| Aperture Diameter|
| Aperture Diameter|
The resolution of a sensor is given by
Δxr = R λs / ds
A spacecraft's passive sensor array consists of a set of telescopes operating in the infrared to visible range. A global set of low-resolution, low-sensitivity scopes is used to track all possible targets (typically such an array can detect the waste heat of a fusion reactor several AUs distant). Target tracking and identification is carried out by a smaller number of larger telescopes (usually in the 50cm size range, but as large as 2m for major vessels; even larger for habitat cylinders). An OASIS (Optical Aperture Synthesis Imaging System) arrangement may be the range at which detailed target identification and imaging may be carried out. Vibrations caused by operating drive systems limits the use of OASIS to ships that are not accelerating.
On warships, there is a second type of important sensor: the x-ray telescope. Operating fusion drives discharge plasma at a temperature of around a hundred million kelvin. The initial cooling of this drive plume is by x-ray emission, and x-rays are also emitted from the fusing regions. A pulsed or continuous fusion drive produces terawatts of power, much of which ends up as x-rays, or extreme ultraviolet. As well as detecting the high-temperature drive plume, x-ray calorimetry can give information on the drive geometry and reaction conditions. Such data can help to indentify the class of ship (or design of drive). The cylindrical mirrors used on x-ray telescopes are of much poorer quality than the mirrors used in near-optical telescopes, so the x-ray detectors are of only limited use for attaining firing solutions.
Passive sensors can track targets at a much greater range than the range at which indentification becomes possible. The plume from a fusion rocket is so clearly visible that warships spend most of the time on unpowered orbits, or accelerating using a secondary drive at much lower power, or accelerating in short bursts. The fusion drive is only used when stealth has ceased to be a significant factor (i.e. when actively engaging enemy vessels).
The future of Ad Astra