Lucky lucky lucky.
Firstly, kudos to kevinbreen who gave me a heads up for good seeing which tempted me to attempt Saturn in the very low dawn sky. Its still very badly placed in the UK, but the jetstream gods were truly onside and the seeing was remarkable for ~23 degrees altitude. I used Baader R and 610 filters and QHY5III200m, 12" newtonian.
Just happened to check the moon positions in CdC the night before (hoping against hope for a Titan transit) and noticed a grey streak across the A ring at 0300. Quickly realised it was the shadow of Tethys. Cant be common right? So fingers crossed went for it - and got it! I even collimated on a nearby star beforehand, which was needed as the collimation shifts between Mars in the west (usual target) and saturn here in the east.
The AS stacks are 70% culls, having used PIPP to reject worst 30% frames. Animation uses 43% stacks for the bright run-through, and 17% stacks for the 'disk' images. Maybe a faint equatorial belt structure visible moving across. Animation covers 45 mins - the same data repeats twice at different stretches, one for the rings, one for the disk.
But the main thing is the dark diagonal streak of Tethys' shadow moving to the left across the lower left ansa. Subtle but once you see it, pretty clear.
Images are noisier at the end as the sky was quite bright - it was blue throughout.
WinJupos gets the shadow position quite wrong - not massively so in the scheme of things, perhaps by a Tethys diameter (~500km) or so, but it puts the shadow on the far side of the rings. CdC appears spot on at a casual glance.
I think Tethys is one of the few moons that can do this (Iapetus certainly can) as the orbit is tilted by a full 1 degree - enough to allow a shadow on the rings even when the system is tilted, but small enough that its far from a round shadow (as per Iapetus). I think most of the other big moons are much closer to the equatorial plane, but now i type this, I realise i havent checked.
Edited by happylimpet, 24 June 2025 - 02:01 PM.
