Orwell Astronomical Society (Ipswich)
Comet C/2025 A6 (Lemmon), 25 September - 02 November 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 comet 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!
In electronics we are familiar with breaking a complex waveform into a series of simpler (sine/cosine) waveforms via a Fourier transform. For pictures a familiar equivalent is the jpeg algorithm, used to compress file size. But there are other formulations, one of which is called wavelets. Rather than being infinite in extent like sine/cosine, these functions are space-limited and resemble a Mexican hat. The wavelet formulation decomposes a picture into several panes, each showing detail of differing spatial frequency ranges, from the high frequency noise-like structures, down to the big broad low frequency fuzzy detail.
For the comet I did not know if there was an optimum set of frequencies, so I used the lot! For each still frame six panes were generated, each containing detail of a limited frequency range. Then I created six images by adding each pane one at a time, to create a series with ever more coarse detail within. Then, of course, I created a movie from each one! The first pane is just full of the noise detail, so instead I include the original image.
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!
The Megrez refractor and colour mirrorless camera revealed a tail extending more than 3°. Individual exposures were limited to 15 seconds due to the background sky brightness caused by the Moon. The tail is only approximately 1% of the sky brightness and some artefacts crept into the image: they would not have appeared in a truly dark sky!
The comet has now moved so as to be not visible from my observatory. The night of 24 October was sufficiently clear to try to record the comet from my front garden with a standard camera lens. The following image, taken with a Sony A7S camera on a driven mount, shows the comet with a prolonged ion tail. Unfortunately this camera did not have a light-pollution filter, and increasing the exposure did not produce any real improvement. (My garden is a relatively light-polluted setting.)
The following image, taken with a second A7S camera on a driven mount, this camera fitted with a light pollution filter, unfortunately does not offer much improvement. To record the faint tail, you really need to be in the countryside, far from the street lights!
Besides the two driven cameras, I had a static camera to record the motion of the comet. I limited exposures to five seconds to prevent blurring due to the rotation of the Earth. The following video shows the comet moving towards the horizon. The ever-present Starlink satellites give the impression that the sequence was recorded over Baghdad in 1991!
To stack a set of images on the comet, it is necessary to rotate as well as shift each subsequent frame. The following is a stack of 30 frames (lens distortions prevented any more frames being added.)
By 02 November, the comet had moved sufficiently to be visible again from my observatory! However, the night was moonlit, the comet was at an altitude of only approximately 15°, above the light pollution caused by Ipswich and observing opportunities were limited between the sky becoming sufficiently dark and the comet being hidden by my neighbour's trees.
Four minute exposure with a Seestar S50 "smart" telescope.
Image captured with a Seestar S50 "smart" telescope mounted on one of the balconies at the observatory. Taken during the first eveving of OASI's 2025 open evenings.
Image captured using a Nikon D850 with 500 mm f/5.6 lens at f/5.6, ISO circa 2000, exposure circa 12 s.
The image on the LHS was captured with a handheld iPhone, 4 s exposure. The image on the RHS was taken with a Dwarf 3, 100 two-second frames.
iPhone
Dwarf 3
https://theskylive.com/c2025a6-info.