This project is concerned with detection and characterisation of optically variable sources in the sky, and does not discriminate between natural (e.g.stars/galaxies) or artificial (e.g.satellites/space debris) sources. The DT is sensitive on timescales of seconds to months to all variable and moving sources.

Astrophysics: Many important and often violent astronomical objects are characterised by explosive events which result in unpredictable and rapid optical brightening. At the heart of many of these explosions are compact stellar remnants that can display surface explosions, detonate completely in supernovae, merge violently and drive relativistic outflows and jets. Their short dynamical timescales leads to large-scale variability on astronomically short timescales. Of particular interest in terms of the physics that drives these events is the very early stages, where most of the current uncertainties lie. Whilst we have developed a reasonable understanding of, for example, the nuclear-powered ejecta of SNe that evolve on timescales of weeks to months, the core collapse, formation of the remnant and initial production of collimated particle and emission outflows and shocks remains elusive (Waxman and Katz 2017). This phase is not just theoretically challenging, but it is difficult to secure observations since our strategies rely on responding to events once they have been spotted, usually during their peak brightness stages. Traditional survey astronomy approaches require the cadence to be optimised for a specific timescale of variability and at the expense of sky coverage (Bellm 2016). Serendipitous coverage (e.g.the detection of a GRB in field monitored by the TESS satellite (Fausnaugh et al. 2023)) is rare. The DT sidesteps this problem by offering instantaneous detections of any associated optical signals and being able to cover a wide dynamic range of variability timescales in one go. The next decade will see a step change in coverage of the variable EM sky with facilities such as SKA, CTA, VRO and many high-energy space missions. The DT offers a crucial capability to this multi-wavelength and indeed multi-messenger era.

Space Domain Awareness: Since the dawn of the space age we have launched∼10⁴ satellites, of which approximately half are still functioning. However, with the advent of the so-called mega-constellations, we are entering a new era where near-space will contain >10⁵ operational satellites (and many non-functioning spacecraft). Currently our best efforts have led to a capability to track <3% of the satellite killing debris population (Tarran 2021). This situation will further deteriorate in the future, as more satellite launches occur, and more debris accumulates (ESA Space Debris Office 2022). Most of these resident space objects (RSOs) are in Low Earth Orbit (LEO) and are visible through reflected sunlight for only a few hours after sunset or before sunrise, but RSOs in orbits of >1000 km can be visible through the entire night during summer months. Some satellite operators are planning to deploy satellites to these higher altitudes (e.g.OneWeb) to avoid the congestion of lower orbits. As use of the near-space environment is essentially unregulated, the main challenge in this area is monitoring and characterising the population of RSOs which can be achieved through orbit determination and light curve analysis (e.g. Chote et al. 2019, Shrive et al 2024). From an astronomical perspective, the advent of the mega-constellations complicates traditional wide field astronomy as images will often be contaminated by trails from satellites (Hainaut and Williams 2020).

Traditional astronomical surveys have used a single (or small number of) individual telescope/detector systems to survey the sky serially: the exposure time is fixed, and the sky coverage grows with time as tiles of the sky are observed. The Digital Telescope takes a different approach: the sky coverage is fixed (and is the entire visible sky for the envisaged full system), and the sensitivity grows with time based on the number of exposures that are stacked. This allows the observation cadence and sensitivity limits to be determined on an object-by-object basis depending on its brightness and expected variability timescales.This makes for a significantly more versatile instrument and allows the same data set to be used for a much wider range of science cases.While the concept of a DT has grown from our work in the SDA area over the last few years, it has been inspired by other astronomical facilities. MASCARA (Talens et al. 2017) was designed to search for bright, naked eye, host stars with transiting exoplanets. This instrument was composed of 5 short focus camera lenses equipped with (interline) CCD cameras, all mounted on a single stationary structure. The tiled field of view covered the entire visible sky. Despite the challenges involved (e.g.eclipses with depths of<2%), MASCARA was successful in finding candidates and a handful were shown to be exoplanet hosts (Talens et al. 2018). MASCARA data was also used to track some bright LEO satellites (Flohrer et al. 2019). Evryscope (Ratzloffet al. 2019) has an 8000-square-degree imaging system composed of 24 separate optical systems with an angular scale of 13 arcsec per pixel, reaching 16th magnitude (3σ) in a single exposure (sky limited) with a 120 sec cadence. It is built on a movable mount which sidereal tracks for a 2 hour period and then resets. Evryscope generates data for 100–150 million targets per night and recently detected the flare event on Proxima Centauri (Howard et al. 2018). Evryscope has been used as a test-bed for the more ambitious 900–1000 telescope concept called the Argus Array (Law et al. 2022a,b). This is more comparable to our DT concept as it uses 20 cm optics but these telescopes are again located on a rotating platform. Argus is currently at a prototype phase with funding obtained for the construction of a 38 telescope system. The full Argus array when it is built will be more expensive and complicated; the DT system has much simpler mounting and enclosure hardware, lowering the cost and complexity, and allowing staged construction. At the heart of the DT concept is ability to rapidly read out large format, low noise CMOS detectors and their availability at consumer prices.Scientific CMOS detectors have high quantum efficiencies and low read out noise. When incorporated into a camera, CMOS sensors are often used to take imaging data similar to that from CCD detectors and they are often consider similar to these devices. In fact CMOS and CCD detectors have quite different characteristics and noise qualities.In the DT, detectors are attached to stationary telescopes and images read out continuously at sub-second cadence. As celestial objects will trail at sidereal rates through the field of view, exposures times can be matched to effectively freeze the motion to one pixel - maximising the signal to noise (the angular pixel size depends on the optical system and detector characteristics). Given this, then we track the sidereal motion of sources in software instead of using a mechanical mount. Improved sensitivity can be achieved through “stacking” images (possible due to the low noise of the CMOS data).