I liked this topic two years ago, so since it's continuing...
1 -- For an imaging challenge to be ascertainable, it cannot be a transient event (like a dust storm). It should be predictable, even if rare, like the moon eclipsing Saturn or the elusive Rima Tenuis (if that's even possible, without opening that debate).
2 -- The point of the challenge would to be photograph the target in such a manner as to make it clear to an unfamiliar observer what they're seeing -- cratelets in Plato, detail in the GRS, albedo features on a moon of Jupiter, etc.
3 -- What is considered a 'challenge' needs to evolve with the times. The "hobby" of astronomy often gets in the away of progress in the art of astrophotography. Things that were barely possible 10 years ago for the most skilled and experienced astrophotographers are now fodder for rank amateurs (for example, when I started taking pictures of planets, the prevailing thought was that there was no point in imaging Mars below 10" in apparent diameter -- I think we can all agree that this "guidelines" has been obviated by technology).
4 -- A true challenge is one that requires careful planning, timing, and execution (along, obviously with a lot of luck in the clouds and seeing department). An eclipse is a great example, as are features that are only apparent during certain lunar phases/librations.)
Finally, I have another addition to the list -- the Spokes on Saturn's Rings. Good luck with this one, as I don't think it's going to be possible to do this again for another 10 years or so, when we approach another ring-plane crossing).
P.S. Here are the Martian Volcanos.
Edited by WayneJ, 10 September 2014 - 09:12 AM.