Orwell Astronomical Society (Ipswich)
Comet C/2025 A6 (Lemmon), 25 September - 01 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.
My first effort at observing the comet was on 25 September with the comet below naked eye visibility. The sky was variable so, 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. In stacking the stills into blocks and making a movie, I was pleased with what I could see. Not only does the the comet appear to move across the sky but, for the first time, I have recorded 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 for the still image below tends to blur all the fine detail.
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 nice movie sequence showing the tail changing with time.
Clearer images can be obtained by first summing blocks of subframes. Below are 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.
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.
A finder chart for the comet is available at https://theskylive.com/c2025a6-info.
Nigel Evans