Shooting the Rosette Nebula – a 2 Year Journey

“Follow the light and we can journey from the confines of our planet to other worlds that orbit the Sun without ever dreaming of spacecraft. To look up is to look back in time, because the ancient beams of light are messengers from the Universe’s distant past.”

Brian Cox

One of the first targets I recall sinking some major time into was the Rosette Nebula, catalogue entry Caldwell 49, in the constellation Monoceros, back at the start of 2019.

By “major time”, back then it was anything more than an hour and a half. How foolish, how naive, I was back then to think that it was possible to produce a decent image with such low acquisition times. Of course, depending on your target and other factors, it IS possible to produce a respectable image. M42, the Orion Nebula, for example, lends itself quite well to a decent image with limited data, as do the likes of M31, The Andromeda Galaxy.

Nowadays, I will think nothing of putting in 6 hours plus on a single target. With the UK weather being what it is, it’s difficult to image that long on a single night, especially in the city where you have trees, neighbours rooftops etc to contend with. It’s been even more difficult this past year, due to lockdowns, to get away from the city lights and into darker skies. So you take what you can get, when you can, which is often not much and often not enough.

So join me, as I take you on a journey of 2 years, shooting across 7 separate nights, to capture the Rosette Nebula.

February 2019

Not long after I bought my portable rig, the SkyWatcher Star Adventurer, and the SkyWatcher 72ED, we were into the long, dark, cold winter nights. I’d also not long obtained a fully modded Canon 450D. When we use the term “modded” in reference to cameras used for astrophotography it means that the stock IR cut filter has been removed. This is done in order to expose the sensor to more of the red part of the spectrum, which is so important when imaging emission nebulae. Do we need an astro modded camera? Yes and no. You can still capture the red part of the spectrum, but you need to put a lot more time into the data acquisition in order to obtain an equivalent amount of data. Even then the images are often not as good or as clear.

Having recently successfully imaged the Orion Nebula, I was keen to shoot something else. Hunting around, I came across several images of Caldwell 49, or the Rosette Nebula, pretty much right next door to Orion. The Rosette is a bright Hii emission nebula, with an apparent magnitude of 9, measuring approximately 130 light years across, and makes for an ideal target for a modded camera, with its vast hydrogen alpha fields.

To put that distance into perspective, our nearest stellar neighbour, Proxima Centauri, is a mere 4 light years from us. It would take us around 6300 years to get there using current technology. Multiply that by 32 (the number of times rounded down to the nearest whole number that 4 goes into 130) to give you approximately 201,000. That’s the number of years it would take to traverse the Rosette Nebula, using current earthbound technology. Up until recently, 200,000 years was approximately how long “modern man” has been around on this planet. Remember when I said space was vast and the distances were immense? Hopefully that now gives some sense of this.

Bearing in mind that this was long before I’d discovered the joys of manual platesolving (which I cover in my post on using the SkyWatcher Star Adventurer), I was still star-hopping to get onto target at this point. In theory the Rosette was pretty easy for this one – from the top most star of Orion, Meissa (or Lambda Orionis), follow a straight line through Betelgeuse* (Alpha Orionis) for approximately the same distance again as that between Meissa and Betelgeuse, and you’re in the ballpark. The cluster at the centre of the Rosette, NGC 2244, is generally easy to spot as it has the appearance of a short ladder. Get that cluster in the centre of your screen and you’re good to go.

This theory, is all well and good until you’re trying to put it into practice in minus February temperatures, whilst not knocking your rig out of alignment, not tripping over cables etc. But, in all honesty, all of that hard work makes what I do feel more worthwhile, and there’s certainly no denying the sense of satisfaction when you see the first sub coming in.

To do this hobby, this passion, you often need the patience of a saint. Not least because this is a passion that denies instant gratification, unlike so much else these days. One thing that astrophotography has taught me, is to slow down and appreciate the universe we live in. Not everything has to happen at once. Not everything is about here and now. If it were then I definitely have the wrong interest, because what I do involves time travel of sorts, so “here and now” doesn’t even exist.

I’m looking back years into the past, sometimes tens of millions of years. The light from what I, and many others like me, capture, has travelled for all those millions of years. We’re seeing galaxies as they were when dinosaurs roamed the planet. We’re taking images of nebulae and what we see now is what they looked like when they nailed some guy to a tree for saying how great it would be to be nice to people for a change. This is how vast space is. And it’s only getting bigger!

For me it’s never been about any sort of instant gratification. Of course I love seeing those first subs coming in on the screen, and I can’t help but get excited when I’m imaging something so elusively distant. But that’s only a part of it. It’s more about imagining the epic journey that those photons have been on, crossing the vast distances of space, passing through the interstellar medium of dark clouds, speeding past other stars and planets on their way to be captured by me, sat on this rocky planet orbiting around a relatively insignificant G2V class star far out on the Western Spiral Arm of our galaxy. What wonders have those photons seen? What ancient civilisations have observed those similar photons? What non human eyes have lain their gaze on those same galaxies and nebulae? From what different perspectives do they see them, and us? Did those photons perhaps at some point pass by the comet that brought about the end of the dinosaurs? Did they go past the ruins of older civilisations? Has their long trip been warped by the gravities of super dense stars travelling through the curvature of space-time?

