Io is Jupiter's innermost moon. It was discovered by Galilo Galilei in 1610. It has a diameter of 3643 KM, slightly larger than our own moon that has a diameter of 3475KM. Io whips around Jupiter in a bit less than two days. I wasn't looking for a transit but as luck would have it, last night at about 11PM local, (5AM UT on the 4th of July) I noticed a dark spot as I was capturing.
Details: Meade LX200GPS 10" with ASI224 in trail with a 2X barlow and ZWO IR Cut filter.
Capture area 488 by 488
RGB 24
Gain 387
Exposure 0.0065
7378 frames captured in 2 minutes or 61 frames per second net.
51% were stacked in Autostakert, Linked Wavelets in Registak, final processing in PhotoShop
Pushed a bit hard to pick up Io.