Luminary

A Luminary is an incredibly precise type of Covenant sensor reverse-engineered from a Forerunner equivalent.

During the events that led up to the Human-Covenant War, the Covenant's final Hierarchs realized that Luminaries detected humans, marking them with a symbol previously thought to mean "Reclamation" but actually translating as "Reclaimer". This was revealed to the would-be High Prophets by the shard of the Forerunner AI Mendicant Bias within the Keyship Anodyne Spirit.

Design and functionality
A Luminary is a giant mechanical compound eye, composed of billions of sand grain-sized cells. Each cell is a different kind of sensor. There are dedicated sensor cells for everything from radio waves to gamma radiation to gravimetric and even more exotic sensors. Each cell is also capable of isolating wormholes from the quantum foam and expanding them to the point where a few photons can travel through. These sensor cells' resolution is limited, but they are able to look through the wormholes. If the other end of those wormholes are scattered through a volume of space the size of a planet, that means that the Luminary functions as a telescope with a lens the size of a planet.

This is how the Luminary functions in its passive state. It sweeps a solar system again and again from multiple vantage points, catalogs areas of interest, and then it goes active. Instead of scattering a billion nanoscopic wormholes through a volume of space the size of a planet, billions of wormholes are opened up inside a target. Some sensor cells continue to collect passive information, others go active and transmit signals through the wormholes for other sensor cells to pick up. Active scanning builds the complete picture that the Luminary is looking for. Such a complete picture that it has to be massively simplified for display. So much information is collected that it is dangerous to give a smart AI unfettered access to the data stream.

As useful as they are, Luminaries have their limitations. The best way to elude a Luminary is to evade the passive scanning. This is what Prowlers do. They mimic a volume of empty space so well that the Luminaries disregard them. If a Prowler is actively scanned, the Luminary will reveal its position, and keep track of it unless the operator makes a mistake. There are also countermeasures of Forerunner origin which shut down Luminaries by dissolving the wormholes before they can be opened and stabilized. These are extremely rare in the modern day, however.

Luminaries' cells will also burn out from overuse. They can self-repair and replicate if given the raw materials and energy, but there is a limit to how much they can repair. This disincentivized Shipmasters from using the Luminaries at all during the Human-Covenant War, except to map a new system. At the onset of the Human-Covenant War, Luminaries were to be installed on every warship going to the war as they could be used to quickly detect a human presence. This put a considerable strain on the Covenant's manufacturing capability. Shipmasters were keenly aware that if their Luminaries burned out, they would not receive a replacement. Forerunner-built Luminaries do not seem to have limits on their self-repair functions, but they are too few and valuable to use in war. Instead, the Covenant scrape off cells to form the seeds of new Luminaries.

Most Luminaries are mounted in warships, and Shipmasters rarely coordinate operations with ground forces. Almost never do they feed Luminary data to their Fieldmaster bretheren, and the Fieldmasters never ask for it. There are micro-Luminaries small enough to mount in an aircraft called a Familiar, but those rarely appeared in battle.