The most beautiful region of the sky I have ever photographed.
A truly stunning star cluster with a distinct star in the center, beautiful and bright reflection nebulae wherever you look, dusty pillars surrounded by glowing hydrogen and even more hydrogen structures to the right and last but not least, an absolutely fascinating new discovery that Thrayn (Max Kingham) and me (Sophie Paulin) made about a year ago.

Back in 2022 I came across my very fist discovery when I tried to take a picture of the supernova remnants around NGC7129. By pure chance, another tiny little nebula showed up in both H-alpha and OIII and got registered at HASH (https://www.astrobin.com/d3a634/). This excitement led me to look for more unknown objects all over the sky. Soon after, I met Max, who was really passionate about the discovery work as well. Together we looked through a lot of surveys to find hints of potential unknown objects. At the time (February 2023) I also set up my first remote observatory in Spain, which was extremely useful to verify the objects in OIII, since there aren't any OIII surveys yet.
One day Max stumbled upon a tiny structure on a survey image and asked me to double check it with my scope in case there is any OIII emission involved.
The following clear night (27.03.2023), I gathered about 2 hours of oxygen data on it. Since the little structure resides in Canis Major, it was extremely low on the horizon and I had to shoot through a lot of atmosphere at ~20° altitude.
As I stacked and stretched the data and couldn't believe what I saw.
(OIII)

There was a huge shell-like structure looking back at me. Excited about this, I immediately messaged Max and Marcel Drechsler (who was way too kind for taking his time at 1am in the morning) to find out as much information about this as possible.
(SHS)

The SHS H-Alpha survey also showed parts of our structure and we decided to send it to Pascal le Du as potential PN candidate.
Pascal registered our structure as KinPau-Object 1. He however noted that additional data was necessary for it to be moved to the HASH database. Unfortunately at end of March, Canis Major sets really early and due to the rising Moon and weather, I wasn't able to take any further images and the Object vanished behind the Sun.
In the southern hemisphere however, Canis Major passes through the zenith and was still visible in April. So I bought some imaging time on a remote scope hosting site with scopes located in Namibia. But rain season had different plans during new Moon and I was only able to gather another 2 hours of data on our target, which wasn't enough.

At some point at the end of Summer 2023, Pascal removed our entry from his site, as it has not been accepted to HASH and classified it in a different table.
In order to appear in HASH, the object has to be a Planetary Nebula or resemble its structure. Our find could also be a remnant of a supernova due to its incredible size so our next step was to gather H-alpha and SII data on it.

Together with my friends @Jens Unger  and @Torsten Mueller , we planned to set up a remote telescope in Namibia ourselves. This was really exiting and during all those months of planning and testing (July- December), I always had this region in my mind (btw, huge shout out to Jens and Torsten for letting me use our scope for so many of those rare clear nights during rain season on this target, despite them being equally excited to finally image their favorite southern hemisphere targets <3 you're the best!).

Finally, I had all the data I needed on this target.
(SII)
Sulfur shows no Signal so our Object is unlikely to be a supernova remnant. 

In OIII and H-Alpha the structure looks nearly identical.
(H-Alpha)

(OIII continuum substracted)

What stood out to me was the diffuse glow surrounding the structure. At first I thought it could be a flats issue, but it shows up in the uncalibrated image as well.
Also worth noting is the second arc surrounding the star cluster. It seems unrelated to the big bubble-like structure but shows the same matching OIII/H-Alpha characteristics and similar brightness.
Talking about brightness, this target is really bright compared to some of the other discoveries we made. It is already visible in the STF preview of the 10hrs OIII stack.
(OIII STF rotated)

I have linked the raw OIII data above, so feel free to check it out. Usually it is hard to get a feeling for discovery images because you never know how far the data has been pushed and how the data "really" looks. Please excuse the slightly trailed stars, our piggyback setup suffers from slight differential flexure which is noticeable during long exposures.

We wish we could tell you what this is. Every other object in this image has at least a plausible leading theory, such as SSC-11 in the bottom right, which we believe to be a stellar wind nebula around HD 58009. The upper OIII arc is most likely the ionized remnant of the cloud that formed the star cluster NGC 2362.
Our leading theory of the large bubble is that it might be a superbubble caused by multiple supernova explosions of similar stars in the same area. These explosions together sweep away interstellar gas, which forms a dense shell. The shell could additionally be ionized by the bright star UW Canis Major. The vast majority of the objects in the frame are about 4000 ly away, which leads to an approximated size of 550 ly for our bubble. The upper size limit for a planetary nebula is around 7 ly. So it is NOT a PN and it is most likely not an SNR. The super bubble theory is our best attempt at explaining it, even if it's still unlikely. If you happen to know more, please reach out via comments or PMs.

There are a few other interesting and pretty objects in our image that haven't been catalogued yet. Whenever Max and I stumble upon such finds which have no official place to go, we add it to our "Sight Seeing Catalogue" (SSC). You can check it out here: https://docs.google.com/spreadsheets/d/1WM_LbLjJo7TZFs_ba5d79Yc8CSkhMvO45Rf1jbtBsiY/edit?usp=sharing (click on the SSC tab). If you ever find a pretty or interesting target without a catalogue entry, let us know so we can put it in here so it won't be forgotten


@Brian Puhl volunteered to also process the dataset. We both worked on our own and compared our final images to double check for any processing errors.
Here is his version:


And last but not least, if you were wondering why we called it the Shield Nebula:

(Sorry about those horrendous MS Paint skills)