift appears twice, once for the movement of the equator versus the sun (this you would see if you were observing from the planet), and once for the movement of the equator versus the observer (earth), so you see a Doppler shift of twice the rotation speed. Notice that for the molecular absorption bands from Saturn you would only see a single shift, but these bands are so broad that you probably cannot measure their shift.
More on this subject you find here:
http://www.shelyak.com/dossier.php?id_dossier=16

Regards,

Martin

[Terry Bohlsen]

Thanks Martin
That makes sense. Its good then that I can measure a 20km/s velocity change with the LISA.
Terry