The figure presents additional information on the optics of the atomic-absorption-spectroscopy (AAS) system described in the preceding article. The optics include (1) a periscopic optical subsystem for access to a measurement optical axis in a plane different from that of the input and output optical axes and (2) mechanical and optoelectronic features for aligning the input and output optics.

The Source and Receiver Probes Are Aligned by use of several mechanisms, one of which is the locking ball joint on the receiver probe, shown in the detail view.
To recapitulate: the AAS system includes a source probe that contains a lamp plus collimating optics, and a receiver probe that contains collection optics. To make it possible to perform absorption spectral analysis, it is necessary to align the source and receiver probes so that the axis of the optical signal from the source probe coincides with the optical axis of the receiver probe.

Because of design considerations specific to the original rocket-engine-testing application, the source and receiver probe bodies must be mounted on a plane that coincides approximately with the exit plane of the rocket-engine nozzle, while the optical axis for the absorption measurements must lie in a plane 4 in. ( ≈ 10 cm) downstream from the exit plane. The source probe, which requires a large aperture to accommodate the spectral lamp, uses a two-mirror periscopic assembly to produce the required downstream offset. The receiver probe uses an optical fiber to produce the offset.

Since the environment inside the diffuser is a vacuum, the point where each probe penetrates the diffuser must provide both a seal and serve as the primary mount for the probes. Directional adjustments of the probes pivot at these points and are mechanically limited angular adjustments of 15° in the horizontal plane and 3° in the vertical plane. The ideal optical axis would be a line between the two pivot points displaced downward 4 in. ( ≈ 10 cm). However, mechanical obstructions in the diffuser make this difficult. To compensate, a locking, sealed gimbal mount that houses the receiver optics and optical fiber interface is attached to the input end of the receiver probe. The gimbal, which allows a 30° angular adjustment in any direction, allows the alignment of the optical axis of the receiver to be independent of the orientation of the receiver body.

Adjustment is performed in two stages. In the first stage, the probes are adjusted until the beam of light from the source probe illuminates the face of the gimbal locking cap. In the second stage, a laser is operated in a back-lighting arrangement to generate a beam of light that emerges from the input aperture of, and marks the optical axis of, the receiver probe; the receiver probe is adjusted until the laser beam enters the aperture of the source probe.

The principal innovative feature of the system is the locking ball joint in the receiver probe. For a previous version of the system, which did not include the ball joint, it was necessary to alternately adjust and secure one probe and then the other, repeatedly, in an iterative cycle necessitated by dependence of the target alignment angles of each probe on the adjustment of the other probe. The locking ball joint reduces this dependence and thereby aids the alignment process.

This work was done by Gregory P. McVay of Lockheed Martin for Stennis Space Center. For further information, access the Technical Support Package (TSP) free on-line at  under the Physical Sciences category. SSC-00066