Orwell Astronomical Society (Ipswich)
Comet C/2025 A6 (Lemmon), 25 September - 15 October 2025
Comet C/2025 A6 (Lemmon) was discovered by the Mount Lemmon Survey in images obtained on 03 January 2025. It is a non-periodic comet and is predicted to make its closest approach to Earth on 21 October 2025 then to pass perihelion on 08 November 2025 at a distance of 0.53 AU (79 million km) from the Sun. It will pass over the Sun in October (it is in a polar orbit) and become an evening object, possibly brighten considerably (perhaps reaching magnitude 3?) in the late October evening sky.
Two members of OASI, Nigel Evans and Andy Gibbs, reported observations.
My first effort at observing the comet was on 25 September, when it was below naked eye visibility. The sky was variable and, as a result, in the video below the somet appears to pulsate!
The forecast for the morning of 30 September was not promising; however, I awoke around 3:00am to find the sky perfectly clear!
I took a series of stills with a 200 mm Celestron Edge HD until nautical twilight. I was pleased with the results of stacking the stills into blocks and making a movie: it showed not only the comet moving across the sky but, for the first time for me, motion within the tail.
I've looped the video below several times as one can't appreciate all the detail in a single viewing. And I've stretched the contrast so much that a Newton's rings artefact has appeared!
Standard processing to produce the still image below blurs the fine detail visible in the tail in the video.
The composite below goes some way to show how structure within the tail evolves over time.
Meanwhile, I was also imaging with a 90 mm f/4.93 Megrez refractor coupled to a colour mirrorless camera. The below image from this equipment reveals a longer tail (although it does not grow longer still by increasing the exposure from 10 minutes to 80!) The Newton's rings artefact is much more clearly visible!
The morning of 01 October was again clear, so I had another imaging session. As previously, I used a 200 mm Celestron Edge HD with B&W camera, and a 90 mm f/4.93 Megrez refractor with a Sony A7S colour mirrorless camera to provide a wider FoV.
From individual frames captures by the Celestron, I was able to create a movie showing the tail changing with time.
Clearer images can be obtained by first summing blocks of subframes. Below is a movie based on this approach followed by a still with multiple panes.
The following image is a stack of all frames; here, variation in frames over time has been averaged away. The multiple-frame versions make it possible to reduce the effects of things like satellites and aircraft.
My ultimate aim in processing the images was to make visible fine detail in the comet. Revealing fine detail in a moving object with a bright head and lively, faint tail is a significant challenge, and presented an opportunity to try some new techniques, consuming several days of my life!
One approach to processing an image to reveal fine detail in an object is to make a copy of the object, blur it (retain only low frequency variability) then subtract the blurred copy from the original, to reveal high frequency variability. However, blurring a bright, featureless object such as a star produces a spread-out version of the object and subtracting the original then produces a result with the appearance of an ugly doughnut. I used the technique in the following video, aiming to mitigate the problem with bright stars and the head of the comet by clipping their brightness prior to blurring and subtraction.
My next approach was to use wavelets, a technique often used on stills made from videos of planets. The wavelet functionality in IRIS (my video processing platform of choice) is complex but, in essence, generates a set of panes, each containing video, with the videos showing different degrees of detail. I then subtracted the video in each pane from the original video to produce a final set of panes, with each element in the set revealling a different degree of fine detail. The following video includes most of the results, excluding the level 1 pane (top left) which is mostly noise.
The ugly, doughnut-like effect around bright objects is an artifact of the "Mexican hat" function that is the wavelet operator. Overall, I am pleased with the result.
As previously, images taken with the Megrez showed colour fringes (after a significant contrast stretch, partly to overcome the problems of ligh pollution associated with living in town). But one can imagine that the tail does cross the wider field of view.
Images from the Megrez also illustrate clearly the problems caused by satellite constellations. The following image shows the passage of the vehicles in Starlink G10-25, launched on 25 September 2025, only six days earlier so all still at low altitude, bunched together and very bright.
There was a bright Moon and some cirrus cloud. The comet was moving at around 6 arcseconds per minute, so I limited exposures to 10 seconds, and captured some 900 frames! I summed the 10 second frames into 1 minute frames and, while doing so, took the opportunity to use the clipping function to remove satellite trails.
The above approach produced the following video sequence which reveals a lovely event in the tail.
Then I applied a Gaussian blurring and subtracted the resulting frames from the original: this produced the "panda eyes" visible in the following video.
The still version below shows the development of the kink in the tail.
Inverted version of the above image.
The wavelet approach (as described above) makes detail of the tail visible at nearly all scales. The video below is suitable for viewing on a very high-resolution screen!
Four minute exposure with a Seestar S50 "smart" telescope.
https://theskylive.com/c2025a6-info.
Nigel Evans