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.