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6.2 Planning Observations
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6.2.1 Observing Modes
FPI+ is the standard tracking camera for the SOFIA telescope, and therefore does not require background ellimination techniques to be employed. The FPI+ science instrument offers one imaging configuration with three observing modes: FPI_TRACK_SLOW_STARE, FPI_TRACK_MEDIUM_STARE and FAST_STARE. There is no slit for spectroscopy available. The observing modes are described below.
FPI_TRACK_SLOW_STARE (1 MHz)
Allows image acquisition while tracking on the same images. The parallel tracking requires a resasonably bright guide star in the field of view and exposure times should not exceed 4000 milliseconds.
The higher readout rate results in a higher associated readout noise but also in faster frame rates. The parallel tracking requires a resasonably bright guide star in the field of view and exposure times should not exceed 4000 milliseconds.
FPI_TRACK_FAST_STARE (10 MHz)
The fastest readout currently possible with FPI+. This mode does not allow FPI+ tracking on the science images--however FFI tracking can be used.
The two slower observing modes offer, but do not 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 arcsec 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.
The sensitivities of the FPI+ with all spectral filter options have been implemented into the SOFIA Instrument Time Estimator (SITE). This online tool allows the estimation of exposure times to achieve a desired signal to noise ratio.
Generally, observations can be set up very efficiently and overheads are small. For each observation, bias frames and dark frames will be acquired for image calibration. These calibration acquisitions will result in additional overheads.
The overhead is the instrument set-up time per observation in minutes. It is calculated with:
Overhead = A * Repeat + B
where A = 0.013 sec, B = 200 sec, and Repeat is the total number of acquisitions with the specified exposure time.
The entire duration of the observation is:
Duration = ExposureTime * Repeat + Overhead
Observations are done in the frame transfer mode. This means parameter A represents the time for the charge transfer on the sensor and has a value of 1 to 3.4 µs.
The setup time is less than 20 seconds and approximately 3 minutes should be planned for the acquisition of bias and dark calibration frames.