NGC 6820...

Ha/Ha/SII/OIII = LRGB

Object: NGC 6820, Sh2-86 (containing star cluster NGC 6823)
Type: Emission nebula with open cluster
Constellation: Vulpecula
Distance: 6,000 light years

Date: July 10th/11th, 18th/19th. 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 16 x 600s Ha, 8 x 600s SII, OIII (2x2 binned) no flats/darks (hot pixel removal in Astroart).

Strictly speaking, NGC 6820 is a small reflection nebula near open cluster NGC 6823, discovered on August 7, 1864 by Albert Marth. The reflection nebula and cluster are embedded in a large faint emission nebula called Sh2-86, but the whole area of nebulosity is usually referred to as NGC 6820. Cluster NGC 6823 was discovered on July 17, 1785 by William Herschel

Map showing approximate image field of view

The rather obscure constellation of Vulpecula lies overhead during UK summer evenings, bounded within the three bright stars of Deneb, Vega and Altair, together called the "Summer Triangle". The whole area is rich in deep sky objects, lying as it does on the plane of the Milky Way, which itself can be glimpsed as a faint hazy band of light on those rare clear summer evenings. Vulpecula itself, although small, contains several objects of note, including M27, the Dumbbell Nebula.

Open star cluster NGC 6823 is about 50 light years across and lies about 6000 light years away. The centre of the cluster formed about two million years ago and is dominated in brightness by a host of bright young blue stars packed in a Trapezium-formed region about 1.3 x 0.7 light-years across. Outer parts of the cluster contain even younger stars. It forms the core of the Vulpecula OB1 stellar association.

The most striking feature in the image above is the trunk-like pillar of dust and gas protruding from the eastern side of the nebula towards the adjacent star cluster. The huge pillars of gas and dust are formed by surrounding gas and dust being pushed and eroded away by stellar winds and radiation from the brightest cluster stars. Dark globules of gas and dust (Bok globules) are also visible in the nebula.

Bok globules, named after the Dutch astronomer Bart Bok (who proposed their existence in the 1940′s) are dark clouds of dense cosmic dust and gas within star-forming regions in which usually star formation takes place. They most commonly result in the formation of double or multiple star systems.

I nearly didn’t bother processing the Ha data. The seeing on the night of July 10^th/11^th was terrible, and it was very hard to get a sharp focus on a star. The nebula itself also wasn’t as bright as I thought it might be and so retrospectively, 600 second unbinned exposures against a less-than-dark summer sky was perhaps rather optimistic. Sure enough, the nebula itself was faint and rather fuzzy due to poor focus and required a hefty stretch such that the result was overwhelmed by bloated stars (see below)...

Original Ha stack, with bloated stars

Just as an experiment, I decided to run the above image through Starnet, a piece of freeware that has the ability to remove stars from images of nebulae. I was rather surprised and pleased with the result (below).

Starnet rendition of stretched Ha stack...

I think that completely starless images of nebulae do look a bit weird however, so I blended it with an unstretched copy of the original stack that just more or less showed stars. The lack of stretching meant the starts hadn’t blown out and after a mild Gaussian blur (0.8, just to avoid a “painted-in” look), I dropped the star layer back onto the starless version in PSP "lighten" mode to produce what became the luminance/red channel (see image below).

NGC 6820 (Hydrogen alpha only)...

I finally got the SII and OIII subframes (600s binned) last nightin between a few clouds (18th/19th July) that enabled me to complete a colour rendition of this object. Colour RGB image was produced in Astroart (red - Ha, green - SII, blue - OIII) and finished in PSP, layering the Ha stack back over the RGB image as a luminance layer as the OIII/SII stars were a bit blown out. There wasn't much in the way of SII data at all, just a sort of general glow in the central area of the nebula.

References:

https://en.wikipedia.org/wiki/NGC_6820_and_NGC_6823

http://annesastronomynews.com/photo-gallery-ii/nebulae-clouds/ngc-6820-6823-by-robert-gendler/

https://cseligman.com/text/atlas/ngc68.htm#6820