http://messenger.jhuapl.edu/webcam/annotatedimages/annotated-20031026.html

MESSENGER Mission News
October 31, 2003

Square and Plumb

Just like a house needs straight walls and correct angles, it is critical that MESSENGER's components are carefully aligned during assembly. The device on the tripod in this week's annotated Webcam image is called an autocollimating telescope; its form and function are similar to a builder's transit. The MESSENGER team uses the telescope to align various components with a reference point on spacecraft's top deck.

Accurate alignment ensures MESSENGER's instruments and sensors will point to where spacecraft operators think they are pointing; mission scientists will also need this accuracy to correlate data that different instruments gather on similar sites.

On Oct. 26, the team checked the alignment of the four Digital Solar Attitude Detectors - also known as Sun Sensors - on the sunshade frame as well as a "DSAD" on the rear of the spacecraft. They also checked the alignment of MESSENGER's Inertial Measurement Unit (IMU); the edge of IMU is just visible from this viewpoint. The IMU, which includes gyroscopes and accelerometers, will accurately determine the spacecraft's orientation.