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| Sharpless 2-115 |
Type: Emission nebula with open cluster
Constellation: Cygnus
Distance: 7,500 light years
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD
High overhead during early autumn evenings flies the constellation of Cygnus. The Milky Way forms a star-strewn backdrop to the celestial swan, where many deep-sky objects can be found. One such object is Sharpless 115, an emission nebula that can be found 2 degrees north-west of Deneb, the alpha star of Cygnus the Swan and easternmost star of the Summer Triangle.
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| Stellarium map showing location of field of view |
Noted in his eponymous 1959 catalogue by astronomer Stewart Sharpless, this faint nebula lies along the edge of one of the outer Milky Way's giant molecular clouds, about 7,500 light-years away. Fluorescing with the light of ionized atoms of hydrogen, sulphur and oxygen, the nebulous glow is powered by hot stars in star cluster Berkeley 90 (the cluster of small stars just below left of centre in the field of view above). The cluster stars are thought to be only 100 million years old or so and are still partly embedded in its parent nebula.
To the northwest of Sh-115 can be seen dim streaks of nebulosity catalogued as LBN 362 (the LBN designation referring to the Lynd’s Bright Nebula catalogue). Embedded within that is a small bright circular emission nebula given the designation SH2-116. This object was first classified as a planetary nebula (hence its alternative designations of Abell 71 or PK85+4.1) but recent studies show it to be an HII emission region instead.
The evening of September 13th offered good “seeing” (the air was steady) but less than perfect transparency. Hydrogen alpha data of this faint object showed up well, but was slightly blurred by what seemed to be very faint high-altitude haze. The stars were a little bloated, even though guiding and focus seemed good. Nevertheless, the miracle of Starnet allowed me to remove the stars from the image so that I could polish up the nebula (mild stretch, denoise, despeckle, unsharp mask) and then add the stars back in afterwards.
(I used a “destretched” version of the Ha stack that just
showed the brighter stars, although I did paste in a bit of stretched stack to
show up the
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| "Starnet" version of Ha data, stars restored |
Needless to say, anything less than perfectly clear air hamstrings any attempt to gather OIII data on such weak sources such as Sh2-115. Indeed, only a trace of signal could be found on 600 second exposures binned at 3x3. As a result, even a stack of 12 of such frames looked pretty awful.
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| Raw OIII data stack |
I ran this though Starnet and was surprised to find that it worked though, just leaving behind some of the more bloated stars that I could manually clean up. A combination of the 3x3 binning and weak signal still left a very grainy stack, so I applied a strong Gaussian blur (radius 5) to the starless frame before adding a star layer back (made from the original OIII stack, with brightness and contrast strongly adjusted to leave just the brightest stars, which were then Gaussian blurred, radius 2 to remove the binned “blockiness”) in “blend lighten” mode.
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| Final OIII stack |
An HOO composite from the Ha (as red), OIII (as blue) and a 70/30 OIII/Ha blend was then used to give an RGB colour image, which I sharpened up a bit by adding the Ha data back over as a 50% luminance layer. At first I couldn’t turn a strong magenta hue in the nebula into the more pleasing OIII-related blues without getting weird colours elsewhere. Eventually, I took the OIII layer and pasted it over a blue background in “hard light” mode, that gave blue OIII nebulosity and white stars on a dark background. This got pasted over the colour frame in “dodge” mode at around 20%, which succeeded in bringing out the blue without overly affecting the rest of the colour balance.
A bit of selective tweaking in curves gave the final colour image above, such as it is.
After I put this post up, an acquaintance showed it to their
astronomy-mad grand-daughter. She thought that the nebula looked like a troll,
a horned and bearded beast who was trying to grab the
I can see where she was coming from, hence the amended title of this post…
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| Sh2-115 with extra added troll... |
References:
https://apod.nasa.gov/apod/ap130614.html
http://www.astromaster.org/oggetti/sharpless_data/Sharpless_r.pdf
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