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...