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29.4 Science Observing Modes

 The FOS employed one of six basic data taking modes to acquire science data:

29.4.1 Spectrophotometry Mode (ACCUM)

 ACCUM was the standard FOS spectrophotometric data taking mode. Data were acquired with NXSTEPS=4 and OVERSCAN=5 only. Spectra were read out at approximate 2 minute (FOS/RD) or 4 minute (FOS/BL) intervals and the accumulated sum after each read was stored and recorded in consecutive groups in the standard output data files (see next chapter for description of these files). Therefore, each consecutive spectrum was made up of the sum of all previous intervals of data in an ACCUM observation.

29.4.2 Rapid Readout Mode (RAPID)

Observations requiring higher time resolution than that provided by ACCUM mode used RAPID mode in which the diode array was read out at a rate set by the observer. Readout rates as fast as 0.033 sec were possible (0.035 sec was the fastest ever implemented). NXSTEPS=1, 2, or 4 and OVERSCAN=5 (or, rarely, 1) were normally used, though the full set of commandable values was available. The actual percentage of specified dwell time that was spent exposing could vary substantially depending upon how instrumental parameters were chosen. Timing precision more accurate than 0.125 seconds requires special analysis of engineering data stream information (see timing discussion in Chapter 33). Unlike ACCUM, the actual spectrum obtained in each individual time interval was recorded in the output data products.

29.4.3 Time-Resolved Spectrophotometry (PERIOD)

As a result of implementation errors there were no fully successful PERIOD mode observations made in the FOS science program. Successful tests were performed during Science Verification.

This mode was designed for objects with known periodicity in the 50 msec to 100 sec range. To maintain the phase information of these observations, the known period (CYCLE-TIME) of the object was divided into bins or slices, where each bin had a duration time = period / bins. The spectra acquired in this mode were stored in the different bins which corresponded to a given phase of the period. The spectral information obtained in each period cycle was added to the appropriate phase bin (so long as the period was known accurately). Onboard memory limitations placed substantial restrictions on the number of diodes that could be read out and the number of phase bins that could be sampled.

29.4.4 Spectropolarimetry Mode

Polarimetry data consist of a number of consecutive ACCUM-like exposures (POLSCAN=16, 8, or 4) with the waveplate set at POLSCAN different angles (all within one target visibility period). The Wollaston prism split the light beam into two spectra corresponding to the orthogonal directions of polarization. Although SINGLE apertures were always used, both spectra illuminated different portions of the photocathode-one above, one below the standard SINGLE aperture Y-base location for the disperser employed. Hence, each exposure consisted of the two orthogonal spectra obtained with a single waveplate angle. The first spectrum corresponded to the first pass direction (ordinary ray), the second to the second pass direction (extraordinary ray). These spectra were deflected alternately (not simultaneously) onto the diode array, recorded as the two pass directions, and stored as a single group in the raw data file. The total exposure time specified in the exposure logsheet was divided equally among each of the POLSCAN steps with one-half of the resultant exposure times spent observing each pass direction. These requirements, combined with large instrument overheads, placed tight limits on the length of any individual polarizer position exposure.

Chapter 32 discusses how Y-base positional uncertainties and GIM effects, coupled with the large pre-COSTAR PSF and the influence of the additional reflections in COSTAR, limited the accuracies of FOS polarimetric observations.

29.4.5 Image Mode

IMAGE mode observations could be made with either the camera MIRROR (as in ACQ images discussed above) or with a disperser in the beam. Normally IMAGE mode observations used NXSTEPS=4 and OVERSCAN=5, but the full set of commandable values were available. In IMAGE mode observations both the number and length of rasters and the y-position of each raster could be specified, so that: 1) a white-light image of all or a portion of the aperture was possible with the MIRROR, and 2) spectra from different portions of the aperture were sampled with a disperser.

ACQ mode images, discussed above, simply used a pre-defined, optimized set of IMAGE mode parameters. Most dispersed light IMAGE mode observations were made for specialized calibration purposes, but this mode was occasionally used for interactive target acquisitions of moving targets (the so-called dispersed light interactive target acquisition technique).

29.4.6 Science Data-Taking with Paired Apertures

Depending upon the value of the STEP-PATT RPS2 (RPSS) parameter, paired aperture observations could invoke a different detector readout pattern from that employed for single apertures for ACCUM and RAPID exposures. In certain circumstances (see next chapter) readout deflections were alternated equally between upper and lower paired apertures at approximate 10 second intervals for the duration of the exposure. An additional, but never used, pattern allowed such upper and lower aperture samplings and an additional background sample halfway between the apertures.


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