THE LOBSTER CLAW NEBULA
Sharpless 2-157 • Emission nebula • Cassiopeia • 11,000 light-years from Earth
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Askar FRA400
📷
ZWO 2600MC Pro
🌃
Bortle 8
⏱️
24 hours
🗓️
June 2022
Overview
The Lobster Claw Nebula, also known as Sh2-157, is a large emission nebula located around 11,000 light years from Earth in the constellation Cassiopeia. It’s dominated by glowing hydrogen gas, with traces of oxygen contributing to its structure and colour in narrowband images. The region sits in a rich part of the sky, close to several other well known deep sky objects, making it a rewarding target for wide field astrophotography.
Background
Having recently imaged four broadband targets, it was good to put my Optolong L-eXtreme filter back in and track down a narrowband nebula. The Lobster Claw is faint (as are most of the targets I image!) but a good size for my 400mm Askar FRA400 telescope and ZWO 2600MC-PRO camera. That combination gave me a wide enough field of view to capture other nearby deep sky objects in the same frame. Shout-out to the ASIAIR Plus‘ framing tool, which was very useful here.
It’s currently Summer, and although the skies don’t get properly dark until very late, if at all, I still find it’s worthwhile gathering data. (See Controversial Opinion #7). This particular imaging project took 20 nights, and most of them were cloudy. Even when the skies were clear, it was only dark enough for imaging between 11:30pm and 3am! Still, following my approach for getting long integration times meant that over those 20 nights I gathered over 30 hours of data, despite it being the peak of Summer in cloudy UK. Using PixInsight‘s SubframeSelector tool, I whittled down those 30 hours to the best 24.
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Science
Close to the Lobster Claw is the better-known Bubble Nebula. It’s smaller, so wasn’t the main focus of my image. Maybe one day I’ll come back to it with a longer focal-length telescope. But it holds up surprisingly well when cropped in very tight, and works well as a bonus image!
Kit list
This is the equipment I used to capture the image.
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Telescope: Askar FRA400
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Camera: ZWO ASI 2600MC Pro
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Guidescope: William Optics 32mm Slide-Base Uniguide
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Guidecam: ZWO ASI 120MM Mini
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Control: ASIAIR Plus
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Filter: Optolong L-eXtreme
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Processing
The biggest problem during processing were halos around several bright stars. This is a side-effect of using the Optolong L-eXtreme. I took the image into Photoshop and cloned out the halos. See the slider image below for a before / after.
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Example source data
Here’s an example single subframe and freshly integrated stack, just with simple stretches applied.
Seestar S50 image
This photo of the Lobster Claw Nebula was taken using my Seestar S50 telescope. Its shape is just about visible after an hour of integration time.
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First of all, thank you for the information and the targets that you share with us.
I follow your workflow with good results and I have a question for you.
When you cloned out the halos of l-extreme in this photo and the artefacts of Starnet++, 2 bright stars on the top-right of the photo were cancelled. I think the same stars were also missing from the “stars-only” image because they are not in the final photo.
How you manage normally this issue?
I tried to draw a black point a bit larger than the core of the stars on the starless layer used to calculate the “stars-only” image but I don’t know if there is a better way (I haven’t Pixingight and I must use Siril and Affinity photo).
Best regards
Paolo
Hi, thanks for your message! Those two stars are missing from the final photo because I cropped very slightly to remove them for aesthetic reasons; I didn’t want very bright stars at the extreme edge of the frame.
Hi there, love reading your articles. You show real dedication in learning about some of your mpressive exposure times over multiple nights.
I have a much more simple rig and use an HEQ5 mount with Rowan belt mod, Skywatcher 72ED, ASIAIR PLUS, recently aquired ZWO585MC camera, 0.8x field flattener and Optolong L-Pro filter (I also have a 2″CLS).
I was wondering if you stack all your lights, darks, flats etc across multiple nights in one go, or do you stack images from each night, then ‘combine’ them in Photoshop? I use Deepskystacker and GIMP at the moment as they are free and relatively easy to use. I can’t seem to find any youtube tutorials on combining astro images for GIMP.
I look forward to your thoughts.
Many thanks,
Dougie
Hi Dougie, thanks for your message. I use PixInsight for my stacking, and throw all the data into one pot — that’s all my good quality Light frames across several weeks, along with Bias, Darks, and Flats. I’m happy that I don’t have to split the data by night, as that would take an age! I don’t use DSS or GIMP so can’t help you with that, unfortunately.