Aligning the star tracker


Scott Heatwole aligning the theodolite with the mirror mounted to the star tracker. The mirror is the small white square in the hole near the bottom of the mirror bulkhead.

During the balloon flight, X-Calibur will be pointed at astrophysical objects by the Wallops Arc Second Pointer (WASP). The orientation of the telescope will be monitored constantly by an infrared camera observing stars. It is crucial that this camera be aligned with the optical axis of the X-ray mirror. After all, it’s the mirror that must be pointed at the objects we want to observe. The mechanical alignment was performed by Scott Heatwole and Tim Clair from Wallops Flight Facility prior to the night-sky pointing test.


Tim Clair pointing the theodolite at the Porro Prism. The prism is at the bottom right  in front of the white container.

The alignment was performed in several steps using a theodolite and a Porro Prism. Flat mirrors were mounted on the X-ray mirror and the star tracker exactly perpendicular to the optical axes of the two devices. The theodolite allowed the engineers to determine the direction perpendicular to the flat mirrors and their orientation relative to the vertical. The Porro Prism serves as an azimuth reference;  the theodolite is pointed towards the prism and rotated it until the reflected light is centered in the theodolite’s scope. The two angles — elevation and azimuth — completely specify the orientation of the flat mirrors. The star tracker’s orientation is then adjusted with shims until its optical axis is parallel to that of the X-ray mirror.

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