• 23rd May 2024 - Friends Observing

    The Sun - Observatory @2.30pm

    The Observatory will be open for Friends to observe the Sun, safely of course, with white light and hydrogen alpha filters.
    The event is weather dependent, a final decision will be made on the day of the event.

  • 20th June 2024 - Friends Event

    Solstice Sky - Observatory 10pm

    The Friends will celebrate the Summer Solstice at the Dome with a twilight observing session starting at 10pm. Fingers crossed for noctilucent clouds!


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 15th April 2024

  • Astronomical twilight ends at 22:14 BST at the start of the week and at 22:30 BST by the end of the week.
  • Comet 12P/Pons-Brooks reaches perihelion on Sunday. It will be at its brightest, at mag +4.4. Look for the comet in Taurus, close to the western horizon after sunset, which is at around 20:10 BST.
  • Jupiter (-2.0) is sinking ever closer to the western horizon, but is still just visible around sunset. It lies close to comet 12P/Pons-Brooks (+4.4) and under one degree away from Uranus (+5.8) this week.
  • The Moon is First Quarter on Monday.
  • The Sun currently has 7 active regions and the sunspot number is 115.
  • There are no visible evening ISS passes this week.
    (For full details about ISS passes click this link: heavens-above-iss-passes to visit the heavens-above website. If you are not in Marlborough, please ensure that you set your location for the most accurate ISS timings).


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Latest News

What's been going on at the Blackett Observatory

15th April

Outreach Observing: 19 Brownies and 2 leaders from the 1st Aldbourne Brownies visited the Dome with GKWJ. It was a bright sunny afternoon with broken cloud. The session started outside with a naked eye view of the First Quarter Moon, the Brownies discovered that they could easily cover the Moon with just their little finger at the end of their outstretched arm. They were reminded of the dangers of observing the Sun and introduced to Eclipse Specs. They then safely observed the Sun through the specs and discovered that they could easily blot out the Sun with their little finger, just like the Moon! The mechanics of eclipses was discussed. The group then moved inside to the 10-inch, where they first observed the Moon with excellent detail visible against a light blue background. The White Light filter was safely installed and the group saw the full disk of the Sun’s busy photosphere, with 10 sunspot groups clearly visible. The new hydrogen alpha filter was then fitted and the session ended with a spectacular view of the chromosphere and a gigantic prominence that looked like a forest of trees!

14th April

Friends Observing: 12 Friends came to the new ‘Spring Galaxies’ observing session with GKWJ and JAG. A clear twilight sky showed great promise and the session started with a look at Jupiter and its four Galilean moons through the 10-inch. Next up was the 35% Waxing Crescent Moon, with excellent detail showing in craters Theophilus, Cyrillus and Catherina on the edge of the Sea of Nectar. Darkness descended quite rapidly and the hunt for ‘faint fuzzies’ started in Ursa Major with M101, but first a calibration visit to Mizar and Alcor, where Mizar A & B were easily split. M101 was faint and underwhelming, but two Friends had set up live stacking rigs on the observing platform and showed a more detailed view of M101. Next to M81, Bode’s Galaxy, and M82, the Cigar Galaxy, two excellent galaxies in the eyepiece, with even more structure in the live stack images. Another calibration, this time on Denebola in Leo, then on to M87, Virgo A, in Virgo, an elliptical fuzz ball! Finally to M104, the Sombrero Galaxy in Virgo, which was low down and quite hazy, but definite elongated structure with a dark line visible in the eyepiece. The live stack images were beautiful. Patchy passing cloud hindered progress on occasion through the evening, but some fine galactic sights were enjoyed. As we departed, full cloud covered the sky.

30th March

Solar Imaging: The Daystar solar hydrogen alpha filter was finally returned after its refurbishment and upgrade to a Quantum PE housing. The sky had sufficient breaks in the cloud to permit GKWJ to test out the new filter, obtaining excellent views of the surface of the Sun with exquisite detail in the chromosphere and prominences. An image of AR 3615 was captured. The filter appears to be performing far better than ever, despite the sub-optimal sky conditions!

22nd March

Friends Observing: At last, the clouds disappeared for a while, allowing GKWJ to call an impromptu extra Friends observing session. The plan was to try and spot Mercury in the fading daylight and then hunt down comet 12P/Pons-Brooks. 6 Friends and JAG came to the Dome as dusk descended. The 10-inch was slewed to Jupiter, which shimmered in the blue sky, at first with no moons visible, but as it darkened, one moon (Ganymede) popped into view. Next on to Mercury, which was easily visible in the dusk, showing an obvious last quarter phase. Outside, the sky was darkening and Jupiter was very clear by naked eye. As the first magnitude stars started to appear, so Mercury could finally be made out by naked eye, a bright spot sinking towards the western horizon. Both planets were easy to see in binoculars. Once it had become sufficiently dark, the 10-inch was slewed to the coordinates of comet 12P/Pons-Brooks. Sure enough, there it was, a fuzzy patch, clearly a comet. Its location with respect to the field stars was noted. Over the course of the next hour, the comet was followed and a Friend set up a portable imaging rig outside. A live stack of short exposures was created, giving an excellent view of the comet’s tail, which was not visible in the eyepiece. The comet proved tricky to find in binoculars, but was eventually tracked down. An overhead ISS pass, fading in to the Earth’s shadow just past the zenith, brought the session to an end, but not before 12P was observed in the eyepiece one last time, clearly having moved with respect to the background stars. A calculation using the first and last live stack images gave a speed of about 56 km/s for the comet across our line of sight. A wonderful and varied extra session at the Dome.



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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 20.8 mpsas, which equates to Bortle 4.5 and a naked eye limiting magnitude of +6.0. 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.