IC4628 - The Prawn Nebula

We finally had another clear night here, so I had the rig all outside and ready to capture tonight’s target - IC4628. I’ve seen some interesting images of this emission nebula, and decided it was time to take the plunge and have a go at it.

I spent time making certain I had the camera aligned correctly, so that I had the longest side of the sensor horizontal. Previously, it was at whatever angle it ended up after I assembled the optical train. Which made for some interesting, and potentially odd-looking images; especially if you are used to seeing the target in a particular orientation…

From the NASA website: “South of Antares, in the tail of the nebula-rich constellation Scorpius, lies emission nebula IC 4628. Nearby hot, massive stars, millions of years young, irradiate the nebula with invisible ultraviolet light, stripping electrons from atoms. The electrons eventually recombine with the atoms to produce the visible nebular glow, dominated by the red emission of hydrogen.

At an estimated distance of 6,000 light-years, the region shown is about 250 light-years across, spanning over three full moons on the sky.

The nebula is also cataloged as Gum 56 for Australian astronomer Colin Stanley Gum.”

From the Constellation-guide website: “The Prawn Nebula was discovered by the American astronomer Edward Emerson Barnard around 1900. It is a stellar nursery that contains a large number of very hot, luminous, young stars, formed out of the surrounding gas. These stars include two large, hot, blue-white giants belonging to the rare spectral class O.

O-type stars have a relatively short life span as they tend to burn out very quickly before ending their lives as supernovae and collapsing into either neutron stars or black holes. The blue giants in IC 4628 will end their lives after only about a million years. The material produced by the supernova events will be used to form new stars in the nebula.”

Of course, it wouldn’t be an astro imaging session if everything went according to plan…

After 2 hours of imaging, the mount carried out it’s meridian flip without any problems - it flipped, Ekos re-centred the telescope on the Prawn, and then the guiding failed. Rather than having a tracking error (commonly called the RMS) between 1 and 2 arc-seconds, it shot to 30 arc-seconds and stayed there.

A couple of frames worked just fine, and then the RMS skyrocketed to about 60 and Ekos stopped taking images. I tried a few things (but not a complete power-down and restart of everything) without success. So I packed everything away for the night and got some sleep.

A couple of nights ago we had clear skies again, so I was itching to capture more data of the Prawn. Another promising night - the images were perfect, the meridian flip ran like and clockwork and the guiding was rock solid at about 1 arc-second RMS all night.

Except for one small, tiny, insignificant problem - the camera was at a different rotation on the optical train. Not a problem, usually, as there’s enough overlap between sessions and SiriL is very good at aligning images. Nope - the angle of rotation was too great to give me a good overlap and the new set of data was useless.

Which leaves me really puzzled - I made NO changes to the rig and it’s configuration, so I am stumped as to why the second set of images was so far off. So it means that the rotator I bought the other month will now become a constant part of the optical train and this problem won’t happen again. (But something else will crop up to frustrate my night.)

I ended up processing the data three different ways and obtained 3 rather different final images. In all three images, I made use of SiriL’s “starnet++” integration to process the nebula and stars as separate images before recombining them into the final image.

Now, can you see why it’s been named ‘The Prawn Nebula’?

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Image data:

• Gain: 100
• Offset: 10
• Temperature: 0 degrees C
• Exposure: 120 seconds
• Frames: 71 Lights, 25 Darks, 50 each of Flats and DarkFlats (2:20 hours integration in total)
• Filter: L-Enhance (Ha + Oiii dual-narrowband)
• Processed with SiriL (stacking and pre-processing) then finished in Affinity Photo v2

Equipment: SW72ED @ 357mm / HEQ5-Pro / ASI183MC-Pro / SV165+SV305 / Kstars/Ekos

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• Astro