Abell 31, aka PK 219+31.1 or Sharpless 290, is one of the largest observable planetary nebula, located in Cancer. The PN was discovered in 1955 by George Abell. The optical diameter is 970″, which is nearly the diameter of the Helix Nebula. It is apparently an ancient planetary that is interacting with the surrounding interstellar medium. As the nebula is so big, its surface brightness is very low, with surface magnitudes of 27 arcsec-1. The nebula is brighter in O3 signal than Ha. The O3 portion sits close to the progenitor star. The top portion is bounded by a wide “bow shock” where the nebular material is apparently interacting with the surrounding medium.
Telescope: 16″ f3.75 Dream Scope
Camera: FLI ML16803
Mount: ASA DDM85
Exposure: 8 hours (40x300s Ha + 40x300s O3 + 6x300s each for RGB)
Date: February – March 2021
Location: Southern Alps, France