Borg 90FL, ZWO ASI 294MM, Edmund Optics 589nm filter, 10nm bandpass in homemade filter holder, SkyWatcher EQ6r-Pro mount
30 x 60 seconds with flats, dark-flats and dark frames
A 24 million km long plume of gas is ejected from Mercury's thin atmosphere due to the sun, very much like a comet.
This is only visible using a narrowband filter that captures the bright yellow sodium light at 589nm.
A tail was predicted in the 1980s, and first discovered in 2001. Many tail details were revealed in Multiple observations by NASA's robotic MESSENGER spacecraft that orbited Mercury between 2011 and 2015 revealed more details of this tail.