Intro, Abstract

On the night of April 28, 2025, an ambitious deep-sky imaging project set out to capture the faint supernova AT 2025ilo in the distant Coma Cluster. What began as a challenging hunt for a 20.6 magnitude point of light turned into a night full of discoveries — from resolving the supernova itself to uncovering an unexpected asteroid (2000 JJ18) crossing the field. This report takes you through the planning, challenges, surprises, and final rewards of an unforgettable astrophotography session, revealing the hidden treasures of one of the richest galaxy clusters in the universe.

Location

On April 26, 2025, AT 2025ilo was discovered in the Coma Cluster, specifically in the massive elliptical galaxy NGC 4874. The host galaxy, and thus AT2025ilo, is located in the constellation Coma Berenices, a region rich in galaxies and part of the well-known Coma Cluster.

The „nearest“ brighter star is HD 112887, a 7.57mag F5 galactic star, located approximately 270 light-years (parallax of 12.1302 mas) away within our own Milky Way. It appears close to the position of NGC 4874 in the sky. This is a purely visual alignment: while the star is relatively close by cosmic standards, supernova 2025ilo and its host galaxy NGC 4874 reside over 320 million light-years away in the distant Coma Cluster. Such line-of-sight coincidences are common in astronomical observations and highlight the vast scales involved when we observe the universe.

With an apparent magnitude of 20.6 AT2025ilo (AT stands for „Astronomical Transient“, so to be precise not sure a supernova…) is an extremely faint object. The faintness of this object poses observational challenges but the conditions on April 28, 2025 are not that bad (seeing about 2,6″, no wind, no clouds).

Of course, utilizing my 10-inch Newtonian that has been reduced to f/3 — resulting in a focal length of 750 mm — is not an ideal setup for locating such an extremely faint object. Nevertheless, an attempt will be made to capture the 20.6 magnitude „bright“ supernova. Despite the optical system being optimized more for wide-field imaging rather than deep, high-resolution observations of such faint targets, long exposure times and precise calibration may still allow the supernova to be detected.

Image Planning and first single frames …

Using the Simbad and Aladin databases, and based on the precise coordinates (R.A. 12:59:35.043 and Dec. +27:56:58.60), the target position was accurately determined. This reference served as the basis for both the framing of the field and the subsequent evaluation of the acquired images, ensuring that any potential detection of the supernova would be correctly aligned with its expected location.

The first test Luminance-exposure, taken with an integration time of 300 seconds, unsurprisingly revealed … absolutely nothing at the expected position of the supernova.

At this point, I was almost tempted to switch to live stacking instead of letting the imaging session run through the entire night — a session that would only last about six hours anyway. Live stacking would have provided immediate feedback, sparing me from waiting until the next morning, after normalization, calibration, and integration, to find out whether the supernova had actually been captured. However, I decided to proceed with the full overnight session. The plan is to acquire 36 frames through the luminance filter, totaling three hours of exposure time, followed by an additional hour each through the red, green, and blue filters to complete the dataset.

Even after applying an aggressive stretch to a single raw frame, no significant signal attributable to the supernova could be identified. The extreme stretching mainly amplified background noise and minor sensor artifacts, without revealing any distinct structure or point source at the expected coordinates …or is there actually something to see on the inverted single frame?

… a wide Galaxy Cluster

The patience to image throughout the entire night — frame by frame, hour by hour — and the care taken in the meticulous processing of the data clearly paid off. From precise calibration and alignment to careful noise reduction and contrast enhancement, every step contributed to bringing out the full potential of the data.

The result speaks for itself: fine details, faint structures, and subtle contrasts that would have remained hidden without the time and effort invested. Nights like these are a reminder that deep-sky astrophotography is as much about perseverance and craftsmanship as it is about technology — and when all of that comes together, the reward is a final image that captures both the beauty and depth of the universe.

The „wide-field image“ (with a field of view of 108 x 72 arc minutes) reveals a stunning section of the sky, deep within the Coma Cluster. Take a moment to enlarge the following image — countless faint galaxies await discovery in every corner of this breathtaking cosmic panorama.