The journey of those photons must indeed be wonderous, having travelled for so long and so far, that it’s almost impossible to comprehend the timespan and the distances involved. For in a universe as vast as ours, there is much to be seen, and it can’t be just our limited human eyes that see these things. As Carl Sagan once said “If we are alone in the Universe, it sure seems like an awful waste of space.” And only our own human arrogance would assume that we are indeed alone. Nature loves life. It loves creation. And if it can be kind and benevolent enough to create it on our own world, why would it not repeat that elsewhere in the universe? We can see in our own biosphere the extremes under which life can exist, so statistically speaking there MUST be life elsewhere, even if it’s in forms that we are currently ill-equipped to identify.

The first run on the Rosette Nebula I undertook in February 2019 across 4 nights using 60 and 90 second subs, at ISO 800 using the modded Canon 450D on the SkyWatcher Star Adventurer, a very portable star tracker and an essential piece of kit for any astrophotographer. I won’t cover how it works here as I discuss it at great length in another post. But suffice to say that it punches far above its weight. And coupled with a small refractor, such as the SkyWatcher 72ED, it opens up a universe of possibilities in imaging.

During that first data run in 2019 I managed to gather 4 hours of data. Sadly, my processing wasn’t nearly as good as it is now, and the final image I felt didn’t do the nebula justice at all.

Caldwell 49 Rosette Nebula
Shot in February 2019

The stars are bloated, there’s a lot of noise in the image from where I’ve pushed the processing too hard to bring out the fainter nebulosity around the outer edges, and I had a distinct lack of calibration frames. But I felt it had potential and kept the data, ready to add to it the following year.

Winter 2020

Unfortunately, the weather during this period was especially bad for me, and that coupled with the fact I had a dirty sensor meant I wasn’t able to get out and image using the 450D. I had the sensor cleaned a number of times locally, but even with a full suite of calibration frames I was unable to get rid of the plethora of dust bunnies riddling my frames. So in essence, 2020 was a write off as far as imaging this was concerned. But still I kept the original dataset, hopeful of another go the following season….

2021, the year it came together

Although most of the previous year was either a washout or a cloudout, the start of 2021 has thankfully had more opportunity. Any dedicated astrophotographer will tell you that we long to get out imaging, and the previous few months had been horrific as far as the weather was concerned. Opportunities had been rare, and the ones we had were all too brief.

This year I’ve managed another 3 sessions on the nebula, and this time I’ve been taking the all important calibration frames at the end of each session. And although last year was pretty bad, I used the time to learn better processing techniques. For this I hit YouTube a lot for tutorials, following the likes of Trevor at Astrobackyard, Ruzeen at Astrofarsography and Nick at Astro Exploring. They’ve shown me ways to process what I capture that a couple of years ago I didn’t think was possible, and I’m eternally grateful to them, and others, for the time they put into teaching those like me who have no clue on how to get the best out of the equipment and the hours we spend dragging the data out of the hours and hours we put into capturing it.

Unless I somehow manage to get a decent run of clear nights over the next month, I’ve now finished my data acquisition on this target, having obtained a total of 419 light frames, plus a total of 60 dark frames, 90 flat frames and 30 bias frames. The light frames, or “subs” consist of 117 x 90 second frames plus 302 x 60 second frames, giving a total of 7 hrs 59 minutes aquisition time. Note that when we talk about total aquisition that we don’t include the calibration frames in that.

The next stage is to “stack” all of these images. We do this in order to increase the signal to noise ratio which, when coupled with the calibration frames, produces a far cleaner final image. It’s this final image that we process in order to “stretch” the hidden data contained within it. There are various ways to do this, and no one way is the “right” way, although there’s methods that are generally acknowledged as being the best ways to pull out the data without sacrificing image quality. I’ve been hugely guilty in the past of over processing an image in order to pull out as much as possible, and the final image has suffered for it, as my original of the Rosette clearly shows. This method of pulling out the hidden detail and making the images “pop” is what we refer to as our workflow. Our workflows change over time as we learn new and better techniques, but for now I seem to have settled on one that seems to work well for me, and I cover this in another post.

I’ve processed, reprocessed and then gone back to the original data and restacked then processed it again more times than I can count, and I’m finally happy with it. I don’t have a narrowband filter set, so bear in mind that this is taken with a DSLR which has been modified for astrophotography.

Thank you for taking the time to read, so for now I’ll leave you with my final image of Caldwell 49, The Rosette Nebula. Clear Skies all