In a nebula, the range of explorer sensors and planetary sensors are diminished. However, the circle for planetary sensors are not redrawn to represent this.
Inaccurate range representation.png
watch tv, do nothing 10 May 2015:
The player's fleets should also get a circle when clicked on that shows their sensor radius so that the player doesn't have to guess. It's also not clear to me how the ranges at which fleets are visible and the ranges at which fleets are countable get determined.
Sometimes fleets are visible outside of any planet or fleet's sensor radius, and sometimes fleets aren't countable when it seems like they should be.