I have been having a ball with my recently acquired Orion 150mm Mak-Cass and enjoying beautiful views of Jupiter and its moons (covered in this thread). Last night, I was trying to catch Io in transit across Jupiter and also its shadow on the planet's disk. Unlike Ganymede and Callisto, observing Io's transit was pretty much impossible with my scope (I was using a Celestron/Agena 8-24mm Zoom EP). I was able to catch its shadow transit across Jupiter, but even that somewhat tenuously. Obviously Io is a fair bit smaller than Ganymede, so I expect it to be harder to discern. I was wondering whether something like Io's transit "should be" easily visible with my scope and EP. The seeing conditions were not spectacular but not terrible either - probably average to slightly less than average perhaps. Am I being too ambitious in trying to see Io's transity clearly with my scope/EP?
I use a SW 150mm Mak so what I managed to see might give you an idea.
Last night I saw a "shadow" and didn't remember that there was supposed to be one. Checking in Skysafari, I realized that it was Callisto itself transiting. Callisto is relatively dark in its colors and I guess this is why it was easily visible on the background of Jupiter in magnifications between 110 and 220, with average or a little more than average seeing.
In August there was a double transit of Europa and Ganymede. I could see both shadows and Ganymede, but couldn't see Europa. Again Ganymede is relatively dark but Europa is smaller and has brighter colors probably similar to the background. I guess I used a magnification of 220 or 270.
I don't remember watching a transit of Io visually, but I took the attached image and animation of images in two different occasions. I think it might be possible visually in excellent conditions only, and maybe not against yellow-orange background.
Edited by YossiZ, 18 September 2021 - 12:25 PM.