The Coma Cluster, also known as Abell 1656, is one of the richest clusters of galaxies in the nearby universe. Located about 320 million light-years away in the constellation Coma Berenices, it contains thousands of galaxies bound together by gravity. Most of these galaxies are elliptical or lenticular, older types with little star formation, glowing faintly in reddish tones. Spiral galaxies, like our Milky Way, are rare here, shaped and stripped by the dense cluster environment.

The cluster’s total mass is dominated by dark matter, invisible but detectable through the gravitational effects it exerts on its surroundings. Between the galaxies, an extremely hot gas fills the space, emitting X-rays and revealing the violent past of countless galactic collisions and interactions.

The Coma Cluster is part of the even larger Coma Supercluster and serves as a key site for studying galaxy evolution, dark matter, and the vast structure of the cosmos. Each glance into this crowded region of space is a look into the grand, dynamic history of the universe itself.

A 20.6 mag Supernova resolved and a bonus feature!

The plan was to capture 36 frames through the luminance filter, totaling three hours of exposure time, followed by an additional hour each through the red, green, and blue filters to complete the dataset. This imaging strategy was carried out exactly as intended and yielded truly impressive results.

Honestly, I had not expected to resolve the supernova so clearly, given its faint magnitude of just 20.6. The seeing conditions during the night were quite good, with a stable value around 2.6 arc seconds. The precise execution of the imaging plan meant that every single frame captured during the night turned out to be usable — a rare and highly satisfying outcome in deep-sky astrophotography.

Thanks to the hint from a colleague at the Styrian Astronomical Society (STAV) — many thanks to Gerhard Balda, an expert when it comes to spotting extremely faint and often tiny objects — I was able to discover an unexpected extra : an asteroid! Gerhard had also photographed SN 2025ilo that same night from his location in Trahütten (Koralpe) and noticed the Asteroid 2000 JJ18.

In my integrated image below, the asteroid appears as nothing more than an extremely faint shadow — barely noticeable at first glance. Only when comparing the individual frames does its slow movement become clear. It’s one of those subtle details that would easily go unnoticed without knowing exactly where to look.

A Deeper Look at 2000 JJ18

Asteroid 76719 (2000 JJ18) was discovered in 2000 and is cataloged by the Minor Planet Center. By entering the coordinates (R.A. and Dec.) or the field of view, the MPC can generate a list of objects that are located within the image frame.

2000 JJ18 is classified as a main-belt asteroid, meaning it resides in the vast region between Mars and Jupiter where the majority of known asteroids orbit the Sun. 

Within the Small Body Database Lockup (link), even the orbit is visualized:

To detect an asteroid in a sequence of images, it’s recommended to perform an image integration in PixInsight with the Rejection Map enabled. During integration, any object that moves across the frames — such as an asteroid — is excluded from the final stacked image and will appear only as a faint trace or not at all in the integrated result.

The Rejection Map, however, highlights such moving objects by marking the pixels that were discarded during stacking, making it a useful tool for spotting subtle transient features like asteroids.

In the end

Capturing nights like these show just how rewarding patience and dedication in astrophotography can be — from the right exposure strategy and the unexpectedly good resolution of this faint supernova, to the surprising discovery of an asteroid quietly crossing the field. These images is more than just the results of hours of exposure and detailed processing; they are a visual record of cosmic events playing out far beyond our own solar system, captured thanks to patience, precision, and the right conditions.

The luminance, color data, and the given seeing conditions combined to reveal delicate structures, while the rejection map in PixInsight helped bring the hidden motion of a small asteroid to light — a subtle detail that might have gone unnoticed without the helpful eye of a fellow observer.

Together, these elements create a rich narrative: a fleeting stellar explosion, a passing fragment of our solar system, and a vast galaxy cluster in the background. To witness and document all of this in a single night is a rare privilege — and a powerful reminder of how much the night sky still has to offer when we take the time to look closely