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.
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
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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