Introduction

This deep-field astrophotograph of Messier 51 (the Whirlpool Galaxy) represents one of the most extensive amateur datasets ever collected for this object, with more than 440 hours of combined integration time. While M51 has been imaged countless times, the overwhelming depth of this project - especially the 277 hours of narrowband Hydrogen-alpha - reveals structures rarely seen in amateur or even small professional observatory data. The image exposes vast Hα “cliffs” and filaments curling outward from the galaxy, ultra-faint dust structures and Integrated Flux Nebula (IFN), tidal material, and numerous faint galaxies across the background.

Intentions of the Project

The project aimed to push beyond the typical portrayal of M51. The classical structure of the Whirlpool is well documented; what is not well documented is its deep environment. Two primary goals guided the work: revealing deep Hydrogen-alpha cliffs and streamers, and mapping faint galactic dust around the system. The result is a dataset pushing the limits of amateur deep imaging.

Contributors

In order of amount of data collected:

We are looking for new members! Visit our website to learn more: CAT.

Acquisition

© Collaborative Astrophotography Team - Integration time table

This project brought significant technical challenges due to its extreme depth and multi-imager nature. While the total integration exceeded 440 hours, stacking at this scale required substantial problem-solving. In order to reach our SNR goals for the Ha cliffs, as a team we used on average longer sub exposure lengths than we did for Messier 81-2.

Challenges in the Data

© Collaborative Astrophotography Team - Raw Luminance showing Stacking Artifacts

Combining data from many telescopes introduced slight differences in pixel scale, rotation, field curvature, framing, and sky conditions. Individually negligible, these inconsistencies became major issues across hundreds of hours of integration. The team encountered edge warping, residual alignment seams, distortion mismatches, and inconsistent SNR. These were fixed with synthetic flats although the poor SNR in the corners still meant a crop was required.

One of the hardest challenges involved gradients in the Luminance data. Due to the aforementioned stacking artifacts, that normal methods of background extraction couldn't correct for, synthetic flats were used, enabling removal of these artifacts while preserving the delicate structures in the Luminance. Synthetic flats were carefully tuned to avoid overcorrection by cross-checking structure across other high integration images e.g DSC’s M51. Once gradients and artifacts were controlled, clean L, RGB, and deep Hα masters were produced.

Processing

Processing this target was a great struggle, trying to pry as much as we could from the faint Hydrogen as well as trying to pull all of the IFN we could from our limited luminance channel. The key take aways are:

Iterative stretching of 0.30 or so using GraXpert denoise on the Ha yielded much better results than NoiseXterminator or DeepSNR, allowing us to preserve detail and minimise artifacting around the cliffs.
Pulling up the IFN using GHS and careful masks, while denoising with GraXpert iteratively allowed us to maximise the amount of IFN shown in the image.
The stars in the image were an issue. Normal star extraction processes would leave the small scale stars in the starless image, and they would then get attacked by the denoise. Thanks to Charles Hagan ‘s Extracting Stars Pixel Math, we were able to successfully remove and therefore preserve most of the small stars in the image, while also keeping background galaxies intact.

Interesting Features in the Image

Deep Hydrogen-Alpha Cliffs and Filaments

Ha comparison.png © Collaborative Astrophotography Team - Both the raw and 1.0 denoised Hydrogen Alpha + continuum w/ Watkins discovery image

The defining achievement of this project is the capture of extended Hα “cliffs” - vertical walls and arcs of ionized gas flaring outward from the north face of the galaxy. Its extraordinary faintness typically places it below amateur detection thresholds. According to the Initial Discovery Paper of the “Cliffs”, the Cliffs seem to be doppler shifted Hα, now lying in the NII emission line. This makes it so standard high-end imaging setups with Hα filters of bandwidth ~5nm or less have trouble trying to catch good signal in the “cliffs”. Our team mainly composed of filters between 4.5nm - 7nm, maximising the percentage of emission captured from the “cliffs”, while also taking very long sub exposure lengths to push against read noise limitations of modern cameras.

Ultra-Faint IFN, Tidal Stream and Galactic Cirrus

© Collaborative Astrophotography Team - Inverted Starless (G) of the final Image

The area around M51 contains a soft, dusty, high-altitude galactic cirrus. These clouds appear as subtle gradients and wisps illuminated by the diffuse glow of the Milky Way. Their low relative brightness meant that it was a mission to reveal it in the processing stage.

Beyond its bright spiral arms, M51 possesses a vast stellar tidal stream that is difficult to capture properly. The deep luminance data reveals a smooth, long stream extending well beyond the disk, with asymmetries that reflect the system’s complex dynamical history.

Dozens of faint background galaxies populate the field, made visible through the deep RGB and L channels. Subtle differences in size, colour, and shape are preserved thanks to the high SNR of the master stacks.

Blue Stellar Stream

© Collaborative Astrophotography Team - Close up crop of starless showing the blue stellar stream

North of M51, tucked along the outer reaches of the Whirlpool’s spiral geometry, lies a faint blue stellar stream that often escapes attention. While it’s not as widely recognized as the striking stream near M82, this feature offers its own quiet intrigue. Its origin remains an open question, though current thinking leans toward two possibilities: a lingering trace from past gravitational interactions, or a ribbon of stars born within a large-scale, low-velocity shock.

In the case of M51, the ongoing gravitational interplay between M51a and its companion M51b appears to create precisely the kind of gentle but pervasive shock that can compress and warm interstellar gas just enough to trigger star formation. That burst of young, hot stars is what gives the stream its distinct bluish tint.

The Dimmest Object we Captured

© Collaborative Astrophotography Team - Galaxy DES J132822.39+474752.2 in the Blue filter

unnamed (6).png After capturing 90 hours of RGB and 70 hours of Luminance data, quite a lot of dim regions and objects are resolved. We want to highlight the dimmest object that we have resolved in this image.

This is a far away galaxy, DES J132822.39+474752.2, is part of a galaxy cluster. The magnitude in different photometric filters is shown above. As you can see it is the dimmest in the U and G filter. The U filter covers a band between 320nm and 400nm. The G filter covers a larger part of the visible spectrum of light between 330nm and 1000nm. So of the RGB filters used the object was the dimmest in B (often a bandpass between 400nm and 500nm). Given this information we believe that we have managed to resolve an object with a magnitude >24! The captured object in the B filter is shown above.

Elliptical Galaxies

Artboard 2.png © Collaborative Astrophotography Team - NGC 5173 and NGC 5198, MATLAS-1539 , LEDA 4012049 and ECO 9882

The region around M51 contains a surprising amount of elliptical galaxies. Above are some we wanted to highlight.

Star Clusters

Artboard 1.png © Collaborative Astrophotography Team - Close up crops of Star Clusters in M51

M51 harbours many beautiful HII regions. Among them bright star clusters shine, we were able to resolve many of these in our image. Here we want to showcase some of these star clusters.

Special Thanks

@Trevor Sweigart, for storing, managing and stacking over 2TB of astrophotography data.
Deep Sky Collective, for providing a coherent pre-processing workflow which we used to integrate our data.
@Mayur Ramudit, for an impressive process overcoming all the challenges in the raw masters.