GOTO (telescope array)

The Gravitational-wave Optical Transient Observer (GOTO) is an array of robotic optical telescopes optimized for the discovery of optical counterparts to gravitational wave events and other multi-messenger signals. The array consists of a network of telescope systems, with each system consisting of eight 0.4m telescopes on a single mounting.

As of May 2023 the network consists of two sites, each with two systems. GOTO-N (North) located at the Roque de los Muchachos Observatory (ORM) on the island of La Palma, Spain and GOTO-S (South) located at Siding Spring Observatory (SSO), Australia.

The project is run by an international consortium of universities and other research institutes, including the University of Warwick, Monash University, the University of Sheffield, the University of Leicester, Armagh Observatory, the National Astronomical Research Institute of Thailand, the Instituto de Astrofísica de Canarias, the University of Portsmouth, and the University of Turku.

Telescopes
Each GOTO system can point independently, whilst each unit telescope (UT) has a fixed orientation on the mount so all 8 must be pointed at once. Each UT's pointing is offset from the others to cover the adjacent area of sky, with a small overlap between them. This results in each GOTO system acting as a single large telescope with a very wide field of view (FoV).

The UTs are ASA H400 Newtonian telescopes, each with an aperture of 400mm and a focal length of 960mm (f/2.4). Attached to each telescope is a focuser, filter wheel, and a Finger Lakes Instrumentation (FLI) ML50100 camera, based on the Onsemi KAF-50100 CCD sensor. The fastfocal ratio of f/2.4 and large image sensor result in a relatively large field of view, with each GOTO system having a total FoV of approximately 40 square degrees, around 200x the area of the full Moon in the sky. The fast focal ratio also means that only a small amount of time is needed to observe each area of the sky, with each visit requiring only 3 minutes of exposure time.

Identifying transients
GOTO utilises difference imaging to identify changes of existing objects and the appearance of new transients. Images of the sky are matched to previous observations of the same region, finding the difference between these two images will show only the changes in the new image. Sources within these difference images can then be detected automatically. Using difference imaging in this way produces many thousands of candidate sources per image, the vast majority of which are artefacts of the processing and not real transients. GOTO utilises a convolutional neural network based 'real-bogus' classifier to identify which sources are likely to be real.

Gamma-ray bursts
In addition to follow-up of gravitational wave events, GOTO can respond to detections of gamma-ray bursts.

All-sky survey
GOTO's typical mode of operation when not performing a follow-up campaign is to survey the entire visible sky. As there are sites located in both the northern and southern hemispheres, the visible sky for GOTO is all areas which are visible at night from anywhere on the Earth. If both sites have good weather conditions the entire visible sky can be observed every 2–3 days.

These observations are processed using difference imaging which allows for serendipitous discovery of transients unrelated to multi-messenger events, like supernovae, tidal disruption events, and fast blue optical transients.

History
The first phase of GOTO's development was the deployment of a prototype system located at the planned site of the northern node, consisting of four unit telescopes on a custom-built mount. The prototype system was deployed during the second LIGO-Virgo Collaboration (LVC) observing run (O2), achieving first light in June 2017 with its official inauguration on July 3, 2017.

The prototype system was active during the first half of the third LVC observing run (O3a), which ran between April and October 2019. During this time GOTO was able to respond to gravitational-wave events and begin observing within one minute of alerts being received (if the source region was visible).

In late 2019 funding was awarded to expand the network with two full GOTO systems a duplicate site in Australia. In 2020 the first full system of the northern node was being deployed, with the second system planned for early 2021 and the Australian site planned for later that year.

The deployment of the second northern system was completed in August 2021 and, despite delays due to the 2021 volcanic eruption, the full northern node was completed in December 2021 with the upgrade of the prototype to the final hardware configuration.

By the end of 2022 the site for the second GOTO node (GOTO-S) had been prepared at Siding Spring Observatory (SSO) and the two domes installed. In May 2023 it was announced that both systems at SSO had been successfully installed.

Discoveries
As of June 29, 2024, data from GOTO has been used in the discovery of 562 astronomical transients, of which 88 have been classified as supernovae and one as a tidal disruption event.