M57, the ring nebula, in H-alpha light

The Ring Nebula (M57, NGC 6720) is a planetary nebula in the constellation of Lyra.
The image was taken in H-alpha light, which reveals the very faint outer layer, not seen in amateur telescopes, using the eye and an eyepiece.
Planetary nebulae are formed when a star, during the last stages of its evolution before becoming a white dwarf, expels a vast luminous envelope of ionized gas into the surrounding interstellar space.
This nebula was discovered by the French astronomer Charles Messier while searching for comets in late January 1779.

Telescope: Apertura 6-inch Classic Cassegrain with Baader Alan-Gee reducer, which brings it to f/7.1, 1080mm focal length
Camera: ZWO ASI 294MM Pro Camera, cooled to -10 Celsius
Guidescope: The Bellavia Basic 50mm, f/7.5 refractor
Guide Camera: ZWO ASI 290MC

27x 300 second images, Antlia 4.5nm Ha filter, in BIN2 mode
25 Dark frames, 25 flats, 25 dark-flats for image calibration
Processed in PixInsight