Comet 3I/ATLAS (C2025/N1 (ATLAS)) is somehow special because it is only the third confirmed interstellar object ever observed passing through the Solar System, following 1I/’Oumuamua (2017) and 2I/Borisov (2019). It was first reported by the ATLAS survey from Chile on 1 July 2025 and rapidly confirmed as an extrasolar visitor on a one-time passage through the inner Solar System.

On 29 December 2025, Comet 3I/ATLAS was at a geocentric distance of approximately 1.84 AU, corresponding to about 275 million kilometers from Earth. At the same time, its heliocentric distance was roughly 1.6 AU, as the comet was already past perihelion, which it had reached in late October 2025. The only 5 km small object is moving through the Solar System at a velocity on the order of roughly 60 to 70 km/s, which corresponds to approximately 180,000 to 252,000 km/h. At the time of my observation on 29 December 2025, the interstellar Comet had an apparent visual brightness of approximately magnitude 15 to 16, consistent with its continued post-perihelion fading as it moved farther away from the Sun. With this background, a classically “beautiful” comet image with a well-developed tail cannot be expected; rather, the primary objective of this observation was simply to detect and record the object at all.

• Rect = 9h 54min 54.2s
• Dec = 11° 11`54.19″
• in constellation Leo (near the 10mag star BD +11 2125)

Changing the OTAs in the Observatory

An interstellar object seemed to be a symbolic first light target for my new arrival: the Celestron 9.25 SCT – a Schmidt-Cassegrain Telescope (SCT) that has been on my wish list for quite a while now, mainly because it is well suited for capturing small and compact objects. A few days ago, I found a used one at quite good a price and decided to take this opportunity. The telescope has now been in my setup for only three days bringing of course many new possibilities ( and of course some challenges).

The first challenge was to adapt the controller equipment for the use of two OTA (optical tube assemblies). Until now the 10″ Carbon Newtonian was the only OTA in the observatory and the control PC (the Intel iNUC) was directly mounted upon the Pegasus Astro Ultimate Powerbox on the tube of the Newtonian. Changing the OTA now meant to change also the iNUC and the Powerbox configuration.

My first idea was to control the SCT with the ASIAIR-controller and slave only the observatory dome to the EQ8-R mount via N.I.N.A. This would of course had been too easy and nice … as two instances would now need to have access to the mount, this option was not possible therefore the system was integrated into the N.I.N.A. setup. The solution was to mount the Pegasus Power Box and the iNUC directly on the steel column and therefore make it available for changed OTAs. For proper cable management, further planning is required; at present, the setup is still somewhat chaotic, and it will likely be necessary to provide each telescope with its own dedicated cabling.

Imaging Setup

The data were acquired with the ASI 294MC Pro, which, while well suited for this task, also revealed several challenges during calibration, particularly in achieving a clean and stable correction of instrumental signatures.

Already with the first test on focussing on the bright star α Orionis (Betelgeuse, 0,42 mag) the C9.25 showed some strange artefacts …Such artifacts can, in principle, have a variety of underlying causes. However, after further investigation, I am now fairly confident that in this case the primary cause was an excessively high dew-heater setting applied to the dew ring in front of the Schmidt corrector plate. This can lead to a corresponding deformation of the Schmidt corrector lens …

When configuring the Lacerta Delta T controller, I did not really think much of the heating level, resulting in a setting of approximately 7 °C above ambient temperature. Under these conditions, the artifacts become very dominant at extremely bright stars. Based on the current data, the transition below which these effects are no longer to see appears to occur at stellar brightnesses of approximately 2.4 mag. Ultimately, only subsequent imaging sessions with significantly reduced heater power will confirm whether this was the dominant cause, or whether additional factors within the OTA contributed to the observed behavior.

The exposures on 3I/ATLAS were planned with 30s subframes at Gain 120 and a sensor temperature of -20°C. Total exposure time was set to 30 min only just for a test first light. Seeing conditions were not optimal but guiding performance was quite stable always below 1 arcsec.

Calibration

Calibration when using the ASI294MC Pro is somehow special. Flats shoul in any casae be taken at exposure times at least over 1 second (1.9s in my case), FaltDarks and Darks are essential. Though my Flats looked quite good, the WBPP-Scipt didnt recognise them as can be seen in the first masterframe (left image) after running through the WBPP-script. Also Vignetting was not corrected. It turned out that the file name was not matching in terms of the filter and WBPP therefore failed to calibrate…after setting the filter parameter equally, the script worked fine.

OSC Comet Processing

Some key points regarding the workflow used:

• SPCC for colour calibration
• Comet Alignment to stack on the Comet
• Protection of the comet core using Fame-Mask
• StarXTerminator to get starless comet files
• Image Integration of the starless comet frames
• ColorSat, Stretching
• bringing together comet and stars in Adobe Photoshop
• slight s-curve

First Light

The first light has indeed turned out to be exactly what one would call a genuine first attempt when using a new setup for the very first time. While the image processing has left clearly visible traces, I am nevertheless quite satisfied for an initial trial and, above all, with having managed to capture this prominent interstellar visitor at all.

Image Analysis

The measured median FWHM of 6.3 pixels, corresponding to about 3.0 arcseconds at 0.48″/px, shows relatively soft stellar profiles and image quality consistent with moderate seeing.

In this session the elevated FWHM is very likely influenced by excessive dew-heater power at the Schmidt corrector, which can negatively effect the outcomes and broaden stellar profiles. Next time I´ll try using only minimal, temperature-controlled dew heating to maintain better image sharpness.