Witch’s Head Nebula (IC 2118, NGC 1909)
IC 2118 has been on my list of potential deep sky objects to photograph since I first hear about her. I didn’t think I would have the skills or techniques to do her justice so soon but my plans for shooting M42 with the 300 mm lens were dashed again because of high winds. I studied the area and figured out my desired composition using a 200mm lens and a 1.6 x crop body camera and this is pretty much the result I was hoping for.
Why is this target so difficult for photographing? IC 2118 is known as a reflection nebula, meaning that there aren’t a lot of highly illuminous stars or star formation occurring within this collection of dust and gas. This very dim (apparent magnitude of 13) reflection nebula is primarily illuminated by the 7th brightest star in our sky – Rigel, the left foot in the constellation of Orion. Rigel, located 2.6 degrees to the east of IC 2118, is actually a system of four stars in close proximity. Rigel A is the primary star and is measured to be approximately 120,000 times more luminous than our sun, with an apparent magnitude of 0.13. It is a young star, approximately 8 million years old and has already burned through the hydrogen in its core. It is now burning heavier elements and will one day go supernova – one of the closest stars to us that will do this. When this happens, it is estimated that it will be as visible to us on earth as a quarter moon!
Back to the oh-so-appropriately named Witch’s Head. Due to the blue color of Rigel and the properties of this light scattering off of the gas and dust, this nebula appears blue in color, similar to the reason why our sky is blue on earth. Astronomers are unsure if the nebula is the remnants of an ancient supernova itself or just a collection of dust and gas. Although being close to, or perhaps a part of, the Orion molecular cloud complex, IC 2118 officially lies in the constellation Eridanus. This nebula is approximately 800 light years from earth and of course is absolutely huge. IC 2118 is roughly 1 x 3 degrees in our night sky and roughly 50 light years long. It is not visible to the naked eye from earth, but to give a size comparison of the amount of sky this object would take if we could see it, it would roughly be equivalent in length to six full moons in our night sky.
Collecting the data (27/28 December)
It was nice having two opportunities in December to work on astrophotography. Like I mentioned above, I was hoping to do a closeup of Orion and Running Man nebulas but with 10-12 mph steady winds with gusts above 20 mph, I knew I better not shoot with the 300 mm lens. IC 2118 was definitely on my list and could be captured with the much smaller 200 mm lens. The weather forecast was tricky and one of four weather apps suggested that clouds would ruin my night starting around 01:00. Even if so, which it did, I could still get up to six hours on the target.
I was by myself for this session, Miguel having something else, like sleeping I guess, going on this evening. And I setup at the usual location – Danville Conservation Area. It was truly windy and the temps hovered around the freezing point, which was warmer than the last time we went out.
Being such a dim target presented a significant challenge. Primarily, with a 35% luminated moon, I struggled a bit with getting exposure where I wanted. I would have liked to use ISO 3200, but when I started, this put the histogram peak above the 50% line. So I decided to use ISO 1600 using 30 second exposures. When the moon set at 22:04, I knew the histogram peak would drop and it did to a little less than the 20% mark. This was concerning because I knew this would be too close to get the signal to noise ratio I needed, especially with such a dim target. I contemplated changing the ISO up to 3200 but then I wouldn’t be able to stack the two sets taken at different ISOs with my dark frames while being able to use the process to remove satellites and plane trails. Instead, I opened up the aperture from f/3.5 to f/3.2. This gave me a third stop more light for each sub. I wasn’t sure if this was going to work, especially not being able to see the target in an individual frame!
As I feared, clouds came in heavier than 3 out of 4 weather apps and a meteorologist predicted! So, I shut down around 01:45 and made it home by 03:30 – an early night!
Astro-modified Canon 7D mkii camera, Canon 200mm f/2.8 lens, Skywatcher Star Adventurer tracker without guiding on a William Optics Vixen Wedge Mount. Gitzo CF tripod, Canon shutter release cable, laser pointer to help find Polaris and sky targets, lens warmer to prevent dew and frost on lens, dummy battery to power camera, cart battery to provide power to camera and dew heater, right-angle viewfinder to aid in polar alignment.
Lights taken (30 seconds; ISO 1600; f/3.5 and f/3.2) 671 taken, manually removed bad subs due to tracking errors, winds and clouds for a total of 433 used in integration.
Bias and Flats: Not taken. Removed most vignetting and some chromatic aberration while converting RAW images to TIF.
Not knowing for sure if my individual sub-exposures were going to be accurate, I was eager to get to the processing. After removing obviously bad sub-exposures, I plugged the 433 photos into Deep Sky Stacker and told it to use the best 90% of those, giving me a total of 3.25 hours of integration time.
It’s amazing how I can get sucked into processing these DSO images. This one only took me about four hours from start to finish but it seemed like no time at all. I also used GraXpert to remove gradients and various steps in Photoshop CS6.
Problems and learnings
This is definitely an object you want to shoot without light pollution and with as much time as you can possibly get on her. With roughly half my night lit by the moon and not getting as much time as I had hoped for, I am very pleased with the outcome. I hope to try this one again someday. Being a winter target, it is possible to get 8-10 hours on this target in a single night. This would help bring out the surrounding dust and provide better definition of the target herself. I did wind up using some subs that had light clouds, providing the halo around Rigel that normally wouldn’t be there. I don’t think this hurt the image, however. I could also shoot her with the 300 mm lens but this would eliminate Rigel in the frame and I don’t think would be nearly as interesting.
This is the second image of five I hope to make around the Orion molecular cloud complex. I did not expect to shoot the witch this soon but I am pleased that I have learned enough to make a competent image of this dim and challenging subject. After doing this a few months in a row, I am much more confident in what I am doing and using my kit has almost become old hat. As long as the weather gods bless me, I am feeling much more confident in being able to capture and process the targets that are within my capabilities. I hope to upgrade my tracking mount within the next year or two but I will continue with what I have at the present.