Another one of the 4 nights clear run we had during the 3rd week of January 2023. I had the Witches Head Nebula on my bucket list for a couple of years now, the images I had seen really do show a definite ghostly image of the Witch staring out at bright star Rigel in Orion. It looks awesome! There are a couple of characteristics of this nebula that makes it particularly difficult to take from my back garden here in the suburbs of Sheffield, UK:-
It is very faint, multi minutes are really required to see anything at all
It is a reflection nebula, it is illuminated by reflecting light from the star Rigel.
This being a reflection nebula my Optolong L-Enhance filter was going to have to be removed. I had ordered, a few days prior, the Optolong UV/IR Cut filter. In effect a piece of glass that lets everything your eye can see. The trouble is what my eye can see is light pollution. Without a goto mount at my field of view (1.8º) it would be a nonsense trying to even frame this as a single 2 minute image showed nothing. However, I trusted to N.I.N.A.'s Slew and Sync and set up and advanced sequencer to grab as much as I could under the skies I have been given. I knew from my experience of the DSLR without a Dual Band filter than 2 minutes was about as good as I could do before being completely fogged out so lots of 2 minutes exposures it was.
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Not possible to see with the naked eye but the nebula is to the right of the bright star Rigel, the right-most star making up Orion's tunic, it is actually listed as being part of the Eridanus constellation. The formal name is IC 2118, as previously mentioned it is a faint reflection nebula being illuminated by Rigel and is 900ish light years away. On better images than mine there is a distinct blue tinge to her, caused by the dust particles reflecting blue light better than red.
Capture
Jan 20th 2023
I chose 120s exposures given the UV/IR cut filter allowing about 3 times the light in of my L-Enhance filter. In my skies 2 minutes is about the limiting factor before I am fogged out by Light pollution. I would run from about 6pm to 11:20pm, at which point neighbouring roofs stop play.
146 * 2m Exposures = 4h52m
Capture using my new IR/UV Cut filter with the ZWO ASI533MC Pro camera running at -10º, gain 100 Offset 40.
I was able to use a 120s library file for a master dark I had taken for an earlier project.
30 Flats and 30 DarkFlats were also taken - exposure to get a 50% histogram of 0.74s.
Pre-Processing
I had two attempts at pre-processing this object one without performing background extraction on each light taken and one with. I got better results with the background extract on every light before moving on to registration.
After registration 146 Lights available
Removed for FWHM > 4.6
Removed wFWHM > 5
Removed small number of frames at the very end due to clipping the roof of neighbours roof.
Leaving 134 of 146, 134*2m = 4h28m of exposure time
Post Processing
Performed normal Siril Post Processing:-
Minor Crop to remove stacking artefacts
Deconvolution (Radius 1.08, ran for 72 iterations), this rather exact figure took some experimentation and ws the best I could do to get the bloated stars under control.
Colour Calibration Photometry
Asinh Stretch - 3 iterations with stretches of (15,10,5)
Used Photoshop to perform a levels and curve stretches - this was a gentle procedure to get to a first gently stretch
Photoshop Processing
Once this initial stretching was complete I came out of PS, converted the mage to Fits then ran the image through starnet++ to remove the stars, Pixel Math used to generate the stars version. Now back in PS with the Starless version:-
Raw - Highlights and Contrast
Raw - Texture
Raw - Clarity
Raw - Noise Reduction, small Noise and small detail enhancement
Raw - Shadow, small Sat, Vibrancy
After I was happy with the starless version, I turned my attention to stars, trying to reduce them a little further with a curves. Then copied them into a layer and screen blended them to the starless Witch.
Created Saved the overall workflow as a Tiff then exported final image to JPG and version for my Social feeds and Astrobin.
Results
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This is at the limit of what is possible from my skies. Sure more integration would definitely help another 5 hours would not go amiss. However, during processing this was a battle all the way. Lots of Texture Reduction, Noise Reduction and Clarity increases where needed. The end result I think shows that this was hard.
Another issue I think was the framing, this object would have suited a wider FoV, I would love to have had Rigel in view.
Some positives, well the fact that I was able to get something recognisable as the Witches Head from my skies! Also, there is some structure visible in those brighter areas (around her chin!).
I think she definitely is having a good Cackle at me!
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