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.

Eastern Veil Nebula (NGC 6992-5)

Eastern Veil Nebula (NGC 6992-5)

Object: Eastern Veil Nebula (NGC 6992-5)
Type: Emission Nebula (supernova remnant)
Constellation: Cygnus
Distance: 1470 light years

Date: September 7th, 2019
Equipment: ATIK 460EX, Skywatcher f5.5 Espirit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 16 x 300s H-alpha, 3 x 300s OIII, no flats/darks (hot pixel removal in Astroart).

The nebula was discovered telescopically on 1784 September 5 by William Herschel.  He described the western end of the nebula as "Extended; passes thro' 52 Cygni... near 2 degree in length", and described the eastern end as "Branching nebulosity... The following part divides into several streams uniting again towards the south."

The Veil Nebula is part of a much larger supernova remnant, believed to have formed when a star 20 times more massive than our Sun, exploded around 8,000 years ago.  The remnants have since expanded to cover an area of the sky roughly 3 degrees in diameter (about 6 times the diameter, or 36 times the area, of the full Moon).   The distance to the nebula is not precisely known, but Far Ultraviolet Spectroscopic Explorer (FUSE) data supports a distance of about 1,470 light-years.

The nebula appears as a streamer of fine filaments. The standard explanation is that the outgoing shock wave from the original supernova is heating a shell of residual hydrogen gas and the wave-front is visible only when viewed exactly edge-on, giving the shell the appearance of a filament.  Given a distance of 1470 LightYears, this gives the radius of the entire nebula as 38.5 Light Years (totalwidth, 77 Light Years). At 1/50,000th of the radius, this places the thickness of each filament at around 4 billion miles, or roughly the distance to Pluto.  Undulations in the surface of the shell leadto multiple filamentary images, which appear to be intertwined.

Even though the nebula has a relatively bright integrated magnitude of 7, it is spread over so large an area that the surface brightness is quite low.  It is very hard to see visually from suburban locations, although use of an OIII filter is said to help.

The brighter segments of the nebula have the New General Catalogue designations NGC 6960, 6974, 6979, 6992, and 6995. The easiest segment to find is the Western Veil Nebula NGC 6960, which runs behind the naked eye star 52 Cygni. NGC 6992 and 6995 are also relatively bright objects on the eastern side of the loop. NGC 6974 and NGC 6979 are visible as knots in an area of nebulosity along the northern rim.  Pickering's Triangle, a triangular area of nebulosity between the east and western nebulae, is much fainter and has no NGC number.  It was discovered photographically in 1904 by Williamina Fleming (after the New General Catalogue was published), but credit went to Edward Charles Pickering, the director of her observatory, as was the custom of the day.

Over ten years ago, I managed to photograph the whole of the "Cygnus Loop" using a 135mm camera lens and my old SXV-H9 CCD camera.  The result can be seen here.

The Eastern Veil Nebula is easy to find, tucked just in front of the eastern wing of Cygnus the Swan. The constellation of Cygnus flies high in the southern sky in late summer, with its brightest star Deneb being one of the three bright stars that form the "Summer Triangle".

I had planned to take 16 300 second exposures through H-alpha and OIII filters, but clouds rolled in and I only got three OIII frames.

The separate channels were stacked in Astroart and then RGB combined, using the H-alpha as the red channel and the OIII data as the green and blue channels. The image was a bit noisy due to the lack of subframes, but using the H-alpha data as a luminance overlay to the smoothed RGB channel helped sharpen up the final image.

This was the first test run of my new Esprit 100ED refractor, coupled with a second-hand Avalon Linear mount. The mount performed pretty well after some initial problems with PHD settings, settling down to give an RMS figure of around 0.6, which was much better than my old NEQ6, which usually could only manage 0.8 at best.

I cannot say that the Esprit performed any better than my old Vixen ED114 in narrowband, however. I had hoped that the relatively small field of view offered by the Atik 460 would mean I didn't need to use a field flattener, with all of the critical requirements of camera spacing that requires. Unfortunately, there was a fair bit of star distortion in the corners of the final image, that I manually processed out.

I will see how the scope performs in RGB, but I could well be reinstating my old Vixen...