SN1181 is a recently discovered Supernova Remnant (SNR) in the constellation of Cassiopeia. It lies at a distance of roughly 7500 light years and stretches some 170 arcseconds in diameter. SN1181 gets its name from its initial mention/discovery by Chinese and Japanese astronomers in early August of 1181 CE, who noted it as a guest star. This is one of only five supernovae in the Milky Way that had been identified in the pre-telescopic records, making it ever so interesting.

Until the 2000’s, the young SNR 3C 58 was being linked to this guest star, however more and better data showed that this is likely not the case, due to several metrics not being in line with the expectations for this kind of SN. Marking a turning point in the search of the associated SNR, 2013 Dana Patchick ( @DanaPatchick ) discovered Pa30 in 2013. This newly discovered object lies not far from 3C 58 and right within the reported area of the 1181 guest star.

Although initially believed to be a Planetary Nebula, subsequent measurements revealed a very hot (~210,000 K) central star with high velocity winds of 16,000 km/s, leading to its classification as a type Iax supernova, a rare subtype of the type Ia supernova. More on this in the SN 1181 section of the blog.
Later on, spectral measurements were carried out, and it was found that Pa30, or SN 1181 mainly emits light in near UV, and barely has any associated optical emissions. Adding to the interesting nature of this remnant, the optical emissions that do exist are very atypical - it mainly emits in Sii, there is no associated Ha and researchers believe there is a faint and diffused Oiii emission nebula associated with it.

This immensely interesting background story and atypical emission profile made this the ideal target for the Deep Sky Collective - in order to reveal the SNR in great detail as well as ensure including some background structures such as Ha cirrus and IFN, our goal was primarily set on Sii, Ha and Lum data. Given the immense difficulty of capturing the SNR in SII and the uncertainty of it even being possible with small amateur telescopes, we decided not to try and capture the Oiii shell and focus all resources on Sii.
With the large FOV group of the DSC we started gathering data in July of 2024 and imaged for about 6 months, followed by several months of pre- and post-processing the data. We had a total of 14 people working on the project - 3 Editors, 2 Stackers , 11 photographers and myself coordinating - in the end we managed to get a staggering total of 882h58m of integration, marking the first amateur image of SN1181 and the longest ever on this field. Note that 560h50m of that total is Sii, 158h5m Ha and 73h24m is Lum.

Before getting into any details in the picture, I want to thank everybody who contributed to this project. As always, it was a pleasure working together with everybody!

Tim Schaeffer - @Tim Schaeffer
Justin P. - @Justin P
Carl Björk - @Palmito
Nicolas Puig - @Nicolas Puig
Steeve Body - https://steevebody.com/
Alpha Zhang - @Alpha Zhang

Jasper Capel - @Jasper C
Logan Carpenter - @Logan Carpenter
John Dziuba - @John Dziuba
Akask Jain - @Meeps51
Florent Herrbach - @Florent Herrbach
Kevin Trillsam - @Kevin Trillsam
Jens Unger - @Jens Unger
Mikhail Vasilev - @Mikhail Vasilev

Special thanks to:

Justin and Carl, who both put in a remarkable effort stacking all the data.
Nicolas, Steeve and Alpha for the amazing edits you see today.
Akash, for single handedly putting in over 250h of data.

Interesting features in our image

SN 1181

As mentioned in the beginning of the text, SN 1181 is the Supernova Remnant (SNR) associated with the guest star that Chinese and Japanese Astronomers noted in early August of 1181 CE. 3C 58 was first suspected to be that guest star’s associated SNR, until measurements refuted this theory and Dana Patchick discovered Pa30 in 2013, which would turn out to be the actual remnant of this event. Below, a map with the rough perimeter of this SN is shown. Note that both 3C58 and SN1181 lie in this circle.

A map of the region around SN1181 - © Andreas Ritter et al.

Dana first noticed Pa30 in a mid-IR survey, which sparked interest in the scientific community. Fast forward, this led to spectral observations in which strong emissions in near UV were found. This atypical emission spectra then led to further investigations which ultimately led to a more refined spectra in which the emissions, both in near UV and Sii, were confirmed. These same measurements also revealed an extremely fast expansion speed of nearly 1100 km/s, along with one of the hottest Wolf-Rayet stars in the galaxy at the center of the SNR.
For interested readers, we recommend reading this blog and this publication for more information.
Furthermore, as mentioned in the beginning of the text, note that SN1181 was a Type Iax supernova. This is a rare subtype of the Type Ia SN, with only 5-30% of those being of Type Iax. These SN leave behind a remnant star, also commonly referred to as zombie star, which in this case is the WR star mentioned above. Note that SN1181 is the only known Type Iax SN in our galaxy.
For further reading, please refer tothis paper, or this paper.

For our DSC project we naturally focused on capturing Pa30’s Sii emissions, as there is no reported Ha emission and only a supposedly extremely faint Oiii shell that likely lies beyond what we can capture with amateur equipment.
Please note that it is absolutely necessary to use extremely long sub exposures to pick up any of the signal - we only used subs of 1200s or more, for a total of over 560h of data. To our surprise Akash’s CDK17 performed best at capturing the SNR despite being on the slower side - one major reason for this is probably star deletion. At lower FL, resolving the Sii filaments becomes impossible and all that is left is a circular smudge of Sii. For this reason we recommend both high FL systems and long exposures.
A comparison between data taken at roughly 3000mm FL and 800mm FL is shown below, highlighting the problem mentioned above. Also included is Pa30 as seen in the final stack (Nicolas’ version) as well as from a professional 2.4m telescope.

From left to right, top to bottom: 87h of data at 3000mm, ©Akash - 100h of data at 800mm, ©Nicolas - final DSC image with 560h of exposure in Sii (blue), ©DSC - 1h40 of data with Hiltner 2.4 m telescope, ©Robert A. Fesen et al.

Another awesome close-up of SN1181 with data in IR, X-Ray and Sii can be found here.

Ha and IFN background

Another aspect making this an even more interesting field is the Ha cirrus and IFN included. The Ha is extremely faint in this region, hence requiring nearly 160h of data to get a decent result. The IFN in this region also turned out to be a major challenge to capture and edit.

Three different edits

For this special image, we include 3 different versions:

The final version, edited by Nicolas: This is a pure LRGBHSS palette, making the Sii blue, making it perhaps the most aesthetically pleasing one.
Steeve’s version: This is a LRGBHS version - the Ha is the usual red, with sulfur blended in in yellow. Note that the Sii in question is actually ‘Sii-Ha’ - this was a choice made to better highlight the composition of the background, as the Sii and Ha background was mostly identical. Hence by using ‘Sii-Ha’, we can efficiently showcase regions of more pronounced Sii. Sii was mapped to yellow as this felt less misleading as blue, given blue is commonly seen as Oiii.
Alpha’s version: This is another LRGBHSS palette.

For any further questions about the project, feel free to leave a reach out to us using the contact form or by leaving a comment!
If you want to see our image in greater detail, feel free to go to our website gallery, where the image is uploaded in full resolution ( 7551×5042 px), enabling you to explore the picture by yourself and being able to zoom in on every tiny detail!
Link to our gallery: https://deepskycollective.com/gallery

Integration overview
The final integration count, from only 11 photographers is:

R: 33h25m
G: 27h59m
B: 28h45m
L: 73h24m
Ha: 158h35m
Sii: 560h50m
––––
TOT: 882h58m

A massive thank you to everyone’s efforts!

We hope that you enjoy this image!
Text written by @Tim Schaeffer , organiser and co-ordinator of the project and fact checked & proofread by the DSC team.