On January 17, 2008 Cassini imaged Rhea from a range of ~500,000 km as the phase angle dropped to almost 0 degrees. This is an animation of 17 calibrated clear filter frames with the phase angle displayed at lower right:

Click to view attachment

As the animation shows Rhea gets significantly brighter when the phase angle drops to almost 0 degrees - this is a very pronounced effect. Note that the phase angle changes at a very uneven rate - the time between the images is variable.

The graph I posted in this thread is based on these same images.

That's pretty neat, Bjorn. Especially since Rhea's apparent diameter is larger than the opposition highlight.
Reminds me of a similar Iapetus sequence that went down to a similarly low phase angle.

Always a fun effect to watch, gives a bit more "life" to these otherwise inert worlds...

Yes, that makes this sequence more interesting. At first I didn't notice this - it's easier to see by defocusing the eyes (or in my case, taking off my glasses ).