An external enclosure, the complex Housing Structure Assembly (HSA), accommodates all ATLID sub-systems and provides the instrument mechanical interfaces to the spacecraft. The HSA also supports all external radiators, the large telescope baffle and the instrument electronics units (mounted inside the HSA).

The picture below shows the instrument internal support bench, known as the 'Stable Structure Assembly', which accommodates alignment of all critical units and sub-systems. These include the redundant transmitters with their long baffles and the receiver chain from primary mirror to Focal Plane Assembly.

The Emitter chain includes:

1. The laser Transmitter Assembly consists of a Power Laser Head (PLH) and its Transmitter Laser Electronics, and a Reference Laser Head, which provides the stable seed laser
2. The Emission Beam Expander (EBEX) is used to enlarge the laser beam from the PLH in order to meet the divergence requirement (beam diffraction is reduced by increasing the emitted beam diameter) and to minimise the laser fluence on the last dioptre exposed to vacuum
3. The emission baffle: This baffle is designed to protect the EBEX output window from external contamination during instrument and satellite assembly and testing, and during flight operations

The ATLID power laser head is sealed and pressurised, which ensures more stable operating conditions for the sensitive components of the laser and isolates the laser internal space from surrounding contaminants during the ground and operational lifetime. Pressure also improves tolerance to laser induced contamination, which is the degradation of an optical surface resulting from the interaction of molecular contamination with a high laser illumination level.

The Receiver chain includes:

1. The receiver telescope: An afocal Cassegrain with 620 mm primary mirror diameter, which collects the backscattered light. It is made of Silicon Carbide to ensure high stability
2. Receiver optics: Connects the telescope output to the fibre optic detector entrance
3. The science channels detection functions: Ensured by the Memory Charge-Coupled Device (CCD) and the Instrument Detection Electronics. The detection chain will measure single photon events to meet the worst-case radiometric performance requirements
4. The control and data management unit