Cromwell Tower Observatory

The Cromwell Tower Observatory (CTO) is the University of Aberdeen's historical observatory. It is at the top of Cromwell Tower in the northeast corner of King's College quadrangle. The photo to the right shows King's College quadrangle from the west side. Cromwell Tower is the building with the flagpole on top, and the observatory occupies the top floor.

The observatory was founded in 1826, and fitted out with its first set of refracting telescopes in the 1860s. These instruments are now part of the University of Aberdeen's historical collections. For many years the main use of the observatory was meteorological rather than astronomical. The history of astronomy at Aberdeen goes back some 400 years, and the CTO itself has a long and interesting history. Both the history of astronomy and the CTO will be expanded upon elsewhere on the Physics website. For now, you can read a history of the CTO at John Reid's webpages.

Layout

The observatory consists of two observing domes. In the photo to the right, the two cone-shaped domes at the bottom are on the roof of the CTO. The south dome is on the left and the north dome is on the right.

Within the observatory, there is a circular observing platform for each dome, which instruments can be mounted on. These extend from the floor up to about 2.7m height (the south dome platform is slightly lower), at or just below the roof. Surrounding each platform is a circular pier that observers can stand on, approximately 1.5m above the level of the room, which is accessible by steps. The images below show scale drawings of the interior made around 2000 by the Aberdeen Astronomical Society (AAS).

2000s refurbishment

By 2000, the only telescope to have been under the domes in recent years was a simple 8" reflecting telescope. In June 2000 a refurbishment programme led by Dr John Reid began, aiming to bring the observatory back into astronomical use with up-to-date reflecting telescopes.

This was no easy undertaking as a lot had to be done to make the observatory suitable for use. The images to the right show the interior of the CTO before refurbishment started. The first task was to empty the room entirely, which involved sifting through and clearing out hundreds of journals and papers left by previous users, moving old equipment, dismantling furnishings, and so on. Contractors were then called in to fix the roof and windows, lubricate the observing domes, and redecorate the interior.

Unfortunately, however, the observatory was found to have a persistently leaking roof. To repair this, along with the rest of the CTO and the roof stairs, would be prohibitively expensive. Other difficulties included risks of structural damage due to prolonged water exposure, the CTO being unsuitable for modern accessibility requirements, limited space to install larger modern telescopes, and floodlighting installed to illuminate King's College facade, which has affected the quality of astronomical observation and considerably reduced the usefulness of the observatory. So the refurbishment was ultimately unsuccessful, and due to structural deterioration the CTO has not been in a functional state since then.

We plan to build a new observatory in the Cruickshank Botanic Garden during 2025, subject to planning consent.

The 10" telescope

The existing CTO telescope is a Meade "Classic" LX200 10" Schmidt-Cassegrain telescope (25.4 cm primary mirror, 2.5 m focal length, f/10). This was installed during the 2000s refurbishment, and at the time was a very popular commerically available instrument.

The image to the right shows Dr John Reid with the 10" telescope under the south dome of the CTO, circa 2000.

Restoration of the telescope

In 2023 the 10" telescope was removed from the CTO to prevent its further deterioration, and repaired and restored by BSc student Christopher Willett under the supervision of Dr Charles Wang.

Initial work was on the electronics, which had not worked for at least 10 years, with the optics to follow later. In principle the telescope could still be used manually if the electronics could not be repaired. The problem was identified as a short-circuited CPU capacitor that had resulted from an early manufacturer's modification to the "Classic" LX200 model to make it run on 18V rather than 12V. The image to the right shows the installation of the replacement component.

Restoration of the optics followed next. Most concerning was whether the actual optics had been destroyed by being left exposed to extreme weather cycles in the CTO for up to 20 years with no heating or air conditioning. The telescope tube was disassembled and a grainy deposit carefully cleaned off the Schmidt corrector plate (the lens at the entrance to the telescope tube). The images below show the lens before and after cleaning.

With these repairs done, the telescope was tested first in the Meston Building and later at Sheddocksley Sports Centre playing fields. The AAS kindly loaned two Akit 4000 CCD cameras, and Stellarium was installed for telescope tracking, Sharpcap for taking photos, and Astrosurface for image editing.

The tests identified a problem with focus or collimation, which was fixed thanks to some additional help from the AAS. One of the three collimating set screws had come out of its housing and was not moving the secondary mirror at all when adjusted. A final fix to the electronics was required when the DEC motor encoder blew (which had been repaired in the past but not replaced correctly).

Observations with the restored telescope

Once restoration was complete, the telescope was able to take some fantastic images, a couple of which are shown below.

Top image: Copernicus crater on the Moon, imaged from Westhill on 24 December 2023. The image is enhanced using the Astrosurface programme by taking multiple images of the same object and then digitally "stacking" them together to improve the signal-to-noise ratio.

Bottom image: Jupiter and the four Galilean moons imaged from Westhill on 26 December 2023. After stacking, the image has been enhanced by modifying the RGB colour balance, saturation, brightness and contrast to produce a finer image without altering or adding elements to the original image.

Sources:

• Dr John Reid's Cromwell Tower Observatory webpage.
• CS Willett (2024) "A History of Astronomy: From the 17th Century to the Cromwell Tower Observatory, Incorporating Recent Work on the Telescope in Aberdeen", BSc Physics project thesis, University of Aberdeen, Chapters 6-7.

Image credits (from top): Dr Roland Young; Timothy Wang; Aberdeen Astronomical Society; Dr John Reid (x3); Dr Charles Wang and Christopher Willett; Dr Charles Wang (x3).