8.2.1 Observing Modes

The FPI+ science instrument offers one imaging configuration with three observing modes. The modes are labeled FPI_TRACK_SLOW_STARE, FPI_TRACK_MEDIUM_STARE and FAST_STARE. There is no slit for spectroscopy available. The two slower observing modes offer, but don’t require, tracking in the FPI+ in parallel to the acquisition of science data. However, simultaneous FPI+ tracking does impose certain restrictions on the camera acquisition setting.

Acquisition setting restrictions for simultaneous FPI+ tracking are as follows:

  • Image frame size: Full frame
  • Pixel binning: 1x1, 2x2, 4x4
  • Exposure time: between 100 and 4,000 ms for best tracking performance
  • Target:Track star available in FPI+ field of view

With simultaneous FPI+ tracking, a target position accuracy of 0.17’’ rms has been measured over a two hour time period. There is no positional drift of the target evident.

Alternatively, all of the three observing modes can be used with tracking in the Fine Field Imager (FFI; the tracking camera on the telescope front ring) or without any tracking at all. This allows the selection of an arbitrary sub-frame and binning factors along with the choice of very short (or longer) exposure times in the FPI+. With FFI tracking, positional drifts of the target of 3.9 arcsec per hour have been observed.