The Grating Carrousel Mechanism Of The Goddard High Resolution Spectrograph For The Hubble Space Telescope

The active optical elements of the Goddard High Resolution Spectrograph (GHRS) consist of five plane diffraction gratings, one Echelle and four non-dispersive mirrors. The instrument has eleven modes for ultraviolet spectroscopy and target acquisition. The gratings and mirrors are mounted on a precision mechanism which rotates to position the desired element in the optical path. Fine adjustments select the precise wavelength region of interest and control a variety of scanning options. Aside from a simple two position shutter this is the only moving part involved in normal science observations. The requirements for precise image location, high spectral resolution and efficient autonomous operations place stringent demands on its performance. The carrousel is rotated by a brushless DC torque motor, which produces a slew rate of six degrees per second. The 16 bit position encoder provides a granularity of approximately 20 arc seconds per encoder step. The active electronic control system maintains pointing stability with an RMS jitter of 0.13 arc seconds, which corresponds to 0.033 pixels of image motion at the detector. The repeatability is such that the image returns to within ±0.10 pixels of the same position over 90% of the time. After rotating to a new position the carrousel settles to within the jitter within 3 seconds. This paper describes the mechanical and electronic design, the hardware logic and flight software algorithms which control its operation. Data from pre-flight test programs illustrate the performance of the carrousel.