FUSE is equipped with two Fine Error Sensors, one for each of the LiF mirrors. They were built by COM DEV Ltd. of Cambridge, Ontario, and supplied to FUSE by the Canadian Space Agency (CSA).

Dr. Terry Hughes (left; Program Scientist, CSA Space Science Program Office) and Russ Alexander (right; CSA FES Project Manager) examine a full-scale model of the -Y FES CCD camera, heatstrap, radiator, and controller box.

A full-scale model of the -Y FES components. From left to right, these are: the CCD camera, the radiator and its associated heatstrap, and the controller box. The model was built by Russ Alexander (CSA FES Project Manager). It is currently located on the full-scale mock-up of the FUSE satellite in the rotunda of the Bloomberg Center for Physics and Astronomy at Johns Hopkins University.

Each FES consists of camera with a sensitive charge-coupled device (CCD) detector. These cameras obtain visible-light images of a small patch of sky (about 19.5 x 19.5 arcminutes) surrounding a science target. This light is fed to the FES by the mirrored front surface of a focal-plane assembly (FPA), which also contains the entrance apertures for the far-ultraviolet spectrograph. The FES determines the positions of several field stars in the neighborhood of the target, and feeds this information to the satellite control system in order to keep FUSE centered on the target (to an accuracy of about 0.5 arcseconds) during science exposures. Only one FES is required for operations: the other serves as a redundant backup system.

The CCDs in the FES units have 1024 x 1024 pixels, each of which is 24 microns square. In order to be sufficiently sensitive, they are cooled to approximately -60 degrees C by means of a thermoelectric cooler unit. The excess heat generated by these units is removed from the spectrograph cavity by conduction along a heat strap to a large, flat radiator (see above), which will be mounted on the side of the satellite. The satellite is positioned so that the active FES radiator is kept in the shade.

Photo Credits: Rejean Michaud (CSA Space Science Program Office)