NGC2070 - The Tarantula Nebula

It looks like summer is finally arriving here, and with the clear days come the prospect of clear night skies. (When the moon is not out and illuminating everything.)

With some previous images I captured, there were terrible bands of colour within the stacked images - so much so that I was tempted to throw the data away. (Thankfully, Siril has a ‘Remove Banding’ option that works miracles. So the data is recoverable - except that the images are either not very exciting, or don’t have enough data to be worth proceeding with.)

After doing more reading and research, is seems that this astro camera that I have works best with a gain greater than 50. My previous images were taken with a gain of 25, so there’s the reason I had all that banding. The camera also requires a correct set of calibration images as well - the full set of lights, darks, flats and either bias or dark flats. I’ve read that ZWO do not recommend the use of bias frames, so a set of dark flats it was. (Dark flats are also a lot easier to capture than bias frames, so it was a bit of a no-brainer there.)

I captured the Tarantula Nebula earlier this year, using my DSLR, from a dark sky site. I wasn’t too impressed with how the image came out, so thought I’d have a shot at it again. Why? Because it’s a big, bright target and I needed a big, bright target that was away from the 3/4 full moon. (Have I mentioned how imaging in the week either side of a full moon is a gamble? No? Well, it is - especially if you’re not using narrowband filters to eliminate moonlight and light pollution.)

The hours of daylight here are getting longer, so I can’t actually start imaging until after 10pm. I can do the setup and polar alignment from about 9:30pm, but the sky is still too light for imaging so I ended up sitting and waiting. There’s a lot of waiting in this hobby…

I initially ran a series of 20 second exposures, capturing one hour’s worth of data, before dropping back to 10 second exposures (again, one hour’s worth) to see if that made any difference to the amount of light pollution I captured.

Unfortunately, I messed up the focusing with the longer exposures and ended up throwing all the data away. Note to self - check the focus carefully before you start imaging!

NGC2070 is a difficult target to image well - the core is very bright (prone to being overexposed easily) whilst the nebulosity has a wide dynamic range and is difficult to capture. I suspect a good image could be made from 2 sets of data - a series of long exposure to capture the nebulosity detail at the expense of blowing-out the core, and a series of short exposures to capture the bright objects. The two sets of images would be combined in processing. That will be a project for a later date, when I have a bit more experience.

According to Wikipedia: NGC 2070 (also known as Caldwell 103) is a large open cluster and candidate super star cluster forming the heart of the bright region in the centre-south-east of the Large Magellanic Cloud. It is at the centre of the Tarantula Nebula and produces most of the energy that makes the latter’s gas and dust visible. It’s central core is the star cluster R136, one of the most energetic star clusters known. Among its stars are many of great dimension, including the second most massive star known, R136a1, at 215 time the mass of our sun and 6.16 million time more luminous.

Hmm. That might explain why it’s difficult to image this thing.

Image details:

• Gain: 50
• Offset: 10
• Temperature: 0 degrees C
• Exposure: 10 seconds
• Lights: 360
• Darks: 25
• Flats: 25
• Dark Flats (Bias): 25
• Stacked and pre-proc’d in SiriL, finished in Affinity Photo

---

The first image here came pretty much straight from Siril without much processing (background extraction, star colour calibration, green noise removal, histogram stretch) and then into Affinity for all the hard lifting.

I’m not overly happy with this one, as the core is over-exposed, and I had to accept that shortcoming if I was going to have any sort of detail in the nebula. It’s a crop of the original 5496x3672 image, so magnifies the apparent size of the target.

---

For the second image I used a feature called ‘drizzle’ when stacking them. This doubles the size of the image (and requires 4 times more disk space) to help eliminate noise and to enhance fine detail. The end result from Siril has just short of 1Gb and had a resolution of 10992x7344 pixels.

The base processing I did was pretty much the same as for Image 1, but I concentrated on trying to keep the blue/white colour of the stars whilst separating the nebula from the darker background.

---

Just to compare the new camera with my old DSLR, here’s an image of NGC2070 taken in mid-2022. A different focal length and a colour scheme I am not particularly happy with - it looks far too pale and blue for my liking. (But then again, what is the exact colour of the Tarantula and the LMC?)

The “cloud” at the lower-left is the core of the LMC, with other nebulae scattered around the image.

• Astro