Overview
The Clamshell Nebula (Sh2-119) is around 2200 light-years away in the constellation Cygnus. This photo shows clouds of gas and dust within the central region of the nebula, which are being sculpted by powerful stellar winds and ultraviolet radiation from nearby hot, massive stars — in particular the supergiant 68 Cygni (the bright point in the bottom-right).
Background
I’ve wanted to image the Clamshell Nebula for quite a while. For some reason I’d always assumed it was quite small, so I was surprised to find that it’s actually more of a wide-field target. Still, I wanted to give it a crack, so decided to leverage my Askar 130PHQ’s 1000mm focal length to zero in on a composition around the centre of the nebula.
Imaging details
| Date | 27 September – 26 October 2024 |
| Location | Bristol, UK (Bortle 8) |
| Telescope | Askar 130PHQ Flatfield Astrograph |
| Camera | ZWO ASI 2600MC-PRO |
| Mount | Sky-Watcher EQ6-R PRO |
| Guide | William Optics 50mm Guidescope with 1.25″ RotoLock; ZWO ASI 120MM Mini |
| Control | ASIAIR Plus |
| Software | PixInsight, Lightroom |
| Filters | – No filter (for RGB stars): 30 x 120 seconds (1 hour) – Optolong L-Ultimate (Ha / OIII): 144 x 300 seconds (12 hours) – Askar Colour Magic D2 (SII / OIII): 288 x 300 seconds (24 hours) |
| Total exposure time | 37 hours |
| Image credit | Lee Pullen |
Processing
As usual for these types of nebulae, I used an Optolong L-Ultimate for Ha/OIII data, and an Askar D2 for SII/OIII. I normally use an Optolong L-Quad Enhance for RGB data — in this case just the stars — but I actually forgot to use it this time! It doesn’t really make much difference for the stars though, as they’re bright and use just an hour of integration time.
As for the L-Ultimate, that’s 12 hours of data. I knew the SII/OIII component would be fainter, so I doubled that and collected 24 hours. Unfortunately, the SII/OIII was even weaker than I expected. This caused some issues during processing (outlined below) so I needed to take a slightly different approach.
If I were to tackle this target again, I’d be tempted to forego the SII/OIII aspect, and just aim to get a very high signal-to-noise ratio with the Ha channel. As it stands, it’s a really tricky target with an OSC set-up from a city centre. It may be a more realistic approach with full Mono, and definitely with darker skies!
I don’t have time for a full processing walkthrough this time, but the first steps are the same as I followed for the Wizard Nebula. The main difference came after running DBXtract to make an SHO image from the Ha/OIII and SII/OIII data. The SII/OIII channel was so weak / noisy in comparison to the Ha/OIII, I just couldn’t get a high quality result. So, I decided to try something new. I used NarrowbandHueCombination to mix together the Ha, SII, and OIII channels produced by DBXtract.
Check the image below to see the settings I used. I used red at Contribution 100 for the Ha channel; but Contribution 0.5 for both OIII (blue) and SII (green). This made the stronger Ha channel much more dominant. The end result isn’t as colourful as I’d initially planned, but the actual image quality was higher, in that it wasn’t ridiculously noisy.
After that, I used SelectiveColorCorrection to make the red, orange, and yellow colours a bit more distinct. Then it was a case of my usual processes, including UnsharpMark, NosieXTerminator, and DarkStructureEnhance.
Here are two images I initially made using my regular method. The don’t look so bad when viewed small, but when viewed original size on my screen I just wasn’t happy with the image quality.
Source data
Seestar S50
Here’s an image of the Clamshell Nebula taken from my city centre location using a ZWO Seestar S50 smart telescope. It’s hard to make out much nebulosity, but the bright star 68 Cygni shows up well.
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