The Cygnus Loop...

The Cygnus Loop (Sharpless 2-103)...

Objects NGC 6992-5, IC1340 (the Eastern Veil), NGC 6960 (the Western Veil or "Witches Broom"), "Pickering's Triangle" (wedge of nebulosity at "one-o-clock" position above): also NGC 6974 and NGC 6979 (knots of nebulosity to the left of Pickering's Triangle)

Type: Supernova remnant
Constellation: Cygnus
Distance: 2,400 light years

Equipment: Atik 460/EFW 2, Samyang 135mm lens@ F2, Vixen GPDX mount, guiding with Lodestar X2/PHD
Date: August 8th. 2022

Subframes: 18 x 300s for Ha, 12 x 300s for OIII, flats, no darks (hot pixel removal in Astroart). 

High overhead on a summer’s evening can be found the constellation of Cygnus (The Swan), whose brightest star (Deneb) is the top left star of the isosceles triangle of bright stars that dominates the UK summer night-time skies called the Summer Triangle.

Tucked under the Swan’s eastern wing is an enigmatic object, a supernova remnant called the Cygnus Loop. It consists of a bubble of dust filaments and fluorescent gas over 120 light years in diameter, or about 3 degrees across (6 moon diameters) as viewed from Earth, the remains of a supernova that occurred some 15,000 years ago.

The shock wave from that event is ploughing through and compressing the patchy interstellar medium of gas and dust that surrounded the exploding star, causing the rarefied gasses to fluoresce, hence the nebula we see today.

The brighter sections of the Cygnus Loop, NGC 6992-5 (the Veil Nebula) and NGC 6960 (The “Witches Broom”) are just visible through a large (8-inch aperture or more) telescope on clear, moon-free nights. Details of the central area of the bubble can only be detected on long exposure photography.

The five-minute subframes I used here weren’t really long enough to show up all of the available detail, and the stacks required a hefty tangential stretch to show the wispy central areas of nebulosity.

I used Starnet+ to remove the fog of stars in the stacked hydrogen-alpha and OIII images, which allowed some selective stretching and sharpening of the nebulosity. I then used the PSP “erode” function (“magic wand” to select background > invert > feather (3 pixels) > effects > edge > erode) to reduce the star sizes on the “normal” images, and combined the “starry” and “non-starry” images in “blend lighten” mode to give sharp nebulosity and small stars.

To make the colour image, I used the Ha channel as red, the OIII channel as blue and made an artificial green channel of 60/40 OIII/Ha. Combining the three as an RGB image gave the result at the top of this post.