LUAstro Newsletter - December 2025
Very large telescopes, very small telescopes, and a plurality of Pleiades
Welcome! I’d like to start this month’s edition by congratulating all of you who have successfully made it through the Michaelmas term - it’s now over! Whilst the holiday is, of course, a chance to recap, collate, and revise, do make sure to take a proper break as well, you all deserve it.
This month, we have a fantastic piece by the department’s very own Professor Isobel Hook on her recent trip to Chile, alongside an excellent collection of recent images, and a demonstration of some potential society-led science by Finlay. I hope you enjoy.
Pleiades, Pleiades, Pleiades…
Despite there being 110 Messier catalogue objects and over 7,000 NGC objects to choose from, at least four members of the society decided to spend a considerable chunk of December imaging one of the most commonly targeted and unoriginal, yet stunningly beautiful objects visible in the night sky - the Pleiades star cluster. Known to almost all cultures on earth spanning the past several millennia, the cluster is known by many different, including the Welsh “Tŵr Tewdws” (fat tower), the Maori “Matariki” (mother), and the Japanese “Subaru” (to come together).
Interestingly, the Greek and Aboriginal mythologies surrounding the Pleiades are strikingly similar (they both centre around the idea that Orion is a male hunter who attempts to chase the stars of the Pleiades, personified as six young women). Most notably, they both refer to the cluster as the “Seven Sisters”, despite there only being 6 visually discernible stars in the main cluster, and feature explanations for the disappearance of this seventh “lost Pleiad”. By considering the proper motion of Pleione (one of the stars in the cluster), it can be shown that the Pleiades would in fact have appeared as seven distinct stars to most humans before 100,000 BC. It is therefore likely that the origin of these myths predates our ancestors first leaving Africa and is of an age comparable to that of the human race itself. It seems the stargazers have been at it since the dawn of time…
While transiting the Pleiades cluster on 04/12, the moon presented a 22° halo. Obscured by cloud, the sheer brightness of the full moon leaked through, resulting in this haunting haze.
Galaxies and Other Nebulae
Comets
Finlay has photographed a couple of comets this term, starting with the brilliantly bright C/2025 A6 (Lemmon) in late October and the infamous 3I/ATLAS at its closest approach on the 19th of December.
Even at its closest approach, 3I/ATLAS was so faint that two hours of data had to be stacked to reveal it on a 50mm aperture telescope. As the object does not move at sidereal rate, the background stars trail.
Boxing Day Astronomy
By Finlay
I have been increasingly interested in using telescopes for quantitative data collection, rather than simply producing images. For my first light curve, I chose a variable star with a short period, enabling me to collect enough data within one night. AE Ursae Majoris is such a star that peaks every couple of hours and remains high throughout the night.
On the morning of the 27th of December, I took 1,174 10s exposures of AE UMa between 03:21 and 07:33. To increase the signal-to-noise ratio, I stacked every 20 frames together. Using the Siril software to perform photometry on AE UMa, I subtracted changes in brightness based on four similarly-bright reference stars, removing any uncertainty from sources like clouds briefly passing and obscuring the star.
I plotted my results, and was intrigued to see that the right side of the figure shows an increase in measurement uncertainty during dawn, as the image backgrounds became brighter. I am satisfied with this first light curve, and am eager to produce more next year.
A tour of ESO telescopes in Chile
By Professor Isobel Hook
Professor Hook’s research is focused on using distant supernovae for cosmology, specifically to measure the effects of Dark Energy in the universe. She is also involved in telescope projects including ESA's Euclid mission, the future Rubin Observatory that will carry out the Legacy Survey of Space and Time (LSST), the 4-metre Multi-Object Spectroscopic Telescope (4MOST) and the European Extremely Large telescope (ELT). Here is a piece which she kindly wrote for us discussing her most recent trip.
In early November I travelled to Chile as a member of the 2025 Visiting Committee of the European Southern Observatory (ESO). ESO is an inter-governmental organisation that builds and operates telescopes on behalf of its 18 member states. Our task was to review ESO’s operations and make recommendations for improvements.
Our first stop was Paranal Observatory in the North of Chile. This is a remote location in in the Atacama Desert, with red/brown, dry land as far as the eye can see. The observatory is at an altitude of 2600m – not super high but enough to feel out of breath if you try to walk fast up stairs! There are four 8m-diameter telescopes at Paranal that make up the Very Large Telescope (VLT). We went inside one of the domes at sunset and saw the telescope being prepared for night-time observing. The MUSE instrument is attached to the side of the telescope, looking like a tangle of tubes and wires. These are the instrument’s cooling system and electronics. Inside the box is one of the most impressive astronomical instruments in the world – an “integral field spectrograph” that can take a spectrum of every point in an image over a 1 arcminute square field of view.
While at Paranal we visited the nearby site where the Extremely Large Telescope (ELT) is being constructed. For me, this was the highlight of the trip. The telescope has a diameter of 39m, and it is really taking shape – all of the steel structure is in place for the telescope and dome. There are no mirrors installed yet, but the scale of the telescope is clear to see. What really surprised me is how high the primary mirror is from the ground. We were viewing the telescope from12m (about 4 storeys) up, which already felt quite high, but the underside of the primary mirror structure was way above us. This is particularly impressive when you realise that every day during operations, two of the roughly 800 hexagonal mirror segments will have to be taken out of the primary mirror for re-coating, and replaced with freshly-coated ones. These delicate segments will have to be handed over between cranes and taken to and from the coating chamber at ground level.
The final stop of our tour was the ALMA observatory. This is an interferometer that operates at sub-mm wavelengths (very short-wavelength radio waves). There are 66 dishes in the array, with diameters either 12m or 7m. The telescopes’ signals are combined electronically by a correlator computer. The main thing you notice when visiting is the altitude – ALMA is at 5000m. We all had to pass a medical check before going up, and we were given supplemental oxygen that to breathe from tanks in our backpacks. The site is also very dry, which is important because water in the atmosphere absorbs radiation at the wavelengths of ALMA’s observing range.
Overall it was a wonderful trip. I hope to go back one day when the ELT is finished!
That’s all from us
We hope you’ve enjoyed this month’s edition, and that you’ve also enjoyed our first term. All of us on the committee have had a great time, and we’re already looking forward to next year’s socials and observing nights. In the meantime, please continue sharing your images, and feel free to get in touch with any ideas or suggestions.
From all of us here at LUAstro, have a wonderful new year!
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