BLACKETT OBSERVATORY
Marlborough College
Welcome to the Marlborough College Blackett Observatory, home to the largest refracting telescope in Wiltshire. The Observatory is a key facility for the study of Astronomy by pupils at Marlborough College. It is open to the public through the Friends of the Marlborough Telescope membership scheme. School and Society visits are welcome, please contact the Director to arrange your visit.
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What's Up!
Week of 17th August 2026
- Astronomical Twilight ends at 22:45 BST at the start of the week and at 22:25 BST by the end of the week.
- Venus (-4.3) sets at around 21:30 BST this week; spot the bright planet in the western evening sky. Saturn is now an evening object, rising at 21:58 BST. Early risers can enjoy a view of Mars in the east as it rises around 01:22 BST.
- We look to the eastern sky for our Deep Sky Challenge this week, starting with the open cluster Messier 52 (+6.9) in Cassiopeia. Then down to our first view of M31, the Andromeda Galaxy (+3.4), this season. Finish with perennial favourite the Double Cluster (+5.3), catalogued as NGC 869 and NGC 884, in Perseus on the border with Cassiopeia.
- The Moon is First Quarter on Thursday.
- The Sun currently has 8 active regions and the sunspot number is 111.
- There are no visible evening ISS passes this week (click here for full ISS pass details).
Click here for the What’s Up! archive
Latest News
What's been going on at the Blackett Observatory
22nd June
Radcliffe Society: Traditionally, the summer meeting of the Radcliffe Society is our opportunity to launch rockets on the sports fields by the Observatory. However, ‘adverse atmospheric conditions and excess precipitation’ meant that Mission Control was forced to cancel all launches. Instead, 6 pupils came to the Dome and discussed all things Space and Astronomy with GKWJ, touching on the eclipse on August 12th, the trouble with near light speed space travel, and the dark universe. The group also enjoyed their final KitKats and hot chocolate of this academic year.
21st June
Friends Observing: 13 Friends joined GKWJ at the Dome for the Summer Solstice Celebration observing session. Hazy twilight meant that we started the session with a view of the First Quarter Moon in the 10-inch and the 127mm Maksutov that a Friend set up outside. We moved on to Venus, showing a clear gibbous phase. As the twilight slowly edged towards almost darkness and stars started to appear, we moved to the Double Double in Lyra, at 95x magnification, an obvious double, then at 140x and 220x, becoming an easy split Double Double. The theme of double stars continued with Albireo, then followed by a quick stop at M57, there, but very faint in the not dark sky, on to M13, a better sight, but lacking good contrast. Back to doubles, with Alpha Herculis, Rasalgethi, an easy split, and Zeta Herculis, a much tougher challenge. By this time some had departed, leaving the hardy few to keep going towards midnight, rewarded with a clearing and darker sky, we went to ‘the other globular’ in Hercules, M92, which was stunning, so much so, that we returned to M13, which now showed as the punchy ‘sprinkling of salt’ in the eyepiece. Then the challenge really started, with a Friend suggesting that we try for the Veil Nebula in Cygnus; in a first for the 10-inch, at least in recent times, we found the clear hazy veil structure of ‘The Witches Broom’, the western Veil Nebula, which really ‘popped’ with the OIII filter. Our final target was NGC 6826, the Blinking Planetary Nebula, a clear planet like ‘blob’ through the OIII, then with no filter, its name became apparent – stare straight at the object and you see the small, bright, central white dwarf star piercing through, but when you look to the side, the white dwarf vanishes and averted vision reveals the fainter, larger nebula, flick between the two views and the white dwarf blinks on and off. A fabulous, fun evening on the ‘worst night for astronomy’!!
19th June
Solar Observing: Two teachers visited GKWJ to discover the secrets of teaching Astronomy GCSE as they prepare to offer the course at their schools next year. The visit included a Remove Astronomy lesson held at the Observatory for some solar observing. The photosphere was seen in the Celestron 8-inch with a white light filter, the Lunt 50mm showed the full disk in hydrogen alpha, while the 10-inch, also with the hydrogen alpha filter, was zoomed in to reveal detailed structure in a number of prominences. The spectroscope was also used to see the solar spectrum with clear Fraunhofer lines.
10th June
Promo Filming: GKWJ assembled a group of 6 Remove pupils at the Observatory for a film shoot to be part of the latest promotional video for Science at the College and the refurbished Science buildings. The group delighted at views of the Sun with the hydrogen alpha filter in the 10-inch, with a range of crisp prominences and active regions on display. The film crew delighted at their first view of the Sun in such detail! The final film should be out in the autumn.
All Sky Eye
The Latest View from the MCBO
The All Sky Camera operates from 30 minutes after sunset through the night until 30 minutes before sunrise. The latest image is updated automatically every 5 minutes and the time lapse video is refreshed each morning at around sunrise. Click the still image to view a large version
Latest Still Image
Most recent time lapse video
Sky Quality
The Latest SQM Chart from the MCBO
Thanks to generous funding from the North Wessex Downs National Landscape, the Blackett Observatory now has a fixed Sky Quality Meter (SQM) in place, measuring sky darkness every night throughout the year.
This is the latest chart of SQM readings, measured in magnitudes per square arcsecond (mpsas). The darkest reading to date is 21.1 mpsas, which equates to Bortle 4 and a naked eye limiting magnitude of +6.3. The sky will now be monitored constantly to see how the effect of light pollution is changing over time. A project to monitor sky quality across our local area is underway using NWD funded handheld SQM devices and volunteers from the Friends of the Marlborough Telescope.
The SQM chart updates every 15 minutes through the hours of darkness.
The yellow circles show the altitude of the Moon, if present.