Theodolite autocollimation metrology continues to play an important part in spacecraft optical alignment. Spacecraft optical alignment is both art and science for using optical instruments to place or determine the orientation and envelope of critical components on spaceflight hardware.

The optical alignment of the Global Precipitation Measurement (GPM) core spacecraft at Goddard Space Flight Center (GSFC) presented extraordinary challenges, such as the alignment of the GPM star trackers with shades in their location on the lower bus structure and the g-negated deployment testing of the full high gain antenna system. In both cases the tasks were successfully completed by the ingenious application of theodolite autocollimation metrology.

A theodolite is a small, movable telescope mounted within two perpendicular axes of rotation, one vertical and one horizontal. The circles of rotation are precisely calibrated to mark the angle of rotation about each axis, thus providing the angular orientation of the telescope. Theodolite autocollimation measurements are used to determine the relative alignment between various components on a test object with respect to a common coordinate system (CS). Generally, the optical axis of each component has been previously related to an external optical reference surface, such as a mirror or an optical reference cube mounted rigidly to the component. Autocollimation occurs when collimated light emanating from the theodolite is returned along the same path after its reflection from the reflective surface (mirror or cube face). A level of skill is required by theodolite operators to gain line-of-sight and to autocollimate on various reflective surfaces at various heights and angles that may be required to measure all required cube faces of components on a given test object. Each theodolite in the system must be critically leveled with respect to gravity before a measurement can be made and every measurement must be referenced to the “primary” theodolite, which acts as the facility or fixed laboratory azimuth reference for all measurements in the system. The theodolite from which light is actually autocollimated on a cube face is called the “subject” theodolite for that measurement. Often, a subject theodolite cannot be referenced directly to the primary theodolite. The go-between is another theodolite called a secondary theodolite.

The initial goal of theodolite measurements is to obtain the roll and zenith of each reflective surface normal (vector) in the CS of the theodolite designated as the primary theodolite. The roll and zenith are used to calculate the direction cosines of each vector. As all the vectors are in a common CS, vector analysis can be used to construct a CS using any two non-parallel vectors, such as the vectors representing two adjacent faces of an optical reference cube fixed with respect to the test object. If so constructed, every measured vector can then be transformed into a CS based on this optical cube, which in turn has been previously related to the axes of a flight instrument or other spacecraft component.

This work was done by Joseph C. McMann and Kyle F. McLean of Goddard Space Flight Center, and Vicki I. Roberts, James E. Gill, Samuel E. Hetherington, and Dean Osgood of QinetiQ North America. GSC-17020-1

Photonics Tech Briefs Magazine

This article first appeared in the November, 2015 issue of Photonics Tech Briefs Magazine.

